The term GF is used for different things:
This tutorial is primarily about the GF program and the GF programming language. It will guide you
The program is open-source free software, which you can download via the
GF Homepage:
http://www.cs.chalmers.se/~aarne/GF
There you can download
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.
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 >.
Now you are ready to try out your first grammar.
We start with one that is not written in GF language, but
in the ubiquitous BNF notation (Backus Naur Form), which GF can also
understand. Type (or copy) the following lines in a file named
paleolithic.cf:
S ::= NP VP ;
VP ::= V | TV NP | "is" A ;
NP ::= "this" CN | "that" CN | "the" CN | "a" CN ;
CN ::= A CN ;
CN ::= "boy" | "louse" | "snake" | "worm" ;
A ::= "green" | "rotten" | "thick" | "warm" ;
V ::= "laughs" | "sleeps" | "swims" ;
TV ::= "eats" | "kills" | "washes" ;
(The name paleolithic refers to a larger package
stoneage,
which implements a fragment of primitive language. This fragment
was defined by the linguist Morris Swadesh as a tool for studying
the historical relations of languages. But as suggested
in the Wiktionary article on
Swadesh list, the
fragment is also usable for basic communication between foreigners.)
The first GF command 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 paleolithic.cf
or
i paleolithic.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.
You can now use GF for parsing:
> parse "the boy eats a snake"
S_NP_VP (NP_the_CN CN_boy) (VP_TV_NP TV_eats (NP_a_CN CN_snake))
> parse "the snake eats a boy"
S_NP_VP (NP_the_CN CN_snake) (VP_TV_NP TV_eats (NP_a_CN CN_boy))
The parse (= p) command takes a string
(in double quotes) and returns an abstract syntax tree - the thing
beginning with S_NP_VP. We will see soon how to make sense
of the abstract syntax trees - now you should just notice that the tree
is different for the two strings.
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"
No success in cf parsing hello world
no tree found
You can also use GF for linearizing
(linearize = l). This is the inverse of
parsing, taking trees into strings:
> linearize S_NP_VP (NP_the_CN CN_boy) (VP_TV_NP TV_eats (NP_a_CN CN_snake))
the boy eats a snake
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
S_NP_VP (NP_this_CN (CN_A_CN A_thick CN_worm)) (VP_V V_sleeps)
Now you can copy the tree and paste it to the linearize command.
Or, more efficiently, feed random generation into parsing by using
a pipe.
> gr | l
this worm is warm
The gibberish code with parentheses returned by the parser does not
look like trees. Why is it called so? Trees are a data structure that
represent <b>nesting</b>: 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 "the green boy eats a warm snake" | vt
Random generation can be quite amusing. So you may want to generate ten strings with one and the same command:
> gr -number=10 | l
this boy is green
a snake laughs
the rotten boy is thick
a boy washes this worm
a boy is warm
this green warm boy is rotten
the green thick green louse is rotten
that boy is green
this thick thick boy laughs
a boy is green
To generate <i>all<i> sentence that a grammar
can generate, use the command generate_trees = gt.
> generate_trees | l
this louse laughs
this louse sleeps
this louse swims
this louse is green
this louse is rotten
...
a boy is rotten
a boy is thick
a boy 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 gr
Quiz. If the command gt generated all
trees in your grammar, it would never terminate. Why?
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
Mks_0 (Mks_7 Mks_10) (Mks_1 Mks_18)
a louse sleeps
Mks_0 (Mks_7 Mks_10) (Mks_1 Mks_18)
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.
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 | l -tr | 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 syntax trees returned by GF's parser in the previous examples
are not so nice to look at. The identifiers of form Mks
are labels of the EBNF rules. To see which label corresponds to
which rule, you can use the print_grammar = pg command
with the printer flag set to cf (which means context-free):
> print_grammar -printer=cf
Mks_10. CN ::= "louse" ;
Mks_11. CN ::= "snake" ;
Mks_12. CN ::= "worm" ;
Mks_8. CN ::= A CN ;
Mks_9. CN ::= "boy" ;
Mks_4. NP ::= "this" CN ;
Mks_15. A ::= "thick" ;
...
A syntax tree such as
Mks_4 (Mks_8 Mks_15 Mks_12)
this thick worm
encodes the sequence of grammar rules used for building the
expression. If you look at this tree, you will notice that Mks_4
is the label of the rule prefixing this to a common noun,
Mks_15 is the label of the adjective thick,
and so on.
<h4>The labelled context-free format<h4>
The labelled context-free grammar format permits user-defined
labels to each rule.
