From bd52e0d9368286bf4d2ed745eb9a8177b1b2203e Mon Sep 17 00:00:00 2001 From: aarne Date: Tue, 3 Jul 2007 15:29:23 +0000 Subject: [PATCH] contents of new tutorial --- doc/tutorial-next/Food.gf | 14 + doc/tutorial-next/FoodEng.gf | 23 + doc/tutorial-next/FoodIta.gf | 22 + doc/tutorial-next/Makefile | 6 + doc/tutorial-next/Tree2.png | Bin 0 -> 1717 bytes doc/tutorial-next/food.cf | 15 + doc/tutorial-next/gf-tutorial2.txt | 2940 ++++++++++++++++++++++++++++ 7 files changed, 3020 insertions(+) create mode 100644 doc/tutorial-next/Food.gf create mode 100644 doc/tutorial-next/FoodEng.gf create mode 100644 doc/tutorial-next/FoodIta.gf create mode 100644 doc/tutorial-next/Makefile create mode 100644 doc/tutorial-next/Tree2.png create mode 100644 doc/tutorial-next/food.cf create mode 100644 doc/tutorial-next/gf-tutorial2.txt diff --git a/doc/tutorial-next/Food.gf b/doc/tutorial-next/Food.gf new file mode 100644 index 000000000..1a2d38d1e --- /dev/null +++ b/doc/tutorial-next/Food.gf @@ -0,0 +1,14 @@ +abstract Food = { + + cat + S ; Item ; Kind ; Quality ; + + fun + Is : Item -> Quality -> S ; + This, That : Kind -> Item ; + QKind : Quality -> Kind -> Kind ; + Wine, Cheese, Fish : Kind ; + Very : Quality -> Quality ; + Fresh, Warm, Italian, Expensive, Delicious, Boring : Quality ; + +} \ No newline at end of file diff --git a/doc/tutorial-next/FoodEng.gf b/doc/tutorial-next/FoodEng.gf new file mode 100644 index 000000000..f75727292 --- /dev/null +++ b/doc/tutorial-next/FoodEng.gf @@ -0,0 +1,23 @@ +concrete FoodEng of Food = { + + lincat + S, Item, Kind, Quality = {s : Str} ; + + lin + Is item quality = {s = item.s ++ "is" ++ quality.s} ; + This kind = {s = "this" ++ kind.s} ; + That kind = {s = "that" ++ kind.s} ; + QKind quality kind = {s = quality.s ++ kind.s} ; + Wine = {s = "wine"} ; + Cheese = {s = "cheese"} ; + Fish = {s = "fish"} ; + Very quality = {s = "very" ++ quality.s} ; + Fresh = {s = "fresh"} ; + Warm = {s = "warm"} ; + Italian = {s = "Italian"} ; + Expensive = {s = "expensive"} ; + Delicious = {s = "delicious"} ; + Boring = {s = "boring"} ; + +} + \ No newline at end of file diff --git a/doc/tutorial-next/FoodIta.gf b/doc/tutorial-next/FoodIta.gf new file mode 100644 index 000000000..5c565037a --- /dev/null +++ b/doc/tutorial-next/FoodIta.gf @@ -0,0 +1,22 @@ +concrete FoodIta of Food = { + + lincat + S, Item, Kind, Quality = {s : Str} ; + + lin + Is item quality = {s = item.s ++ "è" ++ quality.s} ; + This kind = {s = "questo" ++ kind.s} ; + That kind = {s = "quello" ++ kind.s} ; + QKind quality kind = {s = kind.s ++ quality.s} ; + Wine = {s = "vino"} ; + Cheese = {s = "formaggio"} ; + Fish = {s = "pesce"} ; + Very quality = {s = "molto" ++ quality.s} ; + Fresh = {s = "fresco"} ; + Warm = {s = "caldo"} ; + Italian = {s = "italiano"} ; + Expensive = {s = "caro"} ; + Delicious = {s = "delizioso"} ; + Boring = {s = "noioso"} ; + +} diff --git a/doc/tutorial-next/Makefile b/doc/tutorial-next/Makefile new file mode 100644 index 000000000..d226e7348 --- /dev/null +++ b/doc/tutorial-next/Makefile @@ -0,0 +1,6 @@ +html: + txt2tags -thtml --toc gf-tutorial2.txt +tex: + txt2tags -ttex --toc gf-tutorial2.txt + pdflatex gf-tutorial2.tex + pdflatex gf-tutorial2.tex diff --git a/doc/tutorial-next/Tree2.png b/doc/tutorial-next/Tree2.png new file mode 100644 index 0000000000000000000000000000000000000000..f58e56b9586a264dd3bf7c2a81dc4508a4ff860c GIT binary patch literal 1717 zcmeAS@N?(olHy`uVBq!ia0y~yU_8gbz*x-0%)r2K>JDQE0|NtRfKQ0)|Ns9O7#RNl z|9@jf%Wnn-2F8*gzhDN3XE)Lq7}%mbT^vIy=DfXgw(qfq0Q-kpPo36kzLnn_56}4) zr2KGs%z?h!@-G%~SriD*O*k<%--S0WJLTZ=!^1?=&z596oMm?C@x+TJVk>T=<6Y`wT z3LLJ^S3KK%G`#N3W^tHNAvSlepT56+q4vqA>wl$vx&F%ZZtJ>*@jJWsN#1PUSI8O|-t?Y@s^!O1B)Bdywbe z7m10Jyk5C_7Cd1KnQ+C=M9{}E$ogW7f=8D6tS;5q-enC{LG}W^&(_Cz)@Bs_t5W^C z@TJG4tr=CnvVwMB@KIRp`PRvJbNJ+2tXqi8!!zTyY?$X2WY4QR#e3-_s|$yUw5DEZ40^m%Zp)5YMM3sFx?{#F#n@`wqQe$GfmaLdmfrpF+i~{lEor%- zTb8+0_Bo&*W@hdG%IQoh|dJDmUHp4|5&g-bw!z zm${?;IpftckIR-O>;DSfl9;vl+X}^{6`oH|TwCn+b&fZ;p z5&)CyN*BOWrQH{Lt3;&#s{M{dLlh-0eTWd_TXR>v!hi6Mq6EG%oDt z68*N9ne|t0#n}+m7s7xw!Y`-F9_{+!u;>7nwDmUf%;>EDYozifiFQ^jv*>&|9q+Z&-Q#iiWxT}JA6`0}Z4Cg0wvT_|*Nk9%t+ zeXHmE!u|`lcP;qHAsP@SXqt8N;Way(Rc*!Eq5CG7t_kNR-zn_|? z)F;k6{q92Ef`jedwQqlYciVF5?ymE(buX@!Co|u&{j>Pfayvv|_w73K+WVq!GxOb+ zyt>n`pX}#~4h*j-m)w;tzcBhj{L$^=3%Ar(FUo&*_(JW|l8{L0vX)!FW{&?V3Ewr{dhWuKIex1omOftiY>C&!5T@CawOl8!UetXtC{L1otM=IhLJ9Lnb&a5dRG`FT;6&AWrI`**PHdd;$KL7UV6w<}H@ zse2iI3Wz|-i4*Lw%ghBbyW6W+Hq{@qHl8#xL$dFX +% %!postproc(html): -C +% %!postproc(html): t- +% %!postproc(html): -t + + + + +[../gf-logo.png] + + + +%--! +=Introduction= + +==GF = Grammatical Framework== + +The term GF is used for different things: + +- a **program** used for working with grammars +- a **programming language** in which grammars can be written +- a **theory** about grammars and languages + + +This tutorial is primarily about the GF program and +the GF programming language. +It will guide you + +- to use the GF program +- to write GF grammars +- to write programs in which GF grammars are used as components + + + +%--! +==What are GF grammars used for== + +A grammar is a definition of a language. +From this definition, different language processing components +can be derived: + +- parsing: to analyse the language +- linearization: to generate the language +- translation: to analyse one language and generate another + + +A GF grammar can be seen as a declarative program from which these +processing tasks can be automatically derived. In addition, many +other tasks are readily available for GF grammars: + +- morphological analysis: find out the possible inflection forms of words +- morphological synthesis: generate all inflection forms of words +- random generation: generate random expressions +- corpus generation: generate all expressions +- teaching quizzes: train morphology and translation +- multilingual authoring: create a document in many languages simultaneously +- speech input: optimize a speech recognition system for your grammar + + +A typical GF application is based on a **multilingual grammar** involving +translation on a special domain. Existing applications of this idea include + +- [Alfa: http://www.cs.chalmers.se/~hallgren/Alfa/Tutorial/GFplugin.html]: + a natural-language interface to a proof editor + (languages: English, French, Swedish) +- [KeY http://www.key-project.org/]: + a multilingual authoring system for creating software specifications + (languages: OCL, English, German) +- [TALK http://www.talk-project.org]: + multilingual and multimodal dialogue systems + (languages: English, Finnish, French, German, Italian, Spanish, Swedish) +- [WebALT http://webalt.math.helsinki.fi/content/index_eng.html]: + a multilingual translator of mathematical exercises + (languages: Catalan, English, Finnish, French, Spanish, Swedish) +- [Numeral translator http://www.cs.chalmers.se/~bringert/gf/translate/]: + number words from 1 to 999,999 + (88 languages) + + +The specialization of a grammar to a domain makes it possible to +obtain much better translations than in an unlimited machine translation +system. This is due to the well-defined semantics of such domains. +Grammars having this character are called **application grammars**. +They are different from most grammars written by linguists just +because they are multilingual and domain-specific. + +However, there is another kind of grammars, which we call **resource grammars**. +These are large, comprehensive grammars that can be used on any domain. +The GF Resource Grammar Library has resource grammars for 10 languages. +These grammars can be used as **libraries** to define application grammars. +In this way, it is possible to write a high-quality grammar without +knowing about linguistics: in general, to write an application grammar +by using the resource library just requires practical knowledge of +the target language. and all theoretical knowledge about its grammar +is given by the libraries. + + + + +%--! +==Who is this tutorial for== + +This tutorial is mainly for programmers who want to learn to write +application grammars. It will go through GF's programming concepts +without entering too deep into linguistics. Thus it should +be accessible to anyone who has some previous programming experience. + +A separate document has been written on how to write resource grammars. +Nevertheless, we will cover some linguistic problems posed by different +languages and how they are solved in GF. + + +%--! +==The coverage of the tutorial== + +The tutorial gives a hands-on introduction to grammar writing. +We start by building a small grammar for the domain of food: +in this grammar, you can say things like +``` + this Italian cheese is delicious +``` +in English and Italian. + +The first English grammar +[``food.cf`` food.cf] +is written in a context-free +notation (also known as BNF). The BNF format is often a good +starting point for GF grammar development, because it is +simple and widely used. However, the BNF format is not +good for multilingual grammars. While it is possible to +"translate" by just changing the words contained in a +BNF grammar to words of some other +language, proper translation usually involves more. +For instance, the order of words may have to be changed: +``` + Italian cheese ===> formaggio italiano +``` +The full GF grammar format is designed to support such +changes, by separating between the **abstract syntax** +(the logical structure) and the **concrete syntax** (the +sequence of words) of expressions. + +There is more than words and word order that makes languages +different. Words can have different forms, and which forms +they have vary from language to language. For instance, +Italian adjectives usually have four forms where English +has just one: +``` + delicious (wine, wines, pizza, pizzas) + vino delizioso, vini deliziosi, pizza deliziosa, pizze deliziose +``` +The **morphology** of a language describes the +forms of its words. While the complete description of morphology +belongs to resource grammars, this tutorial will explain the +programming concepts involved in morphology. This will moreover +make it possible to grow the fragment covered by the food example. +The tutorial will in fact build a miniature resource grammar in order +to illustrate the module structure of library-based application +grammar writing. + +Thus it is by elaborating the initial ``food.cf`` example that +the tutorial makes a guided tour through all concepts of GF. +While the constructs of the GF language are the main focus, +also the commands of the GF system are introduced as they +are needed. + +To learn how to write GF grammars is not the only goal of +this tutorial. To learn the commands of the GF system means +that simple applications of grammars, such as translation and +quiz systems, can be built simply by writing scripts for the +system. More complicated applications, such as natural-language +interfaces and dialogue systems, also require programming in +some general-purpose language. We will briefly explain how +GF grammars are used as components of Haskell, Java, Javascript, +and Prolog grammars. The tutorial concludes with a couple of +case studies showing how such complete systems can be built. + + + +%--! +==Getting the GF program== + +The GF program is open-source free software, which you can download via the +GF Homepage: +[``http://www.cs.chalmers.se/~aarne/GF`` http://www.cs.chalmers.se/~aarne/GF] + +There you can download +- binaries for Linux, Mac OS X, and Windows +- source code and documentation +- grammar libraries and examples + + +If you want to compile GF from source, you need Haskell and Java +compilers. But normally you don't have to compile, and you definitely +don't need to know Haskell or Java to use GF. + +We are assuming the availability of a Unix shell. Mac OS X users +have it automatically, under the name "terminal". +Windows users are recommended to install Cywgin, the free Unix shell for Windows. + +To start the GF program, assuming you have installed it, just type +``` + % gf +``` +in the shell. +You will see GF's welcome message and the prompt ``>``. +The command +``` + > help +``` +will give you a list of available commands. + +As a common convention in this Tutorial, we will use +- ``%`` as a