-
-
-The Module System of GF
- -- -8/4/2005 - 10/4 - -
- -Aarne Ranta - -
-
-
-- -8/4/2005 - 10/4 - -
- -Aarne Ranta - -
+A GF grammar consists of a set of modules, which can be combined in different ways to build different grammars. -There are several different types of modules: -
abstract
+concrete
+resource
+interface
+instance
+incomplete concrete
+We will go through the module types in this order, which is also -their order of "importance" from the most frequently used to -the more esoteric/advanced ones. - -
- -This document is meant as an appendix to the GF tutorial, and -presupposes knowledge of GF judgements and expressions. It aims -just to tell what module system adds to the old functionality; -some information is repeated to give understanding on how the -module system relates to the already familiar uses of GF grammars. - - - -
+This document presupposes knowledge of GF judgements and expressions, which can +be gained from the GF tutorial. It aims +to give a systamatic description of the module system; +some tutorial information is repeated to make the document +self-contained. +
+ +
Any GF grammar that is used in an application
will probably contain at least one module
-of the abstract module type. Here is an example of
+of the abstract module type. Here is an example of
such a module, defining a fragment of propositional logic.
-
- abstract Logic = {
- cat Prop ;
- fun Conj : Prop -> Prop -> Prop ;
- fun Disj : Prop -> Prop -> Prop ;
- fun Impl : Prop -> Prop -> Prop ;
- fun Falsum : Prop ;
- }
-
-The name of this module is Logic.
-
-- -An abstract module defines an abstract syntax, which +
+
+ abstract Logic = {
+ cat Prop ;
+ fun Conj : Prop -> Prop -> Prop ;
+ fun Disj : Prop -> Prop -> Prop ;
+ fun Impl : Prop -> Prop -> Prop ;
+ fun Falsum : Prop ;
+ }
+
+
+The name of this module is Logic.
+
+An abstract module defines an abstract syntax, which
is a language-independent representation of a fragment of language.
-It consists of two kinds of judgements:
-
cat judgements telling what categories there are
(types of abstract syntax trees)
-fun judgements telling what functions there are
(to build abstract syntax trees)
-
+There can also be def and data judgements in an
abstract syntax.
-
-
-
+The GF grammar compiler expects to find the module Logic in a file named
+Logic.gf. When the compiler is run, it produces
+another file, named Logic.gfc. This file is in the
+format called canonical GF, which is the "machine language"
+of GF. Next time that the module Logic is needed in
+compiling a grammar, it can be read from the compiled (gfc)
+file instead of the source (gf) file, unless the source
has been changed after the compilation.
-
-
-
In order for a GF grammar to describe a concrete language, the abstract -syntax must be completed with a concrete syntax of it. -For this purpose, we use modules of type concrete: for instance, -
- concrete LogicEng of Logic = {
- lincat Prop = {s : Str} ;
- lin Conj a b = {s = a.s ++ "and" ++ b.s} ;
- lin Disj a b = {s = a.s ++ "or" ++ b.s} ;
- lin Impl a b = {s = "if" ++ a.s ++ "then" ++ b.s} ;
- lin Falsum = {s = ["we have a contradiction"]} ;
- }
-
-The module LogicEng is a concrete syntax of the
-abstract syntax Logic. The GF grammar compiler checks that
-the concrete is valid with respect to the abstract syntax of
+syntax must be completed with a concrete syntax of it.
+For this purpose, we use modules of type concrete: for instance,
+
+
+ concrete LogicEng of Logic = {
+ lincat Prop = {s : Str} ;
+ lin Conj a b = {s = a.s ++ "and" ++ b.s} ;
+ lin Disj a b = {s = a.s ++ "or" ++ b.s} ;
+ lin Impl a b = {s = "if" ++ a.s ++ "then" ++ b.s} ;
+ lin Falsum = {s = ["we have a contradiction"]} ;
+ }
+
+
+The module LogicEng is a concrete syntax of the
+abstract syntax Logic. The GF grammar compiler checks that
+the concrete is valid with respect to the abstract syntax of
which it is claimed to be. The validity requires that there has to be
-
lincat judgement for each cat judgement, telling what the
+ linearization types of categories are
+lin judgement for each fun judgement, telling what the
+ linearization functions corresponding to functions are
+
Validity also requires that the linearization functions defined by
-lin judgements are type-correct with respect to the
+lin judgements are type-correct with respect to the
linearization types of the arguments and value of the function.
-
-
- -There can also be lindef and printname judgements in a +
+
+There can also be lindef and printname judgements in a
concrete syntax.