GF recognizes files of this format by the suffix
.cf. It is intermediate between EBNF and full GF format.
Let us include the following rules in the file
paleolithic.cf.
PredVP. S ::= NP VP ;
UseV. VP ::= V ;
ComplTV. VP ::= TV NP ;
UseA. VP ::= "is" A ;
This. NP ::= "this" CN ;
That. NP ::= "that" CN ;
Def. NP ::= "the" CN ;
Indef. NP ::= "a" CN ;
ModA. CN ::= A CN ;
Boy. CN ::= "boy" ;
Louse. CN ::= "louse" ;
Snake. CN ::= "snake" ;
Worm. CN ::= "worm" ;
Green. A ::= "green" ;
Rotten. A ::= "rotten" ;
Thick. A ::= "thick" ;
Warm. A ::= "warm" ;
Laugh. V ::= "laughs" ;
Sleep. V ::= "sleeps" ;
Swim. V ::= "swims" ;
Eat. TV ::= "eats" ;
Kill. TV ::= "kills"
Wash. TV ::= "washes" ;
<h4>Using the labelled context-free format<h4>
The GF commands for the .cf format are
exactly the same as for the .ebnf format.
Just the syntax trees become nicer to read and
to remember. Notice that before reading in
a new grammar in GF you often (but not always,
as we will see later) have first to give the
command (empty = e), which removes the
old grammar from the GF shell state.
> empty
> i paleolithic.cf
> p "the boy eats a snake"
PredVP (Def Boy) (ComplTV Eat (Indef Snake))
> gr -tr | l
PredVP (Indef Louse) (UseA Thick)
a louse is thick
To see what there really is in GF's shell state when a grammar
has been imported, you can give the plain 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 to show how GF's own notation gives you
much more expressive power than the .cf and .ebnf
formats. We will introduce the .gf format by presenting
one more way of defining the same grammar as in
paleolithic.cf and paleolithic.ebnf.
Then we will show how the full GF grammar format enables you
to do things that are not possible in the weaker formats.
A GF grammar consists of two main parts:
The EBNF and CF formats fuse these two things together, but it is possible to take them apart. For instance, the verb phrase predication rule
PredVP. S ::= NP VP ;
is interpreted as the following pair of rules:
fun PredVP : NP -> VP -> S ;
lin PredVP x y = {s = x.s ++ y.s} ;
The former rule, with the keyword fun, belongs to the abstract syntax.
It defines the function
PredVP which constructs syntax trees of form
(PredVP <i>x<i> <i>y<i>).
The latter rule, with the keyword lin, belongs to the concrete syntax.
It defines the linearization function for
syntax trees of form (PredVP <i>x<i> <i>y<i>).
<h4>Judgement forms<h4>
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:
| form | reading |
cat C |
C is a category |
fun f : A |
f is a function of type A |
| 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 grammar paleolithic.cf is
expressed by using modules and judgements.
<h4>Module types<h4>
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.
<h4>Record types, records, and Strs<h4>
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"}
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.
Whenever a record r of type {s : Str} is given,
r.s is an object of type Str. This is of course
a special case of the projection rule, allowing the extraction
of fields from a record.
<h4>An abstract syntax example<h4>
Each nonterminal occurring in the grammar paleolithic.cf is
introduced by a cat judgement. Each
rule label is introduced by a fun judgement.
abstract Paleolithic = {
cat
S ; NP ; VP ; CN ; A ; V ; TV ;
fun
PredVP : NP -> VP -> S ;
UseV : V -> VP ;
ComplTV : TV -> NP -> VP ;
UseA : A -> VP ;
ModA : A -> CN -> CN ;
This, That, Def, Indef : CN -> NP ;
Boy, Louse, Snake, Worm : CN ;
Green, Rotten, Thick, Warm : A ;
Laugh, Sleep, Swim : V ;
Eat, Kill, Wash : TV ;
}
Notice the use of shorthands permitting the sharing of
the keyword in subsequent judgements, and of the type
in subsequent fun judgements.