prompt that marks system commands +- ``>`` as a prompt that marks GF commands + + +Thus you should not type these prompts, but only the lines that +follow them. + + + +%--! +=The .cf grammar format= + +Now you are ready to try out your first grammar. +We start with one that is not written in the GF language, but +in the much more common BNF notation (Backus Naur Form). The GF +program understands a variant of this notation and translates it +internally to GF's own representation. + +To get started, type (or copy) the following lines into a file named +``food.cf``: +``` +Is. S ::= Item "is" Quality ; +That. Item ::= "that" Kind ; +This. Item ::= "this" Kind ; +QKind. Kind ::= Quality Kind ; +Cheese. Kind ::= "cheese" ; +Fish. Kind ::= "fish" ; +Wine. Kind ::= "wine" ; +Italian. Quality ::= "Italian" ; +Boring. Quality ::= "boring" ; +Delicious. Quality ::= "delicious" ; +Expensive. Quality ::= "expensive" ; +Fresh. Quality ::= "fresh" ; +Very. Quality ::= "very" Quality ; +Warm. Quality ::= "warm" ; +``` +For those who know ordinary BNF, the +notation we use includes one extra element: a **label** appearing +as the first element of each rule and terminated by a full stop. + +The grammar we wrote defines a set of phrases usable for speaking about food. +It builds **sentences** (``S``) by assigning ``Quality``s to +``Item``s. ``Item``s are build from ``Kind``s by prepending the +word "this" or "that". ``Kind``s are either **atomic**, such as +"cheese" and "wine", or formed by prepending a ``Quality`` to a +``Kind``. A ``Quality`` is either atomic, such as "Italian" and "boring", +or built by another ``Quality`` by prepending "very". Those familiar with +the context-free grammar notation will notice that, for instance, the +following sentence can be built using this grammar: +``` + this delicious Italian wine is very very expensive +``` + + + +%--! +==Importing grammars and parsing strings== + +The first GF command needed when using a grammar is to **import** it. +The command has a long name, ``import``, and a short name, ``i``. +You can type either +``` + > import food.cf +``` +or +``` + > i food.cf +``` +to get the same effect. +The effect is that the GF program **compiles** your grammar into an internal +representation, and shows a new prompt when it is ready. It will also show how much +CPU time is consumed: +``` + > i food.cf + - parsing cf food.cf 12 msec + 16 msec + > +``` +You can now use GF for **parsing**: +``` + > parse "this cheese is delicious" + Is (This Cheese) Delicious + + > p "that wine is very very Italian" + Is (That Wine) (Very (Very Italian)) +``` +The ``parse`` (= ``p``) command takes a **string** +(in double quotes) and returns an **abstract syntax tree** - the thing +beginning with ``Is``. Trees are built from the rule labels given in the +grammar, and record the ways in which the rules are used to produce the +strings. A tree is, in general, something easier than a string +for a machine to understand and to process further. + +Strings that return a tree when parsed do so in virtue of the grammar +you imported. Try parsing something else, and you fail +``` + > p "hello world" + Unknown words: hello world +``` + + + +%--! +==Generating trees and strings== + +You can also use GF for **linearizing** +(``linearize = l``). This is the inverse of +parsing, taking trees into strings: +``` + > linearize Is (That Wine) Warm + that wine is warm +``` +What is the use of this? Typically not that you type in a tree at +the GF prompt. The utility of linearization comes from the fact that +you can obtain a tree from somewhere else. One way to do so is +**random generation** (``generate_random = gr``): +``` + > generate_random + Is (This (QKind Italian Fish)) Fresh +``` +Now you can copy the tree and paste it to the ``linearize command``. +Or, more conveniently, feed random generation into linearization by using +a **pipe**. +``` + > gr | l + this Italian fish is fresh +``` +Pipes in GF work much the same way as Unix pipes: they feed the output +of one command into another command as its input. + + +%--! +==Visualizing trees== + +The gibberish code with parentheses returned by the parser does not +look like trees. Why is it called so? From the abstract mathematical +point of view, trees are a data structure that +represents **nesting**: trees are branching entities, and the branches +are themselves trees. Parentheses give a linear representation of trees, +useful for the computer. But the human eye may prefer to see a visualization; +for this purpose, GF provides the command ``visualizre_tree = vt``, to which +parsing (and any other tree-producing command) can be piped: + +``` + parse "this delicious cheese is very Italian" | vt +``` + +[Tree2.png] + +This command uses the programs Graphviz and Ghostview, which you +might not have, but which are freely available on the web. + + + +%--! +==Some random-generated sentences== + +Random generation is a good way to test a grammar; it can also +be fun. So you may want to +generate ten strings with one and the same command: +``` + > gr -number=10 | l + that wine is boring + that fresh cheese is fresh + that cheese is very boring + this cheese is Italian + that expensive cheese is expensive + that fish is fresh + that wine is very Italian + this wine is Italian + this cheese is boring + this fish is boring +``` + + +%--! +==Systematic generation== + +To generate //all// sentence that a grammar +can generate, use the command ``generate_trees = gt``. +``` + > generate_trees | l + that cheese is very Italian + that cheese is very boring + that cheese is very delicious + that cheese is very expensive + that cheese is very fresh + ... + this wine is expensive + this wine is fresh + this wine is warm + +``` +You get quite a few trees but not all of them: only up to a given +**depth** of trees. To see how you can get more, use the +``help = h`` command, +``` + help gt +``` +**Quiz**. If the command ``gt`` generated all +trees in your grammar, it would never terminate. Why? + + + +%--! +==More on pipes; tracing== + +A pipe of GF commands can have any length, but the "output type" +(either string or tree) of one command must always match the "input type" +of the next command. + +The intermediate results in a pipe can be observed by putting the +**tracing** flag ``-tr`` to each command whose output you +want to see: +``` + > gr -tr | l -tr | p + + Is (This Cheese) Boring + this cheese is boring + Is (This Cheese) Boring +``` +This facility is good for test purposes: for instance, you +may want to see if a grammar is **ambiguous**, i.e. +contains strings that can be parsed in more than one way. + +**Exercise**. Extend the grammar ``food.cf`` so that it produces ambiguous strings. + + + +%--! +==Writing and reading files== + +To save the outputs of GF commands into a file, you can +pipe it to the ``write_file = wf`` command, +``` + > gr -number=10 | l | write_file exx.tmp +``` +You can read the file back to GF with the +``read_file = rf`` command, +``` + > read_file exx.tmp | p -lines +``` +Notice the flag ``-lines`` given to the parsing +command. This flag tells GF to parse each line of +the file separately. Without the flag, the grammar could +not recognize the string in the file, because it is not +a sentence but a sequence of ten sentences. + + + + +%--! +=The .gf grammar format= + +To see GF's internal representation of a grammar +that you have imported, you can give the command +``print_grammar = pg``, +``` + > print_grammar +``` +The output is quite unreadable at this stage, and you may feel happy that +you did not need to write the grammar in that notation, but that the +GF grammar compiler produced it. + +However, we will now start the demonstration +how GF's own notation gives you +much more expressive power than the ``.cf`` +format. We will introduce the ``.gf`` format by presenting +another way of defining the same grammar as in +``food.cf``. +Then we will show how the full GF grammar format enables you +to do things that are not possible in the context-free format. + + +%--! +==Abstract and concrete syntax== + +A GF grammar consists of two main parts: + +- **abstract syntax**, defining what syntax trees there are +- **concrete syntax**, defining how trees are linearized into strings + + +The context-free format fuses these two things together, but it is always +possible to take them apart. For instance, the sentence formation rule +``` + Is. S ::= Item "is" Quality ; +``` +is interpreted as the following pair of GF rules: +``` + fun Is : Item -> Quality -> S ; + lin Is item quality = {s = item.s ++ "is" ++ quality.s} ; +``` +The former rule, with the keyword ``fun``, belongs to the abstract syntax. +It defines the **function** +``Is`` which constructs syntax trees of form +(``Is`` //item// //quality//). + +The latter rule, with the keyword ``lin``, belongs to the concrete syntax. +It defines the **linearization function** for +syntax trees of form (``Is`` //item// //quality//). + + +%--! +==Judgement forms== + +Rules in a GF grammar are called **judgements**, and the keywords +``fun`` and ``lin`` are used for distinguishing between two +**judgement forms**. Here is a summary of the most important +judgement forms: + + - abstract syntax + + | form | reading | + | ``cat`` C | C is a category + | ``fun`` f ``:`` A | f is a function of type A + + - concrete syntax + + | form | reading | + | ``lincat`` C ``=`` T | category C has linearization type T + | ``lin`` f ``=`` t | function f has linearization t + + + +We return to the precise meanings of these judgement forms later. +First we will look at how judgements are grouped into modules, and +show how the food grammar is +expressed by using modules and judgements. + + +%--! +==Module types== + +A GF grammar consists of **modules**, +into which judgements are grouped. The most important +module forms are + + - ``abstract`` A ``=`` M, abstract syntax A with judgements in + the module body M. + - ``concrete`` C ``of`` A ``=`` M, concrete syntax C of the + abstract syntax A, with judgements in the module body M. + + +%--! +==Basic types and function types== + +The nonterminals of a context-free grammar, i.e. categories, +are called **basic types** in the type system of GF. In addition +to them, there are **function types** such as +``` + Item -> Quality -> S +``` +This type is read "a function from iterms and qualities to sentences". +The last type in the arrow-separated sequence is the **value type** +of the function type, the earlier types are its **argument types**. + + + + +%--! +==Records and strings== + +The linearization type of a category is a **record type**, with +zero of more **fields** of different types. The simplest record +type used for linearization in GF is +``` + {s : Str} +``` +which has one field, with **label** ``s`` and type ``Str``. + +Examples of records of this type are +``` + {s = "foo"} + {s = "hello" ++ "world"} +``` + +Whenever a record ``r`` of type ``{s : Str}`` is given, +``r.s`` is an object of type ``Str``. This is +a special case of the **projection** rule, allowing the extraction +of fields from a record: + +- if //r// : ``{`` ... //p// : //T// ... ``}`` then //r.p// : //T// + + +The type ``Str`` is really the type of **token lists**, but +most of the time one can conveniently think of it as the type of strings, +denoted by string literals in double quotes. + +Notice that +``` "hello world" +is not recommended as an expression of type ``Str``. It denotes +a token with a space in it, and will usually +not work with the lexical analysis that precedes parsing. A shorthand +exemplified by +``` + ["hello world and people"] === "hello" ++ "world" ++ "and" ++ "people" +``` +can be used for lists of tokens. The expression +``` + [] +``` +denotes the empty token list. + + + +%--! +==An abstract syntax example== + +To express the abstract syntax of ``food.cf`` in +a file ``Food.gf``, we write two kinds of judgements: + +- Each category is introduced by a ``cat`` judgement. +- Each rule label is introduced by a ``fun`` judgement, + with the type formed from the nonterminals of the rule. + + +``` + abstract Food = { + + cat + S ; Item ; Kind ; Quality ; + + fun + Is : Item -> Quality -> S ; + This, That : Kind -> Item ; + QKind : Quality -> Kind -> Kind ; + Wine, Cheese, Fish : Kind ; + Very : Quality -> Quality ; + Fresh, Warm, Italian, Expensive, Delicious, Boring : Quality ; + } +``` +Notice the use of shorthands permitting the sharing of +the keyword in subsequent judgements, +``` + cat S ; Item ; === cat S ; cat Item ; +``` +and of the type in subsequent ``fun`` judgements, +``` + fun Wine, Fish : Kind ; === + fun Wine : Kind ; Fish : Kind ; === + fun Wine : Kind ; fun Fish : Kind ; +``` +The order of judgements in a module is free. + + + +%--! +==A concrete syntax example== + +Each category introduced in ``Food.gf`` is +given a ``lincat`` rule, and each +function is given a ``lin`` rule. Similar shorthands +apply as in ``abstract`` modules. +``` + concrete FoodEng of Food = { + + lincat + S, Item, Kind, Quality = {s : Str} ; + + lin + Is item quality = {s = item.s ++ "is" ++ quality.s} ; + This kind = {s = "this" ++ kind.s} ; + That kind = {s = "that" ++ kind.s} ; + QKind quality kind = {s = quality.s ++ kind.s} ; + Wine = {s = "wine"} ; + Cheese = {s = "cheese"} ; + Fish = {s = "fish"} ; + Very quality = {s = "very" ++ quality.s} ; + Fresh = {s = "fresh"} ; + Warm = {s = "warm"} ; + Italian = {s = "Italian"} ; + Expensive = {s = "expensive"} ; + Delicious = {s = "delicious"} ; + Boring = {s = "boring"} ; + } +``` + + +%--! +==Modules and files== + +GF uses suffixes to recognize different file formats. The most +important ones are: +- Source files: Module name + ``.gf`` = file name +- Target files: each module is compiled into a ``.gfc`` file. + + +Import ``FoodEng.gf`` and see what happens: +``` + > i FoodEng.gf + - compiling Food.gf... wrote file Food.gfc 16 msec + - compiling FoodEng.gf... wrote file FoodEng.gfc 20 msec + +``` +The GF program does not only read the file +``FoodEng.gf``, but also all other files that it +depends on - in this case, ``Food.gf``. + +For each file that is compiled, a ``.gfc`` file +is generated. The GFC format (="GF Canonical") is the +"machine code" of GF, which is faster to process than +GF source files. When reading a module, GF decides whether +to use an existing ``.gfc`` file or to generate +a new one, by looking at modification times. + +**Exercise**. What happens when you import ``FoodEng.gf`` for +a second time? Try this in different situations: +- Right after importing it the first time (the modules are kept in + the memory of GF and need no reloading). +- After issuing the command ``empty`` (``e``), which clears the memory + of GF. +- After making a small change in ``FoodEng.gf``, be it only an added space. +- After making a change in ``Food.gf``. + + + +%--! +=Multilingual grammars and translation= + +The main advantage of separating abstract from concrete syntax is that +one abstract syntax can be equipped with many concrete syntaxes. +A system with this property is called a **multilingual grammar**. + +Multilingual grammars can be used for applications such as +translation. Let us build an Italian concrete syntax for +``Food`` and then test the resulting +multilingual grammar. + + + + +%--! +==An Italian concrete syntax== + +``` +concrete FoodIta of Food = { + + lincat + S, Item, Kind, Quality = {s : Str} ; + + lin + Is item quality = {s = item.s ++ "è" ++ quality.s} ; + This kind = {s = "questo" ++ kind.s} ; + That kind = {s = "quello" ++ kind.s} ; + QKind quality kind = {s = kind.s ++ quality.s} ; + Wine = {s = "vino"} ; + Cheese = {s = "formaggio"} ; + Fish = {s = "pesce"} ; + Very quality = {s = "molto" ++ quality.s} ; + Fresh = {s = "fresco"} ; + Warm = {s = "caldo"} ; + Italian = {s = "italiano"} ; + Expensive = {s = "caro"} ; + Delicious = {s = "delizioso"} ; + Boring = {s = "noioso"} ; + +} + +``` + +%--! +==Using a multilingual grammar== + +Import the two grammars in the same GF session. +``` + > i FoodEng.gf + > i FoodIta.gf +``` +Try generation now: +``` + > gr | l + quello formaggio molto noioso è italiano + + > gr | l -lang=FoodEng + this fish is warm +``` +Translate by using a pipe: +``` + > p -lang=FoodEng "this cheese is very delicious" | l -lang=FoodIta + questo formaggio è molto delizioso +``` +Generate a **multilingual treebank**, i.e. a set of trees with their +translations in different languages: +``` + > gr -number=2 | tree_bank + Is (That Cheese) (Very Boring) + quello formaggio è molto noioso + that cheese is very boring + Is (That Cheese) Fresh + quello formaggio è fresco + that cheese is fresh +``` +The ``lang`` flag tells GF which concrete syntax to use in parsing and +linearization. By default, the flag is set to the last-imported grammar. +To see what grammars are in scope and which is the main one, use the command +``print_options = po``: +``` + > print_options + main abstract : Food + main concrete : FoodIta + actual concretes : FoodIta FoodEng +``` + + +%--! +==Translation session== + +If translation is what you want to do with a set of grammars, a convenient +way to do it is to open a ``translation_session = ts``. In this session, +you can translate between all the languages that are in scope. +A dot ``.`` terminates the translation session. +``` + > ts + + trans> that very warm cheese is boring + quello formaggio molto caldo è noioso + that very warm cheese is boring + + trans> questo vino molto italiano è molto delizioso + questo vino molto italiano è molto delizioso + this very Italian wine is very delicious + + trans> . + > +``` + + + +%--! +==Translation quiz== + +This is a simple language exercise that can be automatically +generated from a multilingual grammar. The system generates a set of +random sentences, displays them in one language, and checks the user's +answer given in another language. The command ``translation_quiz = tq`` +makes this in a subshell of GF. +``` + > translation_quiz FoodEng FoodIta + + Welcome to GF Translation Quiz. + The quiz is over when you have done at least 10 examples + with at least 75 % success. + You can interrupt the quiz by entering a line consisting of a dot ('.'). + + this fish is warm + questo pesce è caldo + > Yes. + Score 1/1 + + this cheese is Italian + questo formaggio è noioso + > No, not questo formaggio è noioso, but + questo formaggio è italiano + + Score 1/2 + this fish is expensive +``` +You can also generate a list of translation exercises and save it in a +file for later use, by the command ``translation_list = tl`` +``` + > translation_list -number=25 FoodEng FoodIta +``` +The ``number`` flag gives the number of sentences generated. + + + +%--! +=Grammar architecture= + +==Extending a grammar== + +The module system of GF makes it possible to **extend** a +grammar in different ways. The syntax of extension is +shown by the following example. We extend ``Food`` by +adding a category of questions and two new functions. +``` + abstract Morefood = Food ** { + cat + Question ; + fun + QIs : Item -> Quality -> Question ; + Pizza : Kind ; + + } +``` +Parallel to the abstract syntax, extensions can +be built for concrete syntaxes: +``` + concrete MorefoodEng of Morefood = FoodEng ** { + lincat + Question = {s : Str} ; + lin + QIs item quality = {s = "is" ++ item.s ++ quality.s} ; + Pizza = {s = "pizza"} ; + } +``` +The effect of extension is that all of the contents of the extended +and extending module are put together. We also say that the new +module **inherits** the contents of the old module. + + + +%--! +==Multiple inheritance== + +Specialized vocabularies can be represented as small grammars that +only do "one thing" each. For instance, the following are grammars +for fruit and mushrooms +``` + abstract Fruit = { + cat Fruit ; + fun Apple, Peach : Fruit ; + } + + abstract Mushroom = { + cat Mushroom ; + fun Cep, Agaric : Mushroom ; + } +``` +They can afterwards be combined into bigger grammars by using +**multiple inheritance**, i.e. extension of several grammars at the +same time: +``` + abstract Foodmarket = Food, Fruit, Mushroom ** { + fun + FruitKind : Fruit -> Kind ; + MushroomKind : Mushroom -> Kind ; + } +``` +At this point, you would perhaps like to go back to +``Food`` and take apart ``Wine`` to build a special +``Drink`` module. + + +%--! +==Visualizing module structure== + +When you have created all the abstract syntaxes and +one set of concrete syntaxes needed for ``Foodmarket``, +your grammar consists of eight GF modules. To see how their +dependences look like, you can use the command +``visualize_graph = vg``, +``` + > visualize_graph +``` +and the graph will pop up in a separate window. + +The graph uses + +- oval boxes for abstract modules +- square boxes for concrete modules +- black-headed arrows for inheritance +- white-headed arrows for the concrete-of-abstract relation + + +[Foodmarket.png] + + +Just as the ``visualize_tree = vt`` command, the open source tools +Ghostview and Graphviz are needed. + + +%--! +==System commands== + +To document your grammar, you may want to print the +graph into a file, e.g. a ``.png`` file that +can be included in an HTML document. You can do this +by first printing the graph into a file ``.dot`` and then +processing this file with the ``dot`` program (from the Graphviz package). +``` + > pm -printer=graph | wf Foodmarket.dot + > ! dot -Tpng Foodmarket.dot > Foodmarket.png +``` +The latter command is a Unix command, issued from GF by using the +shell escape symbol ``!``. The resulting graph was shown in the previous section. + +The command ``print_multi = pm`` is used for printing the current multilingual +grammar in various formats, of which the format ``-printer=graph`` just +shows the module dependencies. Use ``help`` to see what other formats +are available: +``` + > help pm + > help -printer + > help help +``` + + + +%--! +=Resource modules= + + +==The golden rule of functional programming== + +In comparison to the ``.cf`` format, the ``.gf`` format looks rather +verbose, and demands lots more characters to be written. You have probably +done this by the copy-paste-modify method, which is a common way to +avoid repeating work. + +However, there is a more elegant way to avoid repeating work than the copy-and-paste +method. The **golden rule of functional programming** says that + +- whenever you find yourself programming by copy-and-paste, write a function instead. + + +A function separates the shared parts of different computations from the +changing parts, its **arguments**, or **parameters**. +In functional programming languages, such as +[Haskell http://www.haskell.org], it is possible to share much more +code with functions than in imperative languages such as C and Java. + + +==Operation definitions== + +GF is a functional programming language, not only in the sense that +the abstract syntax is a system of functions (``fun``), but also because +functional programming can be used to define concrete syntax. This is +done by using a new form of judgement, with the keyword ``oper`` (for +**operation**), distinct from ``fun`` for the sake of clarity. +Here is a simple example of an operation: +``` + oper ss : Str -> {s : Str} = \x -> {s = x} ; +``` +The operation can be **applied** to an argument, and GF will +**compute** the application into a value. For instance, +``` + ss "boy" ===> {s = "boy"} +``` +(We use the symbol ``===>`` to indicate how an expression is +computed into a value; this symbol is not a part of GF) + +Thus an ``oper`` judgement includes the name of the defined operation, +its type, and an expression defining it. As for the syntax of the defining +expression, notice the **lambda abstraction** form ``\x -> t`` of +the function. + + + +%--! +==The ``resource`` module type== + +Operator definitions can be included in a concrete syntax. +But they are not really tied to a particular set of linearization rules. +They should rather be seen as **resources** +usable in many concrete syntaxes. + +The ``resource`` module type can be used to package +``oper`` definitions into reusable resources. Here is +an example, with a handful of operations to manipulate +strings and records. +``` + resource StringOper = { + oper + SS : Type = {s : Str} ; + ss : Str -> SS = \x -> {s = x} ; + cc : SS -> SS -> SS = \x,y -> ss (x.s ++ y.s) ; + prefix : Str -> SS -> SS = \p,x -> ss (p ++ x.s) ; + } +``` +Resource modules can extend other resource modules, in the +same way as modules of other types can extend modules of the +same type. Thus it is possible to build resource hierarchies. + + + +%--! +==Opening a resource== + +Any number of ``resource`` modules can be +**opened** in a ``concrete`` syntax, which +makes definitions contained +in the resource usable in the concrete syntax. Here is +an example, where the resource ``StringOper`` is +opened in a new version of ``FoodEng``. +``` + concrete Food2Eng of Food = open StringOper in { + + lincat + S, Item, Kind, Quality = SS ; + + lin + Is item quality = cc item (prefix "is" quality) ; + This k = prefix "this" k ; + That k = prefix "that" k ; + QKind k q = cc k q ; + Wine = ss "wine" ; + Cheese = ss "cheese" ; + Fish = ss "fish" ; + Very = prefix "very" ; + Fresh = ss "fresh" ; + Warm = ss "warm" ; + Italian = ss "Italian" ; + Expensive = ss "expensive" ; + Delicious = ss "delicious" ; + Boring = ss "boring" ; + + } +``` +**Exercise**. Use the same string operations to write ``FoodIta`` +more concisely. + +**Exercise**. Define an operation ``infix`` analogous to ``prefix``, +such that it allows you to write +``` + lin Is = infix "is" ; +``` + + +%--! +==Partial application== + +GF, like Haskell, permits **partial application** of +functions. An example of this is the rule +``` + lin This k = prefix "this" k ; +``` +which can be written more concisely +``` + lin This = prefix "this" ; +``` +The first form is perhaps more intuitive to write +but, once you get used to partial application, you will appreciate its +conciseness and elegance. The logic of partial application +is known as **currying**, with a reference to Haskell B. Curry. +The idea is that any //n//-place function can be defined as a 1-place +function whose value is an //n-//1 -place function. Thus +``` + oper prefix : Str -> SS -> SS ; +``` +can be used as a 1-place function that takes a ``Str`` into a +function ``SS -> SS``. The expected linearization of ``This`` is exactly +a function of such a type, operating on an argument of type ``Kind`` +whose linearization is of type ``SS``. Thus we can define the +linearization directly as ``prefix "this"``. + + + +%--! +==Division of labour== + +Using operations defined in resource modules is a +way to avoid repetitive code. +In addition, it enables a new kind of modularity +and division of labour in grammar writing: grammarians familiar with +the linguistic details of a language can make their knowledge +available through resource grammar modules, whose users only need +to pick the right operations and not to know their implementation +details. + +In the following sections, we will go through some +such linguistic details. The programming constructs needed when +doing this are useful for all GF programmers, even if they don't +hand-code the linguistics of their applications but get them +from libraries. It is also useful to know something about the +linguistic concepts of inflection, agreement, and parts of speech. + + + + +%--! +=Morphology= + +Suppose we want to say, with the vocabulary included in +``Food.gf``, things like +``` + all Italian wines are delicious +``` +The new grammatical facility we need are the plural forms +of nouns and verbs (//wines, are//), as opposed to their +singular forms. + +The introduction of plural forms requires two things: + +- the **inflection** of nouns and verbs in singular and plural +- the **agreement** of the verb to subject: + the verb must have the same number as the subject + + +Different languages have different rules of inflection and agreement. +For instance, Italian has also agreement in gender (masculine vs. feminine). +We want to express such special features of languages in the +concrete syntax while ignoring them in the abstract syntax. + +To be able to do all this, we need one new judgement form +and many new expression forms. +We also need to generalize linearization types +from strings to more complex types. + + +%--! +==Parameters and tables== + +We define the **parameter type** of number in Englisn by +using a new form of judgement: +``` + param Number = Sg | Pl ; +``` +To express that ``Kind`` expressions in English have a linearization +depending on number, we replace the linearization type ``{s : Str}`` +with a type where the ``s`` field is a **table** depending on number: +``` + lincat Kind = {s : Number => Str} ; +``` +The **table type** ``Number => Str`` is in many respects similar to +a function type (``Number -> Str``). The main difference is that the +argument type of a table type must always be a parameter type. This means +that the argument-value pairs can be listed in a finite table. The following +example shows such a table: +``` + lin Cheese = {s = table { + Sg => "cheese" ; + Pl => "cheeses" + } + } ; +``` +The table consists of **branches**, where a **pattern** on the +left of the arrow ``=>`` is assigned a **value** on the right. + +The application of a table to a parameter is done by the **selection** +operator ``!``. For instance, +``` + table {Sg => "cheese" ; Pl => "cheeses"} ! Pl +``` +is a selection that computes into the value ``"cheeses"``. +This computation is performed by **pattern matching**: return +the value from the first branch whose pattern matches the +selection argument. Thus +``` + table {Sg => "cheese" ; Pl => "cheeses"} ! Pl + ===> "cheeses" +``` + + +%--! +==Inflection tables, paradigms, and ``oper`` definitions== + +All English common nouns are inflected in number, most of them in the +same way: the plural form is obtained from the singular by adding the +ending //s//. This rule is an example of +a **paradigm** - a formula telling how the inflection +forms of a word are formed. + +From the GF point of view, a paradigm is a function that takes a **lemma** - +also known as a **dictionary form** - and returns an inflection +table of desired type. Paradigms are not functions in the sense of the +``fun`` judgements of abstract syntax (which operate on trees and not +on strings), but operations defined in ``oper`` judgements. +The following operation defines the regular noun paradigm of English: +``` + oper regNoun : Str -> {s : Number => Str} = \x -> { + s = table { + Sg => x ; + Pl => x + "s" + } + } ; +``` +The **gluing** operator ``+`` tells that +the string held in the variable ``x`` and the ending ``"s"`` +are written together to form one **token**. Thus, for instance, +``` + (regNoun "cheese").s ! Pl ---> "cheese" + "s" ---> "cheeses" +``` + + + +%--! +==Worst-case functions and data abstraction== + +Some English nouns, such as ``mouse``, are so irregular that +it makes no sense to see them as instances of a paradigm. Even +then, it is useful to perform **data abstraction** from the +definition of the type ``Noun``, and introduce a constructor +operation, a **worst-case function** for nouns: +``` + oper mkNoun : Str -> Str -> Noun = \x,y -> { + s = table { + Sg => x ; + Pl => y + } + } ; +``` +Thus we can define +``` + lin Mouse = mkNoun "mouse" "mice" ; +``` +and +``` + oper regNoun : Str -> Noun = \x -> + mkNoun x (x + "s") ; +``` +instead of writing the inflection tables explicitly. + +The grammar engineering advantage of worst-case functions is that +the author of the resource module may change the definitions of +``Noun`` and ``mkNoun``, and still retain the +interface (i.e. the system of type signatures) that makes it +correct to use these functions in concrete modules. In programming +terms, ``Noun`` is then treated as an **abstract datatype**. + + + +%--! +==A system of paradigms using Prelude operations== + +In addition to the completely regular noun paradigm ``regNoun``, +some other frequent noun paradigms deserve to be +defined, for instance, +``` + sNoun : Str -> Noun = \kiss -> mkNoun kiss (kiss + "es") ; +``` +What about nouns like //fly//, with the plural //flies//? The already +available solution is to use the longest common prefix +//fl// (also known as the **technical stem**) as argument, and define +``` + yNoun : Str -> Noun = \fl -> mkNoun (fl + "y") (fl + "ies") ; +``` +But this paradigm would be very unintuitive to use, because the technical stem +is not an existing form of the word. A better solution is to use +the lemma and a string operator ``init``, which returns the initial segment (i.e. +all characters but the last) of a string: +``` + yNoun : Str -> Noun = \fly -> mkNoun fly (init fly + "ies") ; +``` +The operation ``init`` belongs to a set of operations in the +resource module ``Prelude``, which therefore has to be +``open``ed so that ``init`` can be used. + + + +%--! +==An intelligent noun paradigm using ``case`` expressions== + +It may be hard for the user of a resource morphology to pick the right +inflection paradigm. A way to help this is to define a more intelligent +paradigm, which chooses the ending by first analysing the lemma. +The following variant for English regular nouns puts together all the +previously shown paradigms, and chooses one of them on the basis of +the final letter of the lemma (found by the prelude operator ``last``). +``` + regNoun : Str -> Noun = \s -> case last s of { + "s" | "z" => mkNoun s (s + "es") ; + "y" => mkNoun s (init s + "ies") ; + _ => mkNoun s (s + "s") + } ; +``` +This definition displays many GF expression forms not shown befores; +these forms are explained in the next section. + +The paradigms ``regNoun`` does not give the correct forms for +all nouns. For instance, //mouse - mice// and +//fish - fish// must be given by using ``mkNoun``. +Also the word //boy// would be inflected incorrectly; to prevent +this, either use ``mkNoun`` or modify +``regNoun`` so that the ``"y"`` case does not +apply if the second-last character is a vowel. + + + +%--! +==Pattern matching== + +We have so far built all expressions of the ``table`` form +from branches whose patterns are constants introduced in +``param`` definitions, as well as constant strings. +But there are more expressive patterns. Here is a summary of the possible forms: +- a variable pattern (identifier other than constant parameter) matches anything +- the wild card ``_`` matches anything +- a string literal pattern, e.g. ``"s"``, matches the same string +- a disjunctive pattern ``P | ... | Q`` matches anything that + one of the disjuncts matches + + +Pattern matching is performed in the order in which the branches +appear in the table: the branch of the first matching pattern is followed. + +As syntactic sugar, one-branch tables can be written concisely, +``` + \\P,...,Q => t === table {P => ... table {Q => t} ...} +``` +Finally, the ``case`` expressions common in functional +programming languages are syntactic sugar for table selections: +``` + case e of {...} === table {...