-
-
-
+When a concrete module is successfully compiled, a gfc
+file is produced in the same way as for abstract modules. The
+pair of an abstract and a corresponding concrete module
+is a top-level grammar, which can be used in the GF system to
perform various tasks. The most fundamental tasks are
-
In the current grammar, infinitely many trees and strings are recognized, although no very interesting ones. For example, the tree -
- Impl (Disj Falsum Falsum) Falsum -+ +
+ Impl (Disj Falsum Falsum) Falsum ++
has the linearization -
- if we have a contradiction or we have a contradiction then we have a contradiction -+ +
+ if we have a contradiction or we have a contradiction then we have a contradiction ++
which in turn can be parsed uniquely as that tree. - - -
+When GF compiles the module LogicEng it also has to compile
+all modules that it depends on (in this case, just Logic).
The compilation process starts with dependency analysis to find
all these modules, recursively, starting from the explicitly imported one.
-The compiler then reads either gf or gfc files, in
+The compiler then reads either gf or gfc files, in
a dependency order. The decision on which files to read depends on
time stamps and dependencies in a natural way, so that all and only
those modules that have to be compiled are compiled. (This behaviour can
be changed with flags, see below.)
-
-
-
- i LogicEng.gf -+ + +
+To use a top-level grammar in the GF system, one uses the import
+command (short name i). For instance,
+
+ i LogicEng.gf ++
It is also possible to specify the imported grammar(s) on the command line when invoking GF: -
- gf LogicEng.gf --Various compilation flags can be added to both ways of compiling a module: -
+ gf LogicEng.gf ++
+Various compilation flags can be added to both ways of compiling a module: +
+-src forces compilation form source files
+-v gives more verbose information on compilation
+-s makes compilation silent (except if it fails with an error message)
+
+A complete list of flags can be obtained in GF by help i.
+
+Importing a grammar makes it visible in GF's internal state. To see
+what modules are available, use the command print_options (po).
+You can empty the state with the command empty (e); this is
needed if you want to read in grammars with a different abstract syntax
than the current one without exiting GF.
-
-
- +
+Grammar modules can reside in different directories. They can then be found -by means of a search path, which is a flag such as -
- -path=.:../prelude --given to the import command or the shell command invoking GF. +by means of a search path, which is a flag such as + +
+ -path=.:api/toplevel:prelude ++
+given to the import command or the shell command invoking GF.
(It can also be defined in the grammar file; see below.) The compiler
-writes every gfc file in the same directory as the corresponding
-gf file.
-
-
-
-Parsing and linearization can be performed with the parse
-(p) and linearize (l) commands, respectively.
+writes every gfc file in the same directory as the corresponding
+gf file.
+
+The path is relative to the working directory pwd, so that
+all directories listed are primarily interpreted as subdirectories of
+pwd. Secondarily, they are searched relative to the value of the
+environment variable GF_LIB_PATH, which is by default set to
+/usr/local/share/GF.
+
+Parsing and linearization can be performed with the parse
+(p) and linearize (l) commands, respectively.
For instance,
-
- > l Impl (Disj Falsum Falsum) Falsum - if we have a contradiction or we have a contradiction then we have a contradiction - - > p -cat=Prop "we have a contradiction" - Falsum --Notice that the parse command needs the parsing category + +
+ > l Impl (Disj Falsum Falsum) Falsum + if we have a contradiction or we have a contradiction then we have a contradiction + + > p -cat=Prop "we have a contradiction" + Falsum ++
+Notice that the parse command needs the parsing category
as a flag. This necessary since a grammar can have several
possible parsing categories ("entry points").
-
-
-
-
- concrete LogicFre of Logic = {
- lincat Prop = {s : Str} ;
- lin Conj a b = {s = a.s ++ "et" ++ b.s} ;
- lin Disj a b = {s = a.s ++ "ou" ++ b.s} ;
- lin Impl a b = {s = "si" ++ a.s ++ "alors" ++ b.s} ;
- lin Falsum = {s = ["nous avons une contradiction"]} ;
- }
-
- concrete LogicSymb of Logic = {
- lincat Prop = {s : Str} ;
- lin Conj a b = {s = "(" ++ a.s ++ "&" ++ b.s ++ ")"} ;
- lin Disj a b = {s = "(" ++ a.s ++ "v" ++ b.s ++ ")"} ;
- lin Impl a b = {s = "(" ++ a.s ++ "->" ++ b.s ++ ")"} ;
- lin Falsum = {s = "_|_"} ;
- }
-
-The four modules Logic, LogicEng, LogicFre, and
-LogicSymb together form a multilingual grammar, in which
+
+
+
+One abstract syntax can have several concrete syntaxes.