<h4>A concrete syntax example<h4>
Each category introduced in Paleolithic.gf is
given a lincat rule, and each
function is given a fun rule. Similar shorthands
apply as in abstract modules.
concrete PaleolithicEng of Paleolithic = {
lincat
S, NP, VP, CN, A, V, TV = {s : Str} ;
lin
PredVP np vp = {s = np.s ++ vp.s} ;
UseV v = v ;
ComplTV tv np = {s = tv.s ++ np.s} ;
UseA a = {s = "is" ++ a.s} ;
This cn = {s = "this" ++ cn.s} ;
That cn = {s = "that" ++ cn.s} ;
Def cn = {s = "the" ++ cn.s} ;
Indef cn = {s = "a" ++ cn.s} ;
ModA a cn = {s = a.s ++ cn.s} ;
Boy = {s = "boy"} ;
Louse = {s = "louse"} ;
Snake = {s = "snake"} ;
Worm = {s = "worm"} ;
Green = {s = "green"} ;
Rotten = {s = "rotten"} ;
Thick = {s = "thick"} ;
Warm = {s = "warm"} ;
Laugh = {s = "laughs"} ;
Sleep = {s = "sleeps"} ;
Swim = {s = "swims"} ;
Eat = {s = "eats"} ;
Kill = {s = "kills"} ;
Wash = {s = "washes"} ;
}
<h4>Modules and files<h4>
Module name + .gf = file name
Each module is compiled into a .gfc file.
Import PaleolithicEng.gf and try what happens
> i PaleolithicEng.gf
The GF program does not only read the file
PaleolithicEng.gf, but also all other files that it
depends on - in this case, Paleolithic.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 knows whether
to use an existing .gfc file or to generate
a new one, by looking at modification times.
<h4>Multilingual grammar<h4>
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 buid an Italian concrete syntax for
Paleolithic and then test the resulting
multilingual grammar.
<h4>An Italian concrete syntax<h4>
concrete PaleolithicIta of Paleolithic = {
lincat
S, NP, VP, CN, A, V, TV = {s : Str} ;
lin
PredVP np vp = {s = np.s ++ vp.s} ;
UseV v = v ;
ComplTV tv np = {s = tv.s ++ np.s} ;
UseA a = {s = "è" ++ a.s} ;
This cn = {s = "questo" ++ cn.s} ;
That cn = {s = "quello" ++ cn.s} ;
Def cn = {s = "il" ++ cn.s} ;
Indef cn = {s = "un" ++ cn.s} ;
ModA a cn = {s = cn.s ++ a.s} ;
Boy = {s = "ragazzo"} ;
Louse = {s = "pidocchio"} ;
Snake = {s = "serpente"} ;
Worm = {s = "verme"} ;
Green = {s = "verde"} ;
Rotten = {s = "marcio"} ;
Thick = {s = "grosso"} ;
Warm = {s = "caldo"} ;
Laugh = {s = "ride"} ;
Sleep = {s = "dorme"} ;
Swim = {s = "nuota"} ;
Eat = {s = "mangia"} ;
Kill = {s = "uccide"} ;
Wash = {s = "lava"} ;
}
<h4>Using a multilingual grammar<h4>
Import without first emptying
> i PaleolithicEng.gf
> i PaleolithicIta.gf
Try generation now:
> gr | l
un pidocchio uccide questo ragazzo
> gr | l -lang=PaleolithicEng
that louse eats a louse
Translate by using a pipe:
> p -lang=PaleolithicEng "the boy eats the snake" | l -lang=PaleolithicIta
il ragazzo mangia il serpente
<h4>Translation quiz<h4>
This is a simple language exercise that can be automatically
generated from a multilingual grammar. The system generates a set of
random sentence, 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 PaleolithicEng PaleolithicIta
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 ('.').
a green boy washes the louse
un ragazzo verde lava il gatto
No, not un ragazzo verde lava il gatto, but
un ragazzo verde lava il pidocchio
Score 0/1
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 PaleolithicEng PaleolithicIta
The number flag gives the number of sentences generated.
<h4>The multilingual shell state<h4>
A GF shell is at any time in a state, which
contains a multilingual grammar. One of the concrete
syntaxes is the "main" one, which means that parsing and linearization
are performed by using it. By default, the main concrete syntax is the
last-imported one. As we saw on previous slide, the lang flag
can be used to change the linearization and parsing grammar.
To see what the multilingual grammar is (as well as some other
things), you can use the command
print_options = po:
> print_options
main abstract : Paleolithic
main concrete : PaleolithicIta
all concretes : PaleolithicIta PaleolithicEng
<h4>Extending a grammar<h4>
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.
abstract Neolithic = Paleolithic ** {
fun
Fire, Wheel : CN ;
Think : V ;
}
Parallel to the abstract syntax, extensions can be built for concrete syntaxes:
concrete NeolithicEng of Neolithic = PaleolithicEng ** {
lin
Fire = {s = "fire"} ;
Wheel = {s = "wheel"} ;
Think = {s = "thinks"} ;
}
The effect of extension is that all of the contents of the extended and extending module are put together.