} ! e +``` + + +%--! +==Morphological resource modules== + +A common idiom is to +gather the ``oper`` and ``param`` definitions +needed for inflecting words in +a language into a morphology module. Here is a simple +example, [``MorphoEng`` resource/MorphoEng.gf]. +``` + --# -path=.:prelude + + resource MorphoEng = open Prelude in { + + param + Number = Sg | Pl ; + + oper + Noun, Verb : Type = {s : Number => Str} ; + + mkNoun : Str -> Str -> Noun = \x,y -> { + s = table { + Sg => x ; + Pl => y + } + } ; + + regNoun : Str -> Noun = \s -> case last s of { + "s" | "z" => mkNoun s (s + "es") ; + "y" => mkNoun s (init s + "ies") ; + _ => mkNoun s (s + "s") + } ; + + mkVerb : Str -> Str -> Verb = \x,y -> mkNoun y x ; + + regVerb : Str -> Verb = \s -> case last s of { + "s" | "z" => mkVerb s (s + "es") ; + "y" => mkVerb s (init s + "ies") ; + "o" => mkVerb s (s + "es") ; + _ => mkVerb s (s + "s") + } ; + } +``` +The first line gives as a hint to the compiler the +**search path** needed to find all the other modules that the +module depends on. The directory ``prelude`` is a subdirectory of +``GF/lib``; to be able to refer to it in this simple way, you can +set the environment variable ``GF_LIB_PATH`` to point to this +directory. + + +%--! +==Testing resource modules== + +To test a ``resource`` module independently, you must import it +with the flag ``-retain``, which tells GF to retain ``oper`` definitions +in the memory; the usual behaviour is that ``oper`` definitions +are just applied to compile linearization rules +(this is called **inlining**) and then thrown away. +``` + > i -retain MorphoEng.gf +``` +The command ``compute_concrete = cc`` computes any expression +formed by operations and other GF constructs. For example, +``` + > cc regVerb "echo" + {s : Number => Str = table Number { + Sg => "echoes" ; + Pl => "echo" + } + } +``` + +The command ``show_operations = so``` shows the type signatures +of all operations returning a given value type: +``` + > so Verb + MorphoEng.mkNoun : Str -> Str -> {s : {MorphoEng.Number} => Str} + MorphoEng.mkVerb : Str -> Str -> {s : {MorphoEng.Number} => Str} + MorphoEng.regNoun : Str -> {s : {MorphoEng.Number} => Str} + MorphoEng.regVerb : Str -> { s : {MorphoEng.Number} => Str} +``` +Why does the command also show the operations that form +``Noun``s? The reason is that the type expression +``Verb`` is first computed, and its value happens to be +the same as the value of ``Noun``. + + + +=Using parameters in concrete syntax= + +We can now enrich the concrete syntax definitions to +comprise morphology. This will involve a more radical +variation between languages (e.g. English and Italian) +then just the use of different words. In general, +parameters and linearization types are different in +different languages - but this does not prevent the +use of a common abstract syntax. + + +%--! +==Parametric vs. inherent features, agreement== + +The rule of subject-verb agreement in English says that the verb +phrase must be inflected in the number of the subject. This +means that a noun phrase (functioning as a subject), inherently +//has// a number, which it passes to the verb. The verb does not +//have// a number, but must be able to //receive// whatever number the +subject has. This distinction is nicely represented by the +different linearization types of **noun phrases** and **verb phrases**: +``` + lincat NP = {s : Str ; n : Number} ; + lincat VP = {s : Number => Str} ; +``` +We say that the number of ``NP`` is an **inherent feature**, +whereas the number of ``NP`` is a **variable feature** (or a +**parametric feature**). + +The agreement rule itself is expressed in the linearization rule of +the predication function: +``` + lin PredVP np vp = {s = np.s ++ vp.s ! np.n} ; +``` +The following section will present +``FoodsEng``, assuming the abstract syntax ``Foods`` +that is similar to ``Food`` but also has the +plural determiners ``These`` and ``Those``. +The reader is invited to inspect the way in which agreement works in +the formation of sentences. + + +%--! +==English concrete syntax with parameters== + +The grammar uses both +[``Prelude`` ../../lib/prelude/Prelude.gf] and +[``MorphoEng`` resource/MorphoEng]. +We will later see how to make the grammar even +more high-level by using a resource grammar library +and parametrized modules. +``` +--# -path=.:resource:prelude + +concrete FoodsEng of Foods = open Prelude, MorphoEng in { + + lincat + S, Quality = SS ; + Kind = {s : Number => Str} ; + Item = {s : Str ; n : Number} ; + + lin + Is item quality = ss (item.s ++ (mkVerb "are" "is").s ! item.n ++ quality.s) ; + This = det Sg "this" ; + That = det Sg "that" ; + These = det Pl "these" ; + Those = det Pl "those" ; + QKind quality kind = {s = \\n => quality.s ++ kind.s ! n} ; + Wine = regNoun "wine" ; + Cheese = regNoun "cheese" ; + Fish = mkNoun "fish" "fish" ; + Very = prefixSS "very" ; + Fresh = ss "fresh" ; + Warm = ss "warm" ; + Italian = ss "Italian" ; + Expensive = ss "expensive" ; + Delicious = ss "delicious" ; + Boring = ss "boring" ; + + oper + det : Number -> Str -> Noun -> {s : Str ; n : Number} = \n,d,cn -> { + s = d ++ cn.s ! n ; + n = n + } ; + +} +``` + + + +%--! +==Hierarchic parameter types== + +The reader familiar with a functional programming language such as +[Haskell http://www.haskell.org] must have noticed the similarity +between parameter types in GF and **algebraic datatypes** (``data`` definitions +in Haskell). The GF parameter types are actually a special case of algebraic +datatypes: the main restriction is that in GF, these types must be finite. +(It is this restriction that makes it possible to invert linearization rules into +parsing methods.) + +However, finite is not the same thing as enumerated. Even in GF, parameter +constructors can take arguments, provided these arguments are from other +parameter types - only recursion is forbidden. Such parameter types impose a +hierarchic order among parameters. They are often needed to define +the linguistically most accurate parameter systems. + +To give an example, Swedish adjectives +are inflected in number (singular or plural) and +gender (uter or neuter). These parameters would suggest 2*2=4 different +forms. However, the gender distinction is done only in the singular. Therefore, +it would be inaccurate to define adjective paradigms using the type +``Gender => Number => Str``. The following hierarchic definition +yields an accurate system of three adjectival forms. +``` + param AdjForm = ASg Gender | APl ; + param Gender = Utr | Neutr ; +``` +Here is an example of pattern matching, the paradigm of regular adjectives. +``` + oper regAdj : Str -> AdjForm => Str = \fin -> table { + ASg Utr => fin ; + ASg Neutr => fin + "t" ; + APl => fin + "a" ; + } +``` +A constructor can be used as a pattern that has patterns as arguments. For instance, +the adjectival paradigm in which the two singular forms are the same, +can be defined +``` + oper plattAdj : Str -> AdjForm => Str = \platt -> table { + ASg _ => platt ; + APl => platt + "a" ; + } +``` + + +%--! +==Morphological analysis and morphology quiz== + +Even though morphology is in GF +mostly used as an auxiliary for syntax, it +can also be useful on its own right. The command ``morpho_analyse = ma`` +can be used to read a text and return for each word the analyses that +it has in the current concrete syntax. +``` + > rf bible.txt | morpho_analyse +``` +In the same way as translation exercises, morphological exercises can +be generated, by the command ``morpho_quiz = mq``. Usually, +the category is set to be something else than ``S``. For instance, +``` + > cd GF/lib/resource-1.0/ + > i french/IrregFre.gf + > morpho_quiz -cat=V + + Welcome to GF Morphology Quiz. + ... + + réapparaître : VFin VCondit Pl P2 + réapparaitriez + > No, not réapparaitriez, but + réapparaîtriez + Score 0/1 +``` +Finally, a list of morphological exercises can be generated +off-line and saved in a +file for later use, by the command ``morpho_list = ml`` +``` + > morpho_list -number=25 -cat=V | wf exx.txt +``` +The ``number`` flag gives the number of exercises generated. + + + +%--! +==Discontinuous constituents== + +A linearization type may contain more strings than one. +An example of where this is useful are English particle +verbs, such as //switch off//. The linearization of +a sentence may place the object between the verb and the particle: +//he switched it off//. + +The following judgement defines transitive verbs as +**discontinuous constituents**, i.e. as having a linearization +type with two strings and not just one. +``` + lincat TV = {s : Number => Str ; part : Str} ; +``` +This linearization rule +shows how the constituents are separated by the object in complementization. +``` + lin PredTV tv obj = {s = \\n => tv.s ! n ++ obj.s ++ tv.part} ; +``` +There is no restriction in the number of discontinuous constituents +(or other fields) a ``lincat`` may contain. The only condition is that +the fields must be of finite types, i.e. built from records, tables, +parameters, and ``Str``, and not functions. + +A mathematical result +about parsing in GF says that the worst-case complexity of parsing +increases with the number of discontinuous constituents. This is +potentially a reason to avoid discontinuous constituents. +Moreover, the parsing and linearization commands only give accurate +results for categories whose linearization type has a unique ``Str`` +valued field labelled ``s``. Therefore, discontinuous constituents +are not a good idea in top-level categories accessed by the users +of a grammar application. + + +%--! +==Free variation== + +Sometimes there are many alternative ways to define a concrete syntax. +For instance, the verb negation in English can be expressed both by +//does not// and //doesn't//. In linguistic terms, these expressions +are in **free variation**. The ``variants`` construct of GF can +be used to give a list of strings in free variation. For example, +``` + NegVerb verb = {s = variants {["does not"] ; "doesn't} ++ verb.s ! Pl} ; +``` +An empty variant list +``` + variants {} +``` +can be used e.g. if a word lacks a certain form. + +In general, ``variants`` should be used cautiously. It is not +recommended for modules aimed to be libraries, because the +user of the library has no way to choose among the variants. + + +==Overloading of operations== + +Large libraries, such as the GF Resource Grammar Library, may define +hundreds of names, which can be unpractical +for both the library writer and the user. The writer has to invent longer +and longer names which are not always intuitive, +and the user has to learn or at least be able to find all these names. +A solution to this problem, adopted by languages such as C++, is **overloading**: +the same name can be used for several functions. When such a name is used, the +compiler performs **overload resolution** to find out which of the possible functions +is meant. The resolution is based on the types of the functions: all functions that +have the same name must have different types. + +In C++, functions with the same name can be scattered everywhere in the program. +In GF, they must be grouped together in ``overload`` groups. Here is an example +of an overload group, defining four ways to define nouns in Italian: +``` + oper mkN = overload { + mkN : Str -> N = -- regular nouns + mkN : Str -> Gender -> N = -- regular nouns with unexpected gender + mkN : Str -> Str -> N = -- irregular nouns + mkN : Str -> Str -> Gender -> N = -- irregular nouns with unexpected gender + } +``` +All of the following uses of ``mkN`` are easy to resolve: +``` + lin Pizza = mkN "pizza" ; -- Str -> N + lin Hand = mkN "mano" Fem ; -- Str -> Gender -> N + lin Man = mkN "uomo" "uomini" ; -- Str -> Str -> N +``` + + + + + + +%--! +=More constructs for concrete syntax= + +In this chapter, we go through constructs that are not necessary in simple grammars +or when the concrete syntax relies on libraries, but very useful when writing advanced +concrete syntax implementations, such as resource grammar libraries. + + +%--! +==Local definitions== + +Local definitions ("``let`` expressions") are used in functional +programming for two reasons: to structure the code into smaller +expressions, and to avoid repeated computation of one and +the same expression. Here is an example, from +[``MorphoIta`` resource/MorphoIta.gf]: +``` + oper regNoun : Str -> Noun = \vino -> + let + vin = init vino ; + o = last vino + in + case o of { + "a" => mkNoun Fem vino (vin + "e") ; + "o" | "e" => mkNoun Masc vino (vin + "i") ; + _ => mkNoun Masc vino vino + } ; +``` + + +==Record extension and subtyping== + +Record types and records can be **extended** with new fields. For instance, +in German it is natural to see transitive verbs as verbs with a case. +The symbol ``**`` is used for both constructs. +``` + lincat TV = Verb ** {c : Case} ; + + lin Follow = regVerb "folgen" ** {c = Dative} ; +``` +To extend a record type or a record with a field whose label it +already has is a type error. + +A record type //T// is a **subtype** of another one //R//, if //T// has +all the fields of //R// and possibly other fields. For instance, +an extension of a record type is always a subtype of it. + +If //T// is a subtype of //R//, an object of //T// can be used whenever +an object of //R// is required. For instance, a transitive verb can +be used whenever a verb is required. + +**Contravariance** means that a function taking an //R// as argument +can also be applied to any object of a subtype //T//. + + + +==Tuples and product types== + +Product types and tuples are syntactic sugar for record types and records: +``` + T1 * ... * Tn === {p1 : T1 ; ... ; pn : Tn} + === {p1 = T1 ; ... ; pn = Tn} +``` +Thus the labels ``p1, p2,...