+Here are two new ones for Logic:
+
+ concrete LogicFre of Logic = {
+ lincat Prop = {s : Str} ;
+ lin Conj a b = {s = a.s ++ "et" ++ b.s} ;
+ lin Disj a b = {s = a.s ++ "ou" ++ b.s} ;
+ lin Impl a b = {s = "si" ++ a.s ++ "alors" ++ b.s} ;
+ lin Falsum = {s = ["nous avons une contradiction"]} ;
+ }
+
+ concrete LogicSymb of Logic = {
+ lincat Prop = {s : Str} ;
+ lin Conj a b = {s = "(" ++ a.s ++ "&" ++ b.s ++ ")"} ;
+ lin Disj a b = {s = "(" ++ a.s ++ "v" ++ b.s ++ ")"} ;
+ lin Impl a b = {s = "(" ++ a.s ++ "->" ++ b.s ++ ")"} ;
+ lin Falsum = {s = "_|_"} ;
+ }
+
+
+The four modules Logic, LogicEng, LogicFre, and
+LogicSymb together form a multilingual grammar, in which
it is possible to perform parsing and linearization with respect to any
of the concrete syntaxes. As a combination of parsing and linearization,
-one can also perform translation from one language to another.
-(By language we mean the set of expressions generated by one
+one can also perform translation from one language to another.
+(By language we mean the set of expressions generated by one
concrete syntax.)
-
-
-
Any combination of abstract syntax and corresponding concrete syntaxes
is thus a multilingual grammar. With many languages and other enrichments
(as described below), a multilingual grammar easily grows to the size of
tens of modules. The grammar developer, having finished her job, can
-package the result in a multilingual canonical grammar, a file
-with the suffix .gfcm. For instance, to compile the set of grammars
+package the result in a multilingual canonical grammar, a file
+with the suffix .gfcm. For instance, to compile the set of grammars
described by now, the following sequence of GF commands can be used:
-
- i LogicEng.gf - i LogicFre.gf - i LogicSymb.gf - pm | wf logic.gfcm --The "end user" of the grammar only needs the file logic.gfcm to + +
+ i LogicEng.gf + i LogicFre.gf + i LogicSymb.gf + pm | wf logic.gfcm ++
+The "end user" of the grammar only needs the file logic.gfcm to
access all the functionality of the multilingual grammar. It can be
-imported in the GF system in the same way as .gf files. But
-it can also be used in the Embedded Java Interpreter for GF to
-build Java programs of which the multilingual grammar functionalities
+imported in the GF system in the same way as .gf files. But
+it can also be used in the
+Embedded Java Interpreter for GF
+to build Java programs of which the multilingual grammar functionalities
(linearization, parsing, translation) form a part.
-
-
- +
+In a multilingual grammar, the concrete syntax module names work as names of languages that can be selected for linearization and parsing: -
- > l -lang=LogicFre Impl Falsum Falsum - si nous avons une contradiction alors nous avons une contradiction - - > l -lang=LogicSymb Impl Falsum Falsum - ( _|_ -> _|_ ) - - > p -cat=Prop -lang=LogicSymb "( _|_ & _|_ )" - Conj Falsum Falsum --The option -multi gives linearization to all languages: -
- > l -multi Impl Falsum Falsum - if we have a contradiction then we have a contradiction - si nous avons une contradiction alors nous avons une contradiction - ( _|_ -> _|_ ) --Translation can be obtained by using a pipe from a parser + +
+ > l -lang=LogicFre Impl Falsum Falsum + si nous avons une contradiction alors nous avons une contradiction + + > l -lang=LogicSymb Impl Falsum Falsum + ( _|_ -> _|_ ) + + > p -cat=Prop -lang=LogicSymb "( _|_ & _|_ )" + Conj Falsum Falsum ++
+The option -multi gives linearization to all languages:
+
+ > l -multi Impl Falsum Falsum + if we have a contradiction then we have a contradiction + si nous avons une contradiction alors nous avons une contradiction + ( _|_ -> _|_ ) ++
+Translation can be obtained by using a pipe from a parser to a linearizer: -
- > p -cat=Prop -lang=LogicSymb "( _|_ & _|_ )" | l -lang=LogicEng - if we have a contradiction then we have a contradiction -- - -
+ > p -cat=Prop -lang=LogicSymb "( _|_ & _|_ )" | l -lang=LogicEng + if we have a contradiction then we have a contradiction ++ + +
+The concrete modules shown above would look much nicer if
we used the main idea of functional programming: avoid repetitive
-code by using functions that capture repeated patterns of
