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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
<html><head><title></title></head>
<body bgcolor="#ffffff" text="#000000">
<center>
<img src="../gf-logo.gif">
<h1>Grammatical Framework Tutorial</h1>
<p>
<b>3rd Edition, for GF version 2.2 or later</b>
</p><p>
<a href="http://www.cs.chalmers.se/~aarne">Aarne Ranta</a>
</p>
<p>
<tt>aarne@cs.chalmers.se</tt>
</p></center>
<!-- NEW -->
<h2>GF = Grammatical Framework</h2>
The term GF is used for different things:
<ul>
<li> a <b>program</b> used for working with grammars
<li> a <b>programming language</b> in which grammars can be written
<li> a <b>theory</b> about the concepts of grammars and languages
</ul>
<p>
This tutorial is about the GF program and the GF programming language.
It will guide you
<ul>
<li> to use the GF program
<li> to write GF grammars
<li> to write programs in which GF grammars are used as components
</ul>
<!-- NEW -->
<h3>The GF program</h3>
The program is open-source free software, which you can download from the
GF Homepage:<br>
<a href="http://www.cs.chalmers.se/%7Eaarne/GF">
<tt>http://www.cs.chalmers.se/~aarne/GF</tt></a>
<p>
There you can download
<ul>
<li> ready-made binaries for Linux, Solaris, Macintosh, and Windows
<li> source code and documentation
<li> grammar libraries and examples
</ul>
If you want to compile GF from source, you need Haskell and Java
compilers. But normally you don't have to compile, and you don't
need to know Haskell or Java to use GF.
<p>
To start the GF program, assuming you have installed it, just type
<pre>
gf
</pre>
in the shell. You will see GF's welcome message and the prompt <tt>></tt>.
<!-- NEW -->
<h2>My first grammar</h2>
Now you are ready to try out your first grammar.
We start with one that is not written in GF language, but
in the EBNF notation (Extended Backus Naur Form), which GF can also
understand. Type (or copy) the following lines in a file named
<tt>paleolithic.ebnf</tt>:
<pre>
S ::= NP VP ;
VP ::= V | TV NP | "is" A ;
NP ::= ("this" | "that" | "the" | "a") CN ;
CN ::= A CN ;
CN ::= "boy" | "louse" | "snake" | "worm" ;
A ::= "green" | "rotten" | "thick" | "warm" ;
V ::= "laughs" | "sleeps" | "swims" ;
TV ::= "eats" | "kills" | "washes" ;
</pre>
<!-- NEW -->
<h3>Importing grammars and parsing strings</h3>
The first GF command when using a grammar is to <b>import</b> it.
The command has a long name, <tt>import</tt>, and a short name, <tt>i</tt>.
<pre>
import paleolithic.gf
</pre>
The GF program now <b>compiles</b> your grammar into an internal
representation, and shows a new prompt when it is ready.
<p>
You can use GF for <b>parsing</b>:
<pre>
> parse "the boy eats a snake"
Mks_0 (Mks_6 Mks_9) (Mks_2 Mks_20 (Mks_7 Mks_11))
> parse "the snake eats a boy"
Mks_0 (Mks_6 Mks_11) (Mks_2 Mks_20 (Mks_7 Mks_9))
</pre>
The <tt>parse</tt> (= <tt>p</tt>) command takes a <b>string</b>
(in double quotes) and returns an <b>abstract syntax tree</b> - the thing
with <tt>Mks</tt>s and parentheses. 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.
<p>
Strings that return a tree when parsed do so in virtue of the grammar
you imported. Try parsing something else, and you fail
<pre>
> p "hello world"
No success in cf parsing
no tree found
<pre>
<!-- NEW -->
<h3>Generating trees and strings</h3>
You can also use GF for <b>linearizing</b>
(<tt>linearize = l</tt>). This is the inverse of
parsing, taking trees into strings:
<pre>
> linearize Mks_0 (Mks_6 Mks_11) (Mks_2 Mks_20 (Mks_7 Mks_9))
the snake eats a boy
</pre>
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
<b>random generation</b> (<tt>generate_random = gr</tt>):
<pre>
> generate_random
Mks_0 (Mks_4 Mks_11) (Mks_3 Mks_15)
</pre>
Now you can copy the tree and paste it to the <tt>linearize command</tt>.