`` are hard-coded. + + +==Record and tuple patterns== + +Record types of parameter types are also parameter types. +A typical example is a record of agreement features, e.g. French +``` + oper Agr : PType = {g : Gender ; n : Number ; p : Person} ; +``` +Notice the term ``PType`` rather than just ``Type`` referring to +parameter types. Every ``PType`` is also a ``Type``, but not vice-versa. + +Pattern matching is done in the expected way, but it can moreover +utilize partial records: the branch +``` + {g = Fem} => t +``` +in a table of type ``Agr => T`` means the same as +``` + {g = Fem ; n = _ ; p = _} => t +``` +Tuple patterns are translated to record patterns in the +same way as tuples to records; partial patterns make it +possible to write, slightly surprisingly, +``` + case of { + => t + ... + } +``` + + +%--! +==Regular expression patterns== + +To define string operations computed at compile time, such +as in morphology, it is handy to use regular expression patterns: + - //p// ``+`` //q// : token consisting of //p// followed by //q// + - //p// ``*`` : token //p// repeated 0 or more times + (max the length of the string to be matched) + - ``-`` //p// : matches anything that //p// does not match + - //x// ``@`` //p// : bind to //x// what //p// matches + - //p// ``|`` //q// : matches what either //p// or //q// matches + + +The last three apply to all types of patterns, the first two only to token strings. +As an example, we give a rule for the formation of English word forms +ending with an //s// and used in the formation of both plural nouns and +third-person present-tense verbs. +``` + add_s : Str -> Str = \w -> case w of { + _ + "oo" => w + "s" ; -- bamboo + _ + ("s" | "z" | "x" | "sh" | "o") => w + "es" ; -- bus, hero + _ + ("a" | "o" | "u" | "e") + "y" => w + "s" ; -- boy + x + "y" => x + "ies" ; -- fly + _ => w + "s" -- car + } ; +``` +Here is another example, the plural formation in Swedish 2nd declension. +The second branch uses a variable binding with ``@`` to cover the cases where an +unstressed pre-final vowel //e// disappears in the plural +(//nyckel-nycklar, seger-segrar, bil-bilar//): +``` + plural2 : Str -> Str = \w -> case w of { + pojk + "e" => pojk + "ar" ; + nyck + "e" + l@("l" | "r" | "n") => nyck + l + "ar" ; + bil => bil + "ar" + } ; +``` + + +Semantics: variables are always bound to the **first match**, which is the first +in the sequence of binding lists ``Match p v`` defined as follows. In the definition, +``p`` is a pattern and ``v`` is a value. +``` + Match (p1|p2) v = Match p1 v ++ Match p2 v + Match (p1+p2) s = [Match p1 s1 ++ Match p2 s2 | + i <- [0..length s], (s1,s2) = splitAt i s] + Match p* s = [[]] if Match "" s ++ Match p s ++ Match (p+p) s ++... /= [] + Match -p v = [[]] if Match p v = [] + Match c v = [[]] if c == v -- for constant and literal patterns c + Match x v = [[(x,v)]] -- for variable patterns x + Match x@p v = [[(x,v)]] + M if M = Match p v /= [] + Match p v = [] otherwise -- failure +``` +Examples: +- ``x + "e" + y`` matches ``"peter"`` with ``x = "p", y = "ter"`` +- ``x + "er"*`` matches ``"burgerer"`` with ``x = "burg" + + + + + +%--! +==Prefix-dependent choices== + +Sometimes a token has different forms depending on the token +that follows. An example is the English indefinite article, +which is //an// if a vowel follows, //a// otherwise. +Which form is chosen can only be decided at run time, i.e. +when a string is actually build. GF has a special construct for +such tokens, the ``pre`` construct exemplified in +``` + oper artIndef : Str = + pre {"a" ; "an" / strs {"a" ; "e" ; "i" ; "o"}} ; +``` +Thus +``` + artIndef ++ "cheese" ---> "a" ++ "cheese" + artIndef ++ "apple" ---> "an" ++ "apple" +``` +This very example does not work in all situations: the prefix +//u// has no general rules, and some problematic words are +//euphemism, one-eyed, n-gram//. It is possible to write +``` + oper artIndef : Str = + pre {"a" ; + "a" / strs {"eu" ; "one"} ; + "an" / strs {"a" ; "e" ; "i" ; "o" ; "n-"} + } ; +``` + + +==Predefined types and operations== + +GF has the following predefined categories in abstract syntax: +``` + cat Int ; -- integers, e.g. 0, 5, 743145151019 + cat Float ; -- floats, e.g. 0.0, 3.1415926 + cat String ; -- strings, e.g. "", "foo", "123" +``` +The objects of each of these categories are **literals** +as indicated in the comments above. No ``fun`` definition +can have a predefined category as its value type, but +they can be used as arguments. For example: +``` + fun StreetAddress : Int -> String -> Address ; + lin StreetAddress number street = {s = number.s ++ street.s} ; + + -- e.g. (StreetAddress 10 "Downing Street") : Address +``` +FIXME: The linearization type is ``{s : Str}`` for all these categories. + + + +%--! +=Using the resource grammar library= + +%!include: ../intro-resource.txt + + +%--! +=Overview of the resource grammar library= + +%!include: ../overview-resource.txt + + + +=More concepts of abstract syntax= + +This section is about the use of the type theory part of GF for +including more semantics in grammars. Some of the subsections present +ideas that have not yet been used in real-world applications, and whose +tool support outside the GF program is not complete. + + +==GF as a logical framework== + +In this section, we will show how +to encode advanced semantic concepts in an abstract syntax. +We use concepts inherited from **type theory**. Type theory +is the basis of many systems known as **logical frameworks**, which are +used for representing mathematical theorems and their proofs on a computer. +In fact, GF has a logical framework as its proper part: +this part is the abstract syntax. + +In a logical framework, the formalization of a mathematical theory +is a set of type and function declarations. The following is an example +of such a theory, represented as an ``abstract`` module in GF. +``` +abstract Arithm = { + cat + Prop ; -- proposition + Nat ; -- natural number + fun + Zero : Nat ; -- 0 + Succ : Nat -> Nat ; -- successor of x + Even : Nat -> Prop ; -- x is even + And : Prop -> Prop -> Prop ; -- A and B + } +``` +A concrete syntax is given below, as an example of using the resource grammar +library. + + + +==Dependent types== + +**Dependent types** are a characteristic feature of GF, +inherited from the +**constructive type theory** of Martin-Löf and +distinguishing GF from most other grammar formalisms and +functional programming languages. +The initial main motivation for developing GF was, indeed, +to have a grammar formalism with dependent types. +As can be inferred from the fact that we introduce them only now, +after having written lots of grammars without them, +dependent types are no longer the only motivation for GF. +But they are still important and interesting. + + +Dependent types can be used for stating stronger +**conditions of well-formedness** than non-dependent types. +A simple example is postal addresses. Ignoring the other details, +let us take a look at addresses consisting of +a street, a city, and a country. +``` +abstract Address = { + cat + Address ; Country ; City ; Street ; + + fun + mkAddress : Country -> City -> Street -> Address ; + + UK, France : Country ; + Paris, London, Grenoble : City ; + OxfordSt, ShaftesburyAve, BdRaspail, RueBlondel, AvAlsaceLorraine : Street ; + } +``` +The linearization rules are straightforward, +``` + lin + mkAddress country city street = + ss (street.s ++ "," ++ city.s ++ "," ++ country.s) ; + UK = ss ("U.K.") ; + France = ss ("France") ; + Paris = ss ("Paris") ; + London = ss ("London") ; + Grenoble = ss ("Grenoble") ; + OxfordSt = ss ("Oxford" ++ "Street") ; + ShaftesburyAve = ss ("Shaftesbury" ++ "Avenue") ; + BdRaspail = ss ("boulevard" ++ "Raspail") ; + RueBlondel = ss ("rue" ++ "Blondel") ; + AvAlsaceLorraine = ss ("avenue" ++ "Alsace-Lorraine") ; +``` +Notice that, in ``mkAddress``, we have +reversed the order of the constituents. The type of ``mkAddress`` +in the abstract syntax takes its arguments in a "logical" order, +with increasing precision. (This order is sometimes even used in the +concrete syntax of addresses, e.g. in Russia). + +Both existing and non-existing addresses are recognized by this +grammar. The non-existing ones in the following randomly generated +list have afterwards been marked by *: +``` + > gr -cat=Address -number=7 | l + + * Oxford Street , Paris , France + * Shaftesbury Avenue , Grenoble , U.K. + boulevard Raspail , Paris , France + * rue Blondel , Grenoble , U.K. + * Shaftesbury Avenue , Grenoble , France + * Oxford Street , London , France + * Shaftesbury Avenue , Grenoble , France +``` +Dependent types provide a way to guarantee that addresses are +well-formed. What we do is to include **contexts** in +``cat`` judgements: +``` + cat + Address ; + Country ; + City Country ; + Street (x : Country)(City x) ; +``` +The first two judgements are as before, but the third one makes +``City`` dependent on ``Country``: there are no longer just cities, +but cities of the U.K. and cities of France. The fourth judgement +makes ``Street`` dependent on ``City``; but since +``City`` is itself dependent on ``Country``, we must +include them both in the context, moreover guaranteeing that +the city is one of the given country. Since the context itself +is built by using a dependent type, we have to use variables +to indicate the dependencies. The judgement we used for ``City`` +is actually shorthand for +``` + cat City (x : Country) +``` +which is only possible if the subsequent context does not depend on ``x``. + +The ``fun`` judgements of the grammar are modified accordingly: +``` + fun + mkAddress : (x : Country) -> (y : City x) -> Street x y -> Address ; + + UK : Country ; + France : Country ; + Paris : City France ; + London : City UK ; + Grenoble : City France ; + OxfordSt : Street UK London ; + ShaftesburyAve : Street UK London ; + BdRaspail : Street France Paris ; + RueBlondel : Street France Paris ; + AvAlsaceLorraine : Street France Grenoble ; +``` +Since the type of ``mkAddress`` now has dependencies among +its argument types, we have to use variables just like we used in +the context of ``Street`` above. What we claimed to be the +"logical" order of the arguments is now forced by the type system +of GF: a variable must be declared (=bound) before it can be +referenced (=used). + +The effect of dependent types is that the *-marked addresses above are +no longer well-formed. What the GF parser actually does is that it +initially accepts them (by using a context-free parsing algorithm) +and then rejects them (by type checking). The random generator does not produce +illegal addresses (this could be useful in bulk mailing!). +The linearization algorithm does not care about type dependencies; +actually, since the //categories// (ignoring their arguments) +are the same in both abstract syntaxes, +we use the same concrete syntax +for both of them. + +**Remark**. Function types //without// +variables are actually a shorthand notation: writing +``` + fun PredV1 : NP -> V1 -> S +``` +is shorthand for +``` + fun PredV1 : (x : NP) -> (y : V1) -> S +``` +or any other naming of the variables. Actually the use of variables +sometimes shortens the code, since we can write e.g. +``` + oper triple : (x,y,z : Str) -> Str = ... +``` +If a bound variable is not used, it can here, as elswhere in GF, be replaced by +a wildcard: +``` + oper triple : (_,_,_ : Str) -> Str = ... +``` + + + +==Dependent types in concrete syntax== + +The **functional fragment** of GF +terms and types comprises function types, applications, lambda +abstracts, constants, and variables. This fragment is similar in +abstract and concrete syntax. In particular, +dependent types are also available in concrete syntax. +We have not made use of them yet, +but we will now look at one example of how they +can be used. + +Those readers who are familiar with functional programming languages +like ML and Haskell, may already have missed **polymorphic** +functions. For instance, Haskell programmers have access to +the functions +``` + const :: a -> b -> a + const c _ = c + + flip :: (a -> b -> c) -> b -> a -> c + flip f y x = f x y +``` +which can be used for any given types ``a``,``b``, and ``c``. + +The GF counterpart