+code by using functions that capture repeated patterns of
expressions. A collection of such functions can be a valuable
-resource for a programmer, reusable in many different
-top-level grammars. Thus we introduce the resource
+resource for a programmer, reusable in many different
+top-level grammars. Thus we introduce the resource
module type, with the first example
-
- resource Util = {
- oper SS : Type = {s : Str} ;
- oper ss : Str -> SS = \s -> {s = s} ;
- oper paren : Str -> Str = \s -> "(" ++ s ++ ")" ;
- oper infix : Str -> SS -> SS -> SS = \h,x,y ->
- ss (x.s ++ h ++ y.s) ;
- oper infixp : Str -> SS -> SS -> SS = \h,x,y ->
- ss (paren (infix h x y)) ;
- }
-
-Modules of resource type have two forms of judgement:
-
+ resource Util = {
+ oper SS : Type = {s : Str} ;
+ oper ss : Str -> SS = \s -> {s = s} ;
+ oper paren : Str -> Str = \s -> "(" ++ s ++ ")" ;
+ oper infix : Str -> SS -> SS -> SS = \h,x,y ->
+ ss (x.s ++ h ++ y.s) ;
+ oper infixp : Str -> SS -> SS -> SS = \h,x,y ->
+ ss (paren (infix h x y)) ;
+ }
+
+
+Modules of resource type have two forms of judgement:
+
oper defining auxiliary operations
+param defining parameter types
+
+A resource can be used in a concrete (or another
+resource) by opening it. This means that
all operations (and parameter types) defined in the resource
module become usable in module that opens it. For instance,
-we can rewrite the module LogicSymb much more concisely:
-
- concrete LogicSymb of Logic = open Util in {
- lincat Prop = SS ;
- lin Conj = infixp "&" ;
- lin Disj = infixp "v" ;
- lin Impl = infixp "->" ;
- lin Falsum = ss "_|_" ;
- }
-
-What happens when this variant of LogicSymb is
-compiled is that the oper-defined constants
-of Util are inlined in the
-right-hand-sides of the judgements of LogicSymb,
-and these expressions are partially evaluated, i.e.
-computed as far as possible. The generated gfc file
+we can rewrite the module LogicSymb much more concisely:
+
+
+ concrete LogicSymb of Logic = open Util in {
+ lincat Prop = SS ;
+ lin Conj = infixp "&" ;
+ lin Disj = infixp "v" ;
+ lin Impl = infixp "->" ;
+ lin Falsum = ss "_|_" ;
+ }
+
+
+What happens when this variant of LogicSymb is
+compiled is that the oper-defined constants
+of Util are inlined in the
+right-hand-sides of the judgements of LogicSymb,
+and these expressions are partially evaluated, i.e.
+computed as far as possible. The generated gfc file
will look just like the file generated for the first version
-of LogicSymb - at least, it will do the same job.
-
-
-
-Several resource modules can be opened
+of LogicSymb - at least, it will do the same job.
+
+Several resource modules can be opened
at the same time. If the modules contain same names, the
-conflict can be resolved by qualified opening and
+conflict can be resolved by qualified opening and
reference. For instance,
-
- concrete LogicSymb of Logic = open Util, Prelude in { ...
- } ;
-
-(where Prelude is a standard library of GF) brings
-into scope two definitions of the constant SS.
+
+
+ concrete LogicSymb of Logic = open Util, Prelude in { ...
+ } ;
+
+
+(where Prelude is a standard library of GF) brings
+into scope two definitions of the constant SS.
To specify which one is used, you can write
-Util.SS or Prelude.SS instead of just SS.
+Util.SS or Prelude.SS instead of just SS.
You can also introduce abbreviations to avoid long qualifiers, e.g.
-
- concrete LogicSymb of Logic = open (U=Util), (P=Prelude) in { ...
- } ;
-
-which means that you can write U.SS and P.SS.
-
-
-
+ concrete LogicSymb of Logic = open (U=Util), (P=Prelude) in { ...
+ } ;
+
+
+which means that you can write U.SS and P.SS.
+
+Judgements of param and oper forms may also be used
+in concrete modules, and they are then considered local
+to those modules, i.e. they are not exported.
+
+The compilation of a resource module differs
+from the compilation of abstract and
+concrete modules because oper operations
+do not in general have values in gfc. A gfc
+file is generated, but it contains only
+param judgements (also recall that opers
are inlined in their top-level use sites, so it is not
necessary to save them in the compiled grammar).