<h4>Multiple inheritance<h4>
Specialized vocabularies can be represented as small grammars that only do "one thing" each, e.g.
abstract Fish = {
cat Fish ;
fun Salmon, Perch : Fish ;
}
abstract Mushrooms = {
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 Gatherer = Paleolithic, Fish, Mushrooms ** {
fun
UseFish : Fish -> CN ;
UseMushroom : Mushroom -> CN ;
}
<h4>Visualizing module structure<h4>
When you have created all the abstract syntaxes and
one set of concrete syntaxes needed for Gatherer,
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. It can also
be printed out into a file, e.g. a .gif file that
can be included in an HTML document
> pm -printer=graph | wf Gatherer.dot
> ! dot -Tgif Gatherer.dot > Gatherer.gif
The latter command is a Unix command, issued from GF by using the
shell escape symbol !. The resulting graph is shown in the next 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.
<h4>The module structure of GathererEng<h4>
The graph uses
<img src="Gatherer.gif">
Suppose we want to say, with the vocabulary included in
Paleolithic.gf, things like
the boy eats two snakes
all boys sleep
The new grammatical facility we need are the plural forms of nouns and verbs (<i>boys, sleep<i>), as opposed to their singular forms.
The introduction of plural forms requires two things:
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 precisely in concrete syntax while ignoring them in abstract syntax.
To be able to do all this, we need two new judgement forms, a new module form, and a generalizarion of linearization types from strings to more complex types.
<h4>Parameters and tables<h4>
We define the parameter type of number in Englisn by using a new form of judgement:
param Number = Sg | Pl ;
To express that nouns 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 CN = {s : Number => Str} ;
The table type Number => Str is in many respects similar to
a function type (Number -> Str). The main restriction 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 Boy = {s = table {
Sg => "boy" ;
Pl => "boys"
}
} ;
The application of a table to a parameter is done by the selection
operator !. For instance,
Boy.s ! Pl
is a selection, whose value is "boys".
<h4>Inflection tables, paradigms, and oper definitions<h4>
All English common nouns are inflected in number, most of them in the same way: the plural form is formed from the singular form by adding the ending <i>s<i>. This rule is an example of a paradigm - a formula telling how the inflection forms of a word are formed.
From GF point of view, a paradigm is a function that takes a lemma -
a string 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). Thus we call them operations for the sake of clarity,
introduce one one form of judgement, with the keyword oper. As an
example, the following operation defines the regular noun paradigm of English:
oper regNoun : Str -> {s : Number => Str} = \x -> {
s = table {
Sg => x ;
Pl => x + "s"
}
} ;
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, and the glueing operator + telling that
the string held in the variable x and the ending "s"
are written together to form one token.
<h4>The resource module type<h4>
Parameter and operator definitions do not belong to the abstract syntax. They can be used when defining concrete syntax - but they are not tied to a particular set of linearization rules. The proper way to see them is as auxiliary concepts, as resources usable in many concrete syntaxes.
The resource module type thus consists of
param and oper definitions. Here is an
example.
resource MorphoEng = {
param
Number = Sg | Pl ;
oper
Noun : Type = {s : Number => Str} ;
regNoun : Str -> Noun = \x -> {
s = table {
Sg => x ;
Pl => 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.
Any number of resource modules can be
opened in a concrete syntax, which
makes the parameter and operation definitions contained
in the resource usable in the concrete syntax. Here is
an example, where the resource MorphoEng is
open in (the fragment of) a new version of PaleolithicEng.
concrete PaleolithicEng of Paleolithic = open MorphoEng in {
lincat
CN = Noun ;
lin
Boy = regNoun "boy" ;
Snake = regNoun "snake" ;
Worm = regNoun "worm" ;
}
Notice that, just like in abstract syntax, function application is written by juxtaposition of the function and the argument.
Using operations defined in resource modules is clearly a concise way of giving e.g. inflection tables and other repeated patterns of expression. 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 put this knowledge available through resource grammars, whose users only need to pick the right operations and not to know their implementation details.
<h4>Worst-case macros and data abstraction<h4>
Some English nouns, such as louse, are so irregular that
it makes little 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 macro for nouns:
oper mkNoun : Str -> Str -> Noun = \x,y -> {
s = table {
Sg => x ;
Pl => y
}
} ;
Thus we define
lin Louse = mkNoun "louse" "lice" ;
instead of writing the inflection table explicitly.
The grammar engineering advantage of worst-case macros 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.