of polymorphic functions are **monomorphic** +functions with explicit **type variables**. Thus the above +definitions can be written +``` + oper const :(a,b : Type) -> a -> b -> a = + \_,_,c,_ -> c ; + + oper flip : (a,b,c : Type) -> (a -> b ->c) -> b -> a -> c = + \_,_,_,f,x,y -> f y x ; +``` +When the operations are used, the type checker requires +them to be equipped with all their arguments; this may be a nuisance +for a Haskell or ML programmer. + + + +==Expressing selectional restrictions== + +This section introduces a way of using dependent types to +formalize a notion known as **selectional restrictions** +in linguistics. We first present a mathematical model +of the notion, and then integrate it in a linguistically +motivated syntax. + +In linguistics, a +grammar is usually thought of as being about **syntactic well-formedness** +in a rather liberal sense: an expression can be well-formed without +being meaningful, in other words, without being +**semantically well-formed**. +For instance, the sentence +``` + the number 2 is equilateral +``` +is syntactically well-formed but semantically ill-formed. +It is well-formed because it combines a well-formed +noun phrase ("the number 2") with a well-formed +verb phrase ("is equilateral") and satisfies the agreement +rule saying that the verb phrase is inflected in the +number of the noun phrase: +``` + fun PredVP : NP -> VP -> S ; + lin PredVP np v = {s = np.s ++ vp.s ! np.n} ; +``` +It is ill-formed because the predicate "is equilateral" +is only defined for triangles, not for numbers. + +In a straightforward type-theoretical formalization of +mathematics, domains of mathematical objects +are defined as types. In GF, we could write +``` + cat Nat ; + cat Triangle ; + cat Prop ; +``` +for the types of natural numbers, triangles, and propositions, +respectively. +Noun phrases are typed as objects of basic types other than +``Prop``, whereas verb phrases are functions from basic types +to ``Prop``. For instance, +``` + fun two : Nat ; + fun Even : Nat -> Prop ; + fun Equilateral : Triangle -> Prop ; +``` +With these judgements, and the linearization rules +``` + lin two = ss ["the number 2"] ; + lin Even x = ss (x.s ++ ["is even"]) ; + lin Equilateral x = ss (x.s ++ ["is equilateral"]) ; +``` +we can form the proposition ``Even two`` +``` + the number 2 is even +``` +but no proposition linearized to +``` + the number 2 is equilateral +``` +since ``Equilateral two`` is not a well-formed type-theoretical object. +It is not even accepted by the context-free parser. + +When formalizing mathematics, e.g. in the purpose of +computer-assisted theorem proving, we are certainly interested +in semantic well-formedness: we want to be sure that a proposition makes +sense before we make the effort of proving it. The straightforward typing +of nouns and predicates shown above is the way in which this +is guaranteed in various proof systems based on type theory. +(Notice that it is still possible to form //false// propositions, +e.g. "the number 3 is even". +False and meaningless are different things.) + +As shown by the linearization rules for ``two``, ``Even``, +etc, it //is// possible to use straightforward mathematical typings +as the abstract syntax of a grammar. However, this syntax is not very +expressive linguistically: for instance, there is no distinction between +adjectives and verbs. It is hard to give rules for structures like +adjectival modification ("even number") and conjunction of predicates +("even or odd"). + +By using dependent types, it is simple to combine a linguistically +motivated system of categories with mathematically motivated +type restrictions. What we need is a category of domains of +individual objects, +``` + cat Dom +``` +and dependencies of other categories on this: +``` + cat + S ; -- sentence + V1 Dom ; -- one-place verb with specific subject type + V2 Dom Dom ; -- two-place verb with specific subject and object types + A1 Dom ; -- one-place adjective + A2 Dom Dom ; -- two-place adjective + NP Dom ; -- noun phrase for an object of specific type + Conj ; -- conjunction + Det ; -- determiner +``` +Having thus parametrized categories on domains, we have to reformulate +the rules of predication, etc, accordingly. This is straightforward: +``` + fun + PredV1 : (A : Dom) -> NP A -> V1 A -> S ; + ComplV2 : (A,B : Dom) -> V2 A B -> NP B -> V1 A ; + DetCN : Det -> (A : Dom) -> NP A ; + ModA1 : (A : Dom) -> A1 A -> Dom ; + ConjS : Conj -> S -> S -> S ; + ConjV1 : (A : Dom) -> Conj -> V1 A -> V1 A -> V1 A ; +``` +In linearization rules, +we use wildcards for the domain arguments, +because they don't affect linearization: +``` + lin + PredV1 _ np vp = ss (np.s ++ vp.s) ; + ComplV2 _ _ v2 np = ss (v2.s ++ np.s) ; + DetCN det cn = ss (det.s ++ cn.s) ; + ModA1 cn a1 = ss (a1.s ++ cn.s) ; + ConjS conj s1 s2 = ss (s1.s ++ conj.s ++ s2.s) ; + ConjV1 _ conj v1 v2 = ss (v1.s ++ conj.s ++ v2.s) ; +``` +The domain arguments thus get suppressed in linearization. +Parsing initially returns metavariables for them, +but type checking can usually restore them +by inference from those arguments that are not suppressed. + +One traditional linguistic example of domain restrictions +(= selectional restrictions) is the contrast between the two sentences +``` + John plays golf + golf plays John +``` +To explain the contrast, we introduce the functions +``` + human : Dom ; + game : Dom ; + play : V2 human game ; + John : NP human ; + Golf : NP game ; +``` +Both sentences still pass the context-free parser, +returning trees with lots of metavariables of type ``Dom``: +``` + PredV1 ?0 John (ComplV2 ?1 ?2 play Golf) + PredV1 ?0 Golf (ComplV2 ?1 ?2 play John) +``` +But only the former sentence passes the type checker, which moreover +infers the domain arguments: +``` + PredV1 human John (ComplV2 human game play Golf) +``` +To try this out in GF, use ``pt = put_term`` with the **tree transformation** +that solves the metavariables by type checking: +``` + > p -tr "John plays golf" | pt -transform=solve + > p -tr "golf plays John" | pt -transform=solve +``` +In the latter case, no solutions are found. + +A known problem with selectional restrictions is that they can be more +or less liberal. For instance, +``` + John loves Mary + John loves golf +``` +should both make sense, even though ``Mary`` and ``golf`` +are of different types. A natural solution in this case is to +formalize ``love`` as a **polymorphic** verb, which takes +a human as its first argument but an object of any type as its second +argument: +``` + fun love : (A : Dom) -> V2 human A ; + lin love _ = ss "loves" ; +``` +In other words, it is possible for a human to love anything. + +A problem related to polymorphism is **subtyping**: what +is meaningful for a ``human`` is also meaningful for +a ``man`` and a ``woman``, but not the other way round. +One solution to this is **coercions**: functions that +"lift" objects from subtypes to supertypes. + + +==Case study: selectional restrictions and statistical language models TODO== + + +==Proof objects== + +Perhaps the most well-known idea in constructive type theory is +the **Curry-Howard isomorphism**, also known as the +**propositions as types principle**. Its earliest formulations +were attempts to give semantics to the logical systems of +propositional and predicate calculus. In this section, we will consider +a more elementary example, showing how the notion of proof is useful +outside mathematics, as well. + +We first define the category of unary (also known as Peano-style) +natural numbers: +``` + cat Nat ; + fun Zero : Nat ; + fun Succ : Nat -> Nat ; +``` +The **successor function** ``Succ`` generates an infinite +sequence of natural numbers, beginning from ``Zero``. + +We then define what it means for a number //x// to be //less than// +a number //y//. Our definition is based on two axioms: +- ``Zero`` is less than ``Succ`` //y// for any //y//. +- If //x// is less than //y//, then``Succ`` //x// is less than ``Succ`` //y//. + + +The most straightforward way of expressing these axioms in type theory +is as typing judgements that introduce objects of a type ``Less`` //x y //: +``` + cat Less Nat Nat ; + fun lessZ : (y : Nat) -> Less Zero (Succ y) ; + fun lessS : (x,y : Nat) -> Less x y -> Less (Succ x) (Succ y) ; +``` +Objects formed by ``lessZ`` and ``lessS`` are +called **proof objects**: they establish the truth of certain +mathematical propositions. +For instance, the fact that 2 is less that +4 has the proof object +``` + lessS (Succ Zero) (Succ (Succ (Succ Zero))) + (lessS Zero (Succ (Succ Zero)) (lessZ (Succ Zero))) +``` +whose type is +``` + Less (Succ (Succ Zero)) (Succ (Succ (Succ (Succ Zero)))) +``` +which is the formalization of the proposition that 2 is less than 4. + +GF grammars can be used to provide a **semantic control** of +well-formedness of expressions. We have already seen examples of this: +the grammar of well-formed addresses and the grammar with +selectional restrictions above. By introducing proof objects +we have now added a very powerful technique of expressing semantic conditions. + +A simple example of the use of proof objects is the definition of +well-formed //time spans//: a time span is expected to be from an earlier to +a later time: +``` + from 3 to 8 +``` +is thus well-formed, whereas +``` + from 8 to 3 +``` +is not. The following rules for spans impose this condition +by using the ``Less`` predicate: +``` + cat Span ; + fun span : (m,n : Nat) -> Less m n -> Span ; +``` +A possible practical application of this idea is **proof-carrying documents**: +to be semantically well-formed, the abstract syntax of a document must contain a proof +of some property, although the proof is not shown in the concrete document. +Think, for instance, of small documents describing flight connections: + +//To fly from Gothenburg to Prague, first take LH3043 to Frankfurt, then OK0537 to Prague.// + +The well-formedness of this text is partly expressible by dependent typing: +``` + cat + City ; + Flight City City ; + fun + Gothenburg, Frankfurt, Prague : City ; + LH3043 : Flight Gothenburg Frankfurt ; + OK0537 : Flight Frankfurt Prague ; +``` +This rules out texts saying //take OK0537 from Gothenburg to Prague//. However, there is a +further condition saying that it must be possible to change from LH3043 to OK0537 in Frankfurt. +This can be modelled as a proof object of a suitable type, which is required by the constructor +that connects flights. +``` + cat + IsPossible (x,y,z : City)(Flight x y)(Flight y z) ; + fun + Connect : (x,y,z : City) -> + (u : Flight x y) -> (v : Flight y z) -> + IsPossible x y z u v -> Flight x z ; +``` + + + +==Variable bindings== + +Mathematical notation and programming languages have lots of +expressions that **bind** variables. For instance, +a universally quantifier proposition +``` + (All x)B(x) +``` +consists of the **binding** ``(All x)`` of the variable ``x``, +and the **body** ``B(x)``, where the variable ``x`` can have +**bound occurrences**. + +Variable bindings appear in informal mathematical language as well, for +instance, +``` + for all x, x is equal to x + + the function that for any numbers x and y returns the maximum of x+y + and x*y +``` +In type theory, variable-binding expression forms can be formalized +as functions that take functions as arguments. The universal +quantifier is defined +``` + fun All : (Ind -> Prop) -> Prop +``` +where ``Ind`` is the type of individuals and ``Prop``, +the type of propositions. If we have, for instance, the equality predicate +``` + fun Eq : Ind -> Ind -> Prop +``` +we may form the tree +``` + All (\x -> Eq x x) +``` +which corresponds to the ordinary notation +``` + (All x)(x = x). +``` + + +An abstract syntax where trees have functions as arguments, as in +the two examples above, has turned out to be precisely the right +thing for the semantics and computer implementation of +variable-binding expressions. The advantage lies in the fact that +only one variable-binding expression form is needed, the lambda abstract +``\x -> b``, and all other bindings can be reduced to it. +This makes it easier to implement mathematical theories and reason +about them, since variable binding is tricky to implement and +to reason about. The idea of using functions as arguments of +syntactic constructors is known as **higher-order abstract syntax**. + +The question now arises: how to define linearization rules +for variable-binding expressions? +Let us first consider universal quantification, +``` + fun All : (Ind -> Prop) -> Prop +``` +We write +``` + lin All B = {s = "(" ++ "All" ++ B.