However, since computing the operations over and over
again can be time comsuming, and since type checking
-resource modules also takes time, a third kind
-of file is generated for resource modules: a .gfr
+resource modules also takes time, a third kind
+of file is generated for resource modules: a .gfr
file. This file is written in the GF source code notation,
-but it is type checked and type annotated, and opers
+but it is type checked and type annotated, and opers
are computed as far as possible.
-
-
- -If you look at any gfc or gfr file generated +
+
+If you look at any gfc or gfr file generated
by the GF compiler, you see that all names have been replaced by
their qualified variants. This is an important first step (after parsing)
the compiler does. As for the commands in the GF shell, some output
qualified names and some not. The difference does not always result
from firm principles.
-
-
-
+The typical use is through open in a
+concrete module, which means that
+resource modules are not imported on their own.
However, in the developing and testing phase of grammars, it
-can be useful to evaluate opers with different
+can be useful to evaluate opers with different
arguments. To prevent them from being thrown away after inlining, the
--retain option can be used:
-
- > i -retain Util.gf --The command compute_concrete (cc) +
-retain option can be used:
+
++ > i -retain Util.gf ++
+The command compute_concrete (cc)
can now be used for evaluating expressions that may contain
-operations defined in Util:
-
- > cc ss (paren "foo")
- {s = "(" ++ "foo" ++ ")"}
-
-To find out what opers are available for a given type,
-the command show_operations (so) can be used:
-- > so SS - Util.ss : Str -> SS ; - Util.infix : Str -> SS -> SS -> SS ; - Util.infixp : Str -> SS -> SS -> SS ; -- - -
Util:
+
+
+ > cc ss (paren "foo")
+ {s = "(" ++ "foo" ++ ")"}
+
+
+To find out what opers are available for a given type,
+the command show_operations (so) can be used:
+
+ > so SS + Util.ss : Str -> SS ; + Util.infix : Str -> SS -> SS -> SS ; + Util.infixp : Str -> SS -> SS -> SS ; ++ + +
The most characteristic modularity of GF lies in the division of
-grammars into abstract, concrete, and
-resource modules. This permits writing multilingual
+grammars into abstract, concrete, and
+resource modules. This permits writing multilingual
grammar and sharing the maximum of code between different
languages.
-
-
- -In addition to this special kind of modularity, GF provides inheritance, +
++In addition to this special kind of modularity, GF provides inheritance, which is familiar from other programming languages (in particular, object-oriented ones). Inheritance means that a module inherits all -judgements from another module; we also say that it extends +judgements from another module; we also say that it extends the other module. Inheritance is useful to divide big grammars into smaller units, and also to reuse the same units in different bigger grammars. - -
- +
+The first example of inheritance is for abstract syntax. Let us -extend the module Logic to Arithmetic: -
- abstract Arithmetic = Logic ** {
- cat Nat ;
- fun Even : Nat -> Prop ;
- fun Odd : Nat -> Prop ;
- fun Zero : Nat ;
- fun Succ : Nat -> Nat ;
- }
-
+extend the module Logic to Arithmetic:
+
+
+ abstract Arithmetic = Logic ** {
+ cat Nat ;
+ fun Even : Nat -> Prop ;
+ fun Odd : Nat -> Prop ;
+ fun Zero : Nat ;
+ fun Succ : Nat -> Nat ;
+ }
+
+In parallel with the extension of the abstract syntax -Logic to Arithmetic, we can extend -the concrete syntax LogicEng to ArithmeticEng: -
- concrete ArithmeticEng of Arithmetic = LogicEng ** open Util in {
- lincat Nat = SS ;
- lin Even x = ss (x.s ++ "is" ++ "even") ;
- lin Odd x = ss (x.s ++ "is" ++ "odd") ;
- lin Zero = ss "zero" ;
- lin Succ x = ss ("the" ++ "successor" ++ "of" ++ x.s) ;
- }
-
-Another extension of Logic is Geometry,
-
- abstract Geometry = Logic ** {
- cat Point ;
- cat Line ;
- fun Incident : Point -> Line -> Prop ;
- }
-
+Logic to Arithmetic, we can extend
+the concrete syntax LogicEng to ArithmeticEng:
+
+
+ concrete ArithmeticEng of Arithmetic = LogicEng ** open Util in {
+ lincat Nat = SS ;
+ lin Even x = ss (x.s ++ "is" ++ "even") ;
+ lin Odd x = ss (x.s ++ "is" ++ "odd") ;
+ lin Zero = ss "zero" ;
+ lin Succ x = ss ("the" ++ "successor" ++ "of" ++ x.s) ;
+ }
+
+
+Another extension of Logic is Geometry,
+
+ abstract Geometry = Logic ** {
+ cat Point ;
+ cat Line ;
+ fun Incident : Point -> Line -> Prop ;
+ }
+
+The corresponding concrete syntax is left as exercise. - -
- -Inheritance can be multiple, which means that a module +
+ ++Inheritance can be multiple, which means that a module may extend many modules at the same time. Suppose, for instance, that we want to build a module for mathematics covering both arithmetic and geometry, and the underlying logic. We then write -
- abstract Mathematics = Arithmetic, Geometry ** {
- } ;
-
+
+
+ abstract Mathematics = Arithmetic, Geometry ** {
+ } ;
+
+We could of course add some new judgements in this module, but -it is not necessary to do so. - -
- -The module Mathematics also shows that it is possibe +it is not necessary to do so. If no new judgements are added, the +module body can be omitted: +
++ abstract Mathematics = Arithmetic, Geometry ; ++ +
+The module Mathematics shows that it is possibe
to extend a module already built by extension. The correctness
criterion for extensions is that the same name
-(cat, fun, oper, or param)
+(cat, fun, oper, or param)
may not be defined twice in the resulting union of names.