Or, more efficiently, feed random generation into parsing by using
a <b>pipe</b>.
<pre>
> gr | l
this worm is warm
</pre>
<!-- NEW -->
<h3>Some random-generated sentences</h3>
Random generation can be quite amusing. So you may want to
generate ten strings with one and the same command:
<pre>
> 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
</pre>
<!-- NEW -->
<h3>Systematic generation</h3>
To generate <i>all</i> sentence that a grammar
can generate, use the command <tt>generate_trees = gt</tt>.
<pre>
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
</pre>
You get quite a few trees but not all of them: only up to a given
<b>depth</b> of trees. To see how you can get more, use the
<tt>help = h</tt> command,
<pre>
h gr
</pre>
<b>Quiz</b>. If the command <tt>gt</tt> generated all
trees in your grammar, it would never terminate. Why?
<!-- NEW -->
<h3>More on pipes; tracing</h3>
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.
<p>
The intermediate results in a pipe can be observed by putting the
<b>tracing</b> flag <tt>-tr</tt> to each command whose output you
want to see:
<pre>
> 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)
</pre>
This facility is good for test purposes: for instance, you
may want to see if a grammar is <b>ambiguous</b>, i.e.
contains strings that can be parsed in more than one way.
<!-- NEW -->
<h3>Writing and reading files</h3>
To save the outputs of GF commands into a file, you can
pipe it to the <tt>write_file = wf</tt> command,
<pre>
> gr -number=10 | l | write_file exx.tmp
</pre>
You can read the file back to GF with the
<tt>read_file = rf</tt> command,
<pre>
> read_file exx.tmp | l -tr | p -lines
</pre>
Notice the flag <tt>-lines</tt> 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.
<!-- NEW -->
<h3>Labelled context-free grammars</h3>
The syntax trees returned by GF's parser in the previous examples
are not so nice to look at. The identifiers of form <tt>Mks</tt>
are <b>labels</b> of the EBNF rules. To see which label corresponds to
which rule, you can use the <tt>print_grammar = pg</tt> command
with the <tt>printer</tt> flag set to <tt>cf</tt> (which means context-free):
<pre>
> 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" ;
...
</pre>
A syntax tree such as
<pre>
Mks_4 (Mks_8 Mks_15 Mks_12)
this thick worm
</pre>
encodes the sequence of grammar rules used for building the
expression. If you look at this tree, you will notice that <tt>Mks_4</tt>
is the label of the rule prefixing <tt>this</tt> to a common noun,
<tt>Mks_15</tt> is the label of the adjective <tt>thick</tt>,
and so on.
<!-- NEW -->
<h4>The labelled context-free format</h4>
The <b>labelled context-free grammar</b> format permits user-defined
labels to each rule. GF recognizes files of this format by the suffix
<tt>.cf</tt>. Let us include the following rules in the file
<tt>paleolithic.cf</tt>.
<pre>
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" ;
</pre>
<!-- NEW -->
<h4>Using the labelled context-free format</h4>
The GF commands for the <tt>.cf</tt> format are
exactly the same as for the <tt>.ebnf</tt> 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 (<tt>empty = e</tt>), which removes the
old grammar from the GF shell state.
<pre>
> 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
</pre>
<!-- NEW -->
<h2>The GF grammar format</h2>
To see what there really is in GF's shell state when a grammar
has been imported, you can give the plain command
<tt>print_grammar = pg</tt>.
<pre>
> print_grammar
</pre>
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.
<p>
However, we will now start to show how GF's own notation gives you
much more expressive power than the <tt>.cf</tt> and <tt>.ebnf</tt>
formats. We will introduce the <tt>.gf</tt> format by presenting
one more way of defining the same grammar as in
<tt>paleolithic.cf</tt> and <tt>paleolithic.ebnf</tt>.