<h4>A system of paradigms using Prelude operations<h4>
The regular noun paradigm regNoun can - and should - of course be defined
by the worst-case macro mkNoun. In addition, some more noun paradigms
could be defined, for instance,
regNoun : Str -> Noun = \snake -> mkNoun snake (snake + "s") ;
sNoun : Str -> Noun = \kiss -> mkNoun kiss (kiss + "es") ;
What about nouns like <i>fly<i>, with the plural <i>flies<i>? The already available solution is to use the so-called "technical stem" <i>fl<i> 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 even an existing form of the word. A better solution is to use
the 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 operator init belongs to a set of operations in the
resource module Prelude, which therefore has to be
opened so that init can be used.
<h4>An intelligent noun paradigm using case expressions<h4>
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 paradigms, 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.
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 following section.
The paradigms regNoun does not give the correct forms for
all nouns. For instance, <i>louse - lice<i> and
<i>fish - fish<i> must be given by using mkNoun.
Also the word <i>boy<i> 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.
<h4>Pattern matching<h4>
Expressions of the table form are built from lists of
argument-value pairs. These pairs are called the branches
of the table. In addition to constants introduced in
param definitions, the left-hand side of a branch can more
generally be a pattern, and the computation of selection is
then performed by pattern matching:
_ matches anything
"s", matches the same string
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.
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
<h4>Morphological analysis and morphology quiz<h4>
Even though in GF morphology
is mostly seen as an auxiliary of syntax, a morphology once defined
can be used 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
Similarly to 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,
> i lib/resource/french/VerbsFre.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 and save it in a
file for later use, by the command morpho_list = ml
> morpho_list -number=25 -cat=V
The number flag gives the number of exercises generated.
<h4>Parametric vs. inherent features, agreement<h4>
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), in some sense <i>has<i> a number, which it "sends" 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 parametric.
The agreement rule itself is expressed in the linearization rule of the predication structure:
lin PredVP np vp = {s = np.s ++ vp.s ! np.n} ;
The following page will present a new version of
PaleolithingEng, assuming an abstract syntax
xextended with All and Two.
It also assumes that MorphoEng has a paradigm
regVerb for regular verbs (which need only be
regular only in the present tensse).
The reader is invited to inspect the way in which agreement works in
the formation of noun phrases and verb phrases.
<h4>English concrete syntax with parameters<h4>
concrete PaleolithicEng of Paleolithic = open MorphoEng in {
lincat
S, A = {s : Str} ;
VP, CN, V, TV = {s : Number => Str} ;
NP = {s : Str ; n : Number} ;
lin
PredVP np vp = {s = np.s ++ vp.s ! np.n} ;
UseV v = v ;
ComplTV tv np = {s = \\n => tv.s ! n ++ np.s} ;
UseA a = {s = \\n => case n of {Sg => "is" ; Pl => "are"} ++ a.s} ;
This cn = {s = "this" ++ cn.s ! Sg } ;
Indef cn = {s = "a" ++ cn.s ! Sg} ;
All cn = {s = "all" ++ cn.s ! Pl} ;
Two cn = {s = "two" ++ cn.s ! Pl} ;
ModA a cn = {s = \\n => a.s ++ cn.s ! n} ;
Louse = mkNoun "louse" "lice" ;
Snake = regNoun "snake" ;
Green = {s = "green"} ;
Warm = {s = "warm"} ;
Laugh = regVerb "laugh" ;
Sleep = regVerb "sleep" ;
Kill = regVerb "kill" ;
}
<h4>Hierarchic parameter types<h4>
The reader familiar with a functional programming language such as
<a href="http://www.haskell.org">Haskell<a> 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.
(This restriction 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 (recursion is forbidden). Such parameter types impose a hierarchic order among parameters. They are often useful to define linguistically 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 = Uter | Neuter ;
In pattern matching, a constructor can have 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 = \x -> table {
ASg _ => x ;
APl => x + "a" ;
}
<h4>Discontinuous constituents<h4>
A linearization type may contain more strings than one. An example of where this is useful are English particle verbs, such as <i>switch off<i>. The linearization of a sentence may place the object between the verb and the particle: <i>he switched it off<i>.
The first of the following judgements defines transitive verbs as a discontinuous constituents, i.e. as having a linearization type with two strings and not just one. The second judgement shows how the constituents are separated by the object in complementization.
lincat TV = {s : Number => Str ; s2 : Str} ;
lin ComplTV tv obj = {s = \\n => tv.s ! n ++ obj.s ++ tv.s2} ;
GF currently requires that all fields in linearization records that
have a table with value type Str have as labels
either s or s with an integer index.