-That the names defined in Logic are "inherited twice"
-by Mathematics (via both Arithmetic and
-Geometry) is no violation of this rule; the usual
+That the names defined in Logic are "inherited twice"
+by Mathematics (via both Arithmetic and
+Geometry) is no violation of this rule; the usual
problems of multiple inheritance do not arise, since
the definitions of inherited constants cannot be changed.
-
-
-
+Inheritance can be restricted, which means that only some of +the constants are inherited. There are two dual notations for this: +
++ A [f,g] ++
+meaning that only f and g are inherited from A, and
+
+ A-[f,g] ++
+meaning that everything except f is g are inherited from A.
+
+Constants that are not inherited may be redefined in the inheriting module. +
+ +Inherited judgements are not copied into the inheriting modules. -Instead, an indirection is created for each inherited name, -as can be seen by looking into the generated gfc (and -gfr) files. Thus for instance the names -
- Mathematics.Prop Arithmetic.Prop Geometry.Prop Logic.Prop -+Instead, an indirection is created for each inherited name, +as can be seen by looking into the generated
gfc (and
+gfr) files. Thus for instance the names
+
++ Mathematics.Prop Arithmetic.Prop Geometry.Prop Logic.Prop ++
all refer to the same category, declared in the module -Logic. - - - -
Logic.
+
+
+
+The command visualize_graph (vg) shows the
dependency graph in the current GF shell state. The graph can
also be saved in a file and used e.g. in documentation, by the
-command print_multi -graph (pm -graph).
-
-
-
print_multi -graph (pm -graph).
+
+
+The vg command uses the free software packages Graphviz (commad dot)
+and Ghostscript (command gv).
+
Top-level grammars have a straightforward translation to
-resource modules. The translation concerns
+resource modules. The translation concerns
pairs of abstract-concrete judgements:
-
- cat C ; ===> oper C : Type = T ; - lincat C = T ; - - fun f : A ; ===> oper f : A = t ; - lin f = t ; --Due to this translation, a concrete module -can be opened in the same way as a -resource module; the translation is done + +
+ cat C ; ===> oper C : Type = T ; + lincat C = T ; + + fun f : A ; ===> oper f : A = t ; + lin f = t ; ++
+Due to this translation, a concrete module
+can be opened in the same way as a
+resource module; the translation is done
on the fly (it is computationally very cheap).