Then we will show how the full GF grammar format enables you
to do things that are not possible in the weaker formats.
<!-- NEW -->
<h3>Abstract and concrete syntax</h3>
A GF grammar consists of two main parts:
<ul>
<li> <b>abstract syntax</b>, defining what syntax trees there are
<li> <b>concrete syntax</b>, defining how trees are linearized into strings
</ul>
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
<pre>
PredVP. S ::= NP VP ;
</pre>
is interpreted as the following pair of rules:
<pre>
fun PredVP : NP -> VP -> S ;
lin PredVP x y = {s = x.s ++ y.s} ;
</pre>
The former rule, with the keyword <tt>fun</tt>, belongs to the abstract syntax.
It defines the <b>function</b>
<tt>PredVP</tt> which constructs syntax trees of form
(<tt>PredVP</tt> <i>x</i> <i>y</i>).
<p>
The latter rule, with the keyword <tt>lin</tt>, belongs to the concrete syntax.
It defines the <b>linearization function</b> for
syntax trees of form (<tt>PredVP</tt> <i>x</i> <i>y</i>).
<!-- NEW -->
<h4>Judgement forms</h4>
Rules in a GF grammar are called <b>judgements</b>, and the keywords
<tt>fun</tt> and <tt>lin</tt> are used for distinguishing between two
<b>judgement forms</b>. Here is a summary of the most important
judgement forms:
<ul>
<li> abstract syntax
<ul>
<li> cat C
<li> fun f : A
</ul>
<li> concrete syntax
<ul>
<li> lincat C = T
<li> lin f x ... y = t
</ul>
</ul>
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 <tt>paleolithic.cf</tt> is
expressed by using modules and judgements.
<!-- NEW -->
<h4>Module types</h4>
A GF grammar consists of <b>modules</b>,
into which judgements are grouped. The most important
module forms are
<ul>
<li> <tt>abstract</tt> A = M</tt>, abstract syntax A with judgements in
the module body M.
<li> <tt>concrete</tt> C <tt>of</tt> A = M</tt>, concrete syntax C of the
abstract syntax A, with judgements in the module body M.
</ul>
<!-- NEW -->
<h4>An abstract syntax example</h4>
Each nonterminal occurring in <tt>paleolithic.cf</tt> is
introduced by a <tt>cat</tt> judgement. Each
rule label is introduced by a <tt>fun</tt> judgement.
<pre>
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 ;
}
</pre>
Notice the use of shorthands permitting the sharing of
the keyword in subsequent judgements, and of the type
in subsequent <tt>fun</tt> judgements.
<!-- NEW -->
<h4>A concrete syntax example</h4>
Each category introduced in <tt>Paleolithic.gf</tt> is
given a <tt>lincat</tt> rule, and each
function is given a <tt>fun</tt> rule. Similar shorthands
apply as in <tt>abstract</tt> modules.
<pre>
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"} ;
}
</pre>
<!-- NEW -->
<h4>Modules and files</h4>
Module name + <tt>.gf</tt> = file name
<p>
Each module is compiled into a <tt>.gfc</tt> file.
<p>
Import <tt>PaleolithicEng.gf</tt> and try what happens
<pre>
</pre>
Nothing more than before, except that the GFC files
are generated.
<!-- NEW -->
<h4>An Italian concrete syntax</h4>
<pre>
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"} ;
}
</pre>
<!-- NEW -->
<h4>Using a multilingual grammar</h4>
Import without first emptying
<pre>
</pre>
Try generation now:
<pre>
</pre>
Translate by using a pipe:
<pre>
</pre>
Inspect the shell state (<tt>print_options = po</tt>):
<pre>
> print_options
main abstract : Paleolithic
main concrete : PaleolithicIta
all concretes : PaleolithicIta PaleolithicEng
</pre>
<!-- NEW -->
<h4>Extending the grammar</h4>
Neolithic: fire, wheel, think,...
</body>
</html>
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