$0 ++ ")" ++ B.s} +``` +to obtain the form shown above. +This linearization rule brings in a new GF concept - the ``$0`` +field of ``B`` containing a bound variable symbol. +The general rule is that, if an argument type of a function is +itself a function type ``A -> C``, the linearization type of +this argument is the linearization type of ``C`` +together with a new field ``$0 : Str``. In the linearization rule +for ``All``, the argument ``B`` thus has the linearization +type +``` + {$0 : Str ; s : Str}, +``` +since the linearization type of ``Prop`` is +``` + {s : Str} +``` +In other words, the linearization of a function +consists of a linearization of the body together with a +field for a linearization of the bound variable. +Those familiar with type theory or lambda calculus +should notice that GF requires trees to be in +**eta-expanded** form in order to be linearizable: +any function of type +``` + A -> B +``` +always has a syntax tree of the form +``` + \x -> b +``` +where ``b : B`` under the assumption ``x : A``. +It is in this form that an expression can be analysed +as having a bound variable and a body. + + +Given the linearization rule +``` + lin Eq a b = {s = "(" ++ a.s ++ "=" ++ b.s ++ ")"} +``` +the linearization of +``` + \x -> Eq x x +``` +is the record +``` + {$0 = "x", s = ["( x = x )"]} +``` +Thus we can compute the linearization of the formula, +``` + All (\x -> Eq x x) --> {s = "[( All x ) ( x = x )]"}. +``` + +How did we get the //linearization// of the variable ``x`` +into the string ``"x"``? GF grammars have no rules for +this: it is just hard-wired in GF that variable symbols are +linearized into the same strings that represent them in +the print-out of the abstract syntax. + + +To be able to //parse// variable symbols, however, GF needs to know what +to look for (instead of e.g. trying to parse //any// +string as a variable). What strings are parsed as variable symbols +is defined in the lexical analysis part of GF parsing +``` + > p -cat=Prop -lexer=codevars "(All x)(x = x)" + All (\x -> Eq x x) +``` +(see more details on lexers below). If several variables are bound in the +same argument, the labels are ``$0, $1, $2``, etc. + + + +==Semantic definitions== + +We have seen that, +just like functional programming languages, GF has declarations +of functions, telling what the type of a function is. +But we have not yet shown how to **compute** +these functions: all we can do is provide them with arguments +and linearize the resulting terms. +Since our main interest is the well-formedness of expressions, +this has not yet bothered +us very much. As we will see, however, computation does play a role +even in the well-formedness of expressions when dependent types are +present. + +GF has a form of judgement for **semantic definitions**, +recognized by the key word ``def``. At its simplest, it is just +the definition of one constant, e.g. +``` + def one = Succ Zero ; +``` +We can also define a function with arguments, +``` + def Neg A = Impl A Abs ; +``` +which is still a special case of the most general notion of +definition, that of a group of **pattern equations**: +``` + def + sum x Zero = x ; + sum x (Succ y) = Succ (Sum x y) ; +``` +To compute a term is, as in functional programming languages, +simply to follow a chain of reductions until no definition +can be applied. For instance, we compute +``` + Sum one one --> + Sum (Succ Zero) (Succ Zero) --> + Succ (sum (Succ Zero) Zero) --> + Succ (Succ Zero) +``` +Computation in GF is performed with the ``pt`` command and the +``compute`` transformation, e.g. +``` + > p -tr "1 + 1" | pt -transform=compute -tr | l + sum one one + Succ (Succ Zero) + s(s(0)) +``` + +The ``def`` definitions of a grammar induce a notion of +**definitional equality** among trees: two trees are +definitionally equal if they compute into the same tree. +Thus, trivially, all trees in a chain of computation +(such as the one above) +are definitionally equal to each other. So are the trees +``` + sum Zero (Succ one) + Succ one + sum (sum Zero Zero) (sum (Succ Zero) one) +``` +and infinitely many other trees. + +A fact that has to be emphasized about ``def`` definitions is that +they are //not// performed as a first step of linearization. +We say that **linearization is intensional**, which means that +the definitional equality of two trees does not imply that +they have the same linearizations. For instance, each of the seven terms +shown above has a different linearizations in arithmetic notation: +``` + 1 + 1 + s(0) + s(0) + s(s(0) + 0) + s(s(0)) + 0 + s(0) + s(1) + 0 + 0 + s(0) + 1 +``` +This notion of intensionality is +no more exotic than the intensionality of any **pretty-printing** +function of a programming language (function that shows +the expressions of the language as strings). It is vital for +pretty-printing to be intensional in this sense - if we want, +for instance, to trace a chain of computation by pretty-printing each +intermediate step, what we want to see is a sequence of different +expression, which are definitionally equal. + +What is more exotic is that GF has two ways of referring to the +abstract syntax objects. In the concrete syntax, the reference is intensional. +In the abstract syntax, the reference is extensional, since +**type checking is extensional**. The reason is that, +in the type theory with dependent types, types may depend on terms. +Two types depending on terms that are definitionally equal are +equal types. For instance, +``` + Proof (Odd one) + Proof (Odd (Succ Zero)) +``` +are equal types. Hence, any tree that type checks as a proof that +1 is odd also type checks as a proof that the successor of 0 is odd. +(Recall, in this connection, that the +arguments a category depends on never play any role +in the linearization of trees of that category, +nor in the definition of the linearization type.) + +In addition to computation, definitions impose a +**paraphrase** relation on expressions: +two strings are paraphrases if they +are linearizations of trees that are +definitionally equal. +Paraphrases are sometimes interesting for +translation: the **direct translation** +of a string, which is the linearization of the same tree +in the targer language, may be inadequate because it is e.g. +unidiomatic or ambiguous. In such a case, +the translation algorithm may be made to consider +translation by a paraphrase. + +To stress express the distinction between +**constructors** (=**canonical** functions) +and other functions, GF has a judgement form +``data`` to tell that certain functions are canonical, e.g. +``` + data Nat = Succ | Zero ; +``` +Unlike in Haskell, but similarly to ALF (where constructor functions +are marked with a flag ``C``), +new constructors can be added to +a type with new ``data`` judgements. The type signatures of constructors +are given separately, in ordinary ``fun`` judgements. +One can also write directly +``` + data Succ : Nat -> Nat ; +``` +which is equivalent to the two judgements +``` + fun Succ : Nat -> Nat ; + data Nat = Succ ; +``` + + +==Case study: representing anaphoric reference TODO== + + +=Transfer modules TODO= + +Transfer means noncompositional tree-transforming operations. +The command ``apply_transfer = at`` is typically used in a pipe: +``` + > p "John walks and John runs" | apply_transfer aggregate | l + John walks and runs +``` +See the +[sources ../../transfer/examples/aggregation] of this example. + +See the +[transfer language documentation ../transfer.html] +for more information. + + +=Practical issues TODO= + + +==Lexers and unlexers== + +Lexers and unlexers can be chosen from +a list of predefined ones, using the flags``-lexer`` and `` -unlexer`` either +in the grammar file or on the GF command line. + +Given by ``help -lexer``, ``help -unlexer``: +``` + The default is words. + -lexer=words tokens are separated by spaces or newlines + -lexer=literals like words, but GF integer and string literals recognized + -lexer=vars like words, but "x","x_...","$...$" as vars, "?..." as meta + -lexer=chars each character is a token + -lexer=code use Haskell's lex + -lexer=codevars like code, but treat unknown words as variables, ?? as meta + -lexer=text with conventions on punctuation and capital letters + -lexer=codelit like code, but treat unknown words as string literals + -lexer=textlit like text, but treat unknown words as string literals + -lexer=codeC use a C-like lexer + -lexer=ignore like literals, but ignore unknown words + -lexer=subseqs like ignore, but then try all subsequences from longest + + The default is unwords. + -unlexer=unwords space-separated token list (like unwords) + -unlexer=text format as text: punctuation, capitals, paragraph

+ -unlexer=code format as code (spacing, indentation) + -unlexer=textlit like text, but remove string literal quotes + -unlexer=codelit like code, but remove string literal quotes + -unlexer=concat remove all spaces + -unlexer=bind like identity, but bind at "&+" +``` + + +==Efficiency of grammars== + +Issues: + +- the choice of datastructures in ``lincat``s +- the value of the ``optimize`` flag +- parsing efficiency: ``-fcfg`` vs. others + + +==Speech input and output== + +The``speak_aloud = sa`` command sends a string to the speech +synthesizer +[Flite http://www.speech.cs.cmu.edu/flite/doc/]. +It is typically used via a pipe: +``` generate_random | linearize | speak_aloud +The result is only satisfactory for English. + +The ``speech_input = si`` command receives a string from a +speech recognizer that requires the installation of +[ATK http://mi.eng.cam.ac.uk/~sjy/software.htm]. +It is typically used to pipe input to a parser: +``` speech_input -tr | parse +The method words only for grammars of English. + +Both Flite and ATK are freely available through the links +above, but they are not distributed together with GF. + + +==Multilingual syntax editor== + +The +[Editor User Manual http://www.cs.chalmers.se/~aarne/GF2.0/doc/javaGUImanual/javaGUImanual.htm] +describes the use of the editor, which works for any multilingual GF grammar. + +Here is a snapshot of the editor: + +[../quick-editor.png] + +The grammars of the snapshot are from the +[Letter grammar package http://www.cs.chalmers.se/~aarne/GF/examples/letter]. + + + +==Interactive Development Environment (IDE)== + +Forthcoming. + + +==Communicating with GF== + +Other processes can communicate with the GF command interpreter, +and also with the GF syntax editor. Useful flags when invoking GF are +- ``-batch`` suppresses the promps and structures the communication with XML tags. +- ``-s`` suppresses non-output non-error messages and XML tags. +-- ``-nocpu`` suppresses CPU time indication. + +Thus the most silent way to invoke GF is +``` + gf -batch -s -nocpu +``` + + + +==Embedded grammars in Haskell, Java, and Prolog== + +GF grammars can be used as parts of programs written in the +following languages. The links give more documentation. + +- [Java http://www.cs.chalmers.se/~bringert/gf/gf-java.html] +- [Haskell http://www.cs.chalmers.se/~aarne/GF/src/GF/Embed/EmbedAPI.hs] +- [Prolog http://www.cs.chalmers.se/~peb/software.html] + + +==Alternative input and output grammar formats== + +A summary is given in the following chart of GF grammar compiler phases: +[../gf-compiler.png] + + +=Larger case studies TODO= + +==Interfacing formal and natural languages== + +[Formal and Informal Software Specifications http://www.cs.chalmers.se/~krijo/thesis/thesisA4.pdf], +PhD Thesis by +[Kristofer Johannisson http://www.cs.chalmers.se/~krijo], is an extensive example of this. +The system is based on a multilingual grammar relating the formal language OCL with +English and German. + +A simpler example will be explained here. + + +==A multimodal dialogue system== + +See TALK project deliverables, [TALK homepage http://www.talk-project.org] +