-
-
- +
+Modular grammar engineering often means that some grammarians focus on the semantics of the domain whereas others take care of linguistic details. Thus a typical reuse opens a -linguistically oriented resource grammar, -
- abstract Resource = {
- cat S ; NP ; A ;
- fun PredA : NP -> A -> S ;
- }
- concrete ResourceEng of Resource = {
- lincat S = ... ;
- lin PredA = ... ;
- }
-
-The application grammar, instead of giving linearizations
+linguistically oriented resource grammar,
+
+
+ abstract Resource = {
+ cat S ; NP ; A ;
+ fun PredA : NP -> A -> S ;
+ }
+ concrete ResourceEng of Resource = {
+ lincat S = ... ;
+ lin PredA = ... ;
+ }
+
++The application grammar, instead of giving linearizations explicitly, just reduces them to categories and functions in the resource grammar: -
- concrete ArithmeticEng of Arithmetic = LogicEng ** open ResourceEng in {
- lincat Nat = NP ;
- lin Even x = PredA x (regA "even") ;
- }
-
+
+
+ concrete ArithmeticEng of Arithmetic = LogicEng ** open ResourceEng in {
+ lincat Nat = NP ;
+ lin Even x = PredA x (regA "even") ;
+ }
+
+If the resource grammar is only capable of generating grammatically correct expressions, then the grammaticality of the application grammar is also guaranteed: the type checker of GF is used as @@ -588,382 +727,457 @@ grammar checker. To guarantee distinctions between categories that have the same linearization type, the actual translation used in GF adds to every linearization type and linearization -a lock field, -
- cat C ; ===> oper C : Type = T ** {lock_C : {}} ;
- lincat C = T ;
-
- fun f : C_1 ... C_n -> C ; ===> oper f : C_1 ... C_n -> C = \x_1,...,x_n ->
- lin f = t ; t x_1 ... x_n ** {lock_C = <>};
-
+a lock field,
+
+
+ cat C ; ===> oper C : Type = T ** {lock_C : {}} ;
+ lincat C = T ;
+
+ fun f : C_1 ... C_n -> C ; ===> oper f : C_1 ... C_n -> C = \x_1,...,x_n ->
+ lin f = t ; t x_1 ... x_n ** {lock_C = <>};
+
+
(Notice that the latter translation is type-correct because of
-record subtyping, which means that t can ignore the
+record subtyping, which means that t can ignore the
lock fields of its arguments.) An application grammarian who
only uses resource grammar categories and functions never
needs to write these lock fields herself. Having to do so
serves as a warning that the grammaticality guarantee given
by the resource grammar no longer holds.
-
-
-
+Note. The lock field mechanism is experimental, and may be changed +to a stronger abstraction mechnism in the future. This may result in +hand-written lock fields ceasing to work. +
+ +
+One difference between top-level grammars and resource
modules is that the former systematically separete the
declarations of categories and functions from their definitions.
-In the reuse translation creating and oper judgement,
-the declaration coming from the abstract module is put
-together with the definition coming from the concrete
+In the reuse translation creating and oper judgement,
+the declaration coming from the abstract module is put
+together with the definition coming from the concrete
module.
-
-
- +
+
However, the separation of declarations and definitions is so
useful a notion that GF also has specific modules types that
-resource modules into two parts. In this splitting,
-an interface module corresponds to an abstract syntax,
+resource modules into two parts. In this splitting,
+an interface module corresponds to an abstract syntax,
in giving the declarations of operations (and parameter types).
For instance, a generic markup interface would look as follows:
-
- interface Markup = open Util in {
- oper Boldface : Str -> Str ;
- oper Heading : Str -> Str ;
- oper markupSS : (Str -> Str) -> SS -> SS = \f,r ->
- ss (f r.s) ;
- }
-
-The definitions of the constants declared in an interface
-are given in an instance module (which is always of
-an interface, in the same way as a concrete is always
-of an abstract). The following instances
+
+
+ interface Markup = open Util in {
+ oper Boldface : Str -> Str ;
+ oper Heading : Str -> Str ;
+ oper markupSS : (Str -> Str) -> SS -> SS = \f,r ->
+ ss (f r.s) ;
+ }
+
+
+The definitions of the constants declared in an interface
+are given in an instance module (which is always of
+an interface, in the same way as a concrete is always
+of an abstract). The following instances
define markup in HTML and latex.
-
- instance MarkupHTML of Markup = open Util in {
- oper Boldface s = "<b>" ++ s ++ "</b>" ;
- oper Heading s = "<h2>" ++ s ++ "</h2>" ;
- }
-
- instance MarkupLatex of Markup = open Util in {
- oper Boldface s = "\\textbf{" ++ s ++ "}" ;
- oper Heading s = "\\section{" ++ s ++ "}" ;
- }
-
-Notice that both interfaces and instances may
-open resources (and also reused top-level grammars).
-An interface may moreover define some of the operations it
+
+
+ instance MarkupHTML of Markup = open Util in {
+ oper Boldface s = "<b>" ++ s ++ "</b>" ;
+ oper Heading s = "<h2>" ++ s ++ "</h2>" ;
+ }
+
+ instance MarkupLatex of Markup = open Util in {
+ oper Boldface s = "\\textbf{" ++ s ++ "}" ;
+ oper Heading s = "\\section{" ++ s ++ "}" ;
+ }
+
+
+Notice that both interfaces and instances may
+open resources (and also reused top-level grammars).
+An interface may moreover define some of the operations it
declares; these definitions are inherited by all instances and cannot
be changed in them. Inheritance by module extension
is possible, as always, between modules of the same type.
-
-
-
+An interface or an instance
+can be opened in
+a concrete using the same syntax as when opening
+a resource. For an instance, the semantics
is the same as when opening the definitions together with
-the type signatures - one can think of an interface
-and an instance of it together forming an ordinary
-resource. Opening an interface, however,
+the type signatures - one can think of an interface
+and an instance of it together forming an ordinary
+resource. Opening an interface, however,
is different: functions that are only declared without
having a definition cannot be compiled (inlined); neither
can functions whose definitions depend on undefined functions.
-
-
- -A module that opens an interface is therefore -incomplete, and has to be completed with an -instance of the interface to become complete. To make +
+
+A module that opens an interface is therefore
+incomplete, and has to be completed with an
+instance of the interface to become complete. To make
this situation clear, GF requires any module that opens an
-interface to be marked as incomplete. Thus
+interface to be marked as incomplete. Thus
the module
-
- incomplete concrete DocMarkup of Doc = open Markup in {
- ...
- }
-
-uses the interface Markup to place markup in
+
+
+ incomplete concrete DocMarkup of Doc = open Markup in {
+ ...
+ }
+
+
+uses the interface Markup to place markup in
chosen places in its linearization rules, but the
implementation of markup - whether in HTML or in LaTeX - is
left unspecified. This is a powerful way of sharing
the code of a whole module with just differences in
the definitions of some constants.
-
-
- -Another terminology for incomplete modules is -parametrized modules or functors. -The interface gives the list of parameters +
+
+Another terminology for incomplete modules is
+parametrized modules or functors.
+The interface gives the list of parameters
that the functor depends on.
-
-
-
+To complete an incomplete module, each inteface
+that it opens has to be provided an instance. The following
syntax is used for this:
-
- concrete DocHTML of Doc = DocMarkup with (Markup = MarkupHTML) ; --Instantiation of Markup with MarkupLatex is + +
+ concrete DocHTML of Doc = DocMarkup with (Markup = MarkupHTML) ; ++
+Instantiation of Markup with MarkupLatex is
another one-liner.
-
-
- +
+If more interfaces than one are instantiated, a comma-separated list of equations in parentheses is used, e.g. -
- concrete RulesIta = CategoriesIta ** RulesRomance with - (TypesRomance = TypesIta), (SyntaxRomance = SyntaxIta) ; --(an example from the GF resource grammar library, where languages for -Romance languages share two interfaces). -All interfaces that are opened in the completed model + +
+ concrete MusicIta = MusicI with + (Syntax = SyntaxIta), (LexMusic = LexMusicIta) ; ++
+This example shows a common design pattern for building applications:
+the concrete syntax is a functor on the generic resource grammar library
+interface Syntax and a domain-specific lexicon interface, here
+LexMusic.
+
+All interfaces that are opened in the completed model
must be completed.
-
-
- -Notice that the completion of an incomplete module +
+
+Notice that the completion of an incomplete module
may at the same time extend modules of the same type (which need
-not be completions). But it cannot add new judgements.
-
-
-
+ concrete MusicIta = MusicI - [f] with
+ (Syntax = SyntaxIta), (LexMusic = LexMusicIta) ** {
+
+ lin f = ...
+
+ } ;
+
+
+
+
Interfaces, instances, and parametric modules are purely a
front-end feature of GF: these module types do not exist in
-the gfc and gfr formats. The compiler has
+the gfc and gfr formats. The compiler has
nevertheless to keep track of their dependencies and modification
times. Here is a summary of how they are compiled:
-
interface is compiled into a resource with an empty body
+instance is compiled into a resource in union with its
+ interface
+incomplete module (concrete or resource) is compiled
into a module of the same type with an empty body
-concrete or resource) is compiled
into a module of the same type by compiling its functor so that, instead of
- each interface, its given instance is used
-interface, its given instance is used
+This means that some generated code is duplicated, because those operations that -do have complete definitions in an interface are copied to each of -the instances. - - -
interface are copied to each of
+the instances.
+
+
+Syntax: -
-abstract A = (A1,...,An **)? -{J1 ; ... ; Jm ; } - -
- +
+
+abstract A = (A1,...,An **)?
+{J1 ; ... ; Jm ; }
+
where -
-[f,..,g]
+ or Ai[f,..,g]
+cat, fun, def, data
+Semantic conditions: -
Syntax: -
-incomplete? concrete C of A = -(C1,...,Cn **)? -(open O1,...,Ok in)? -{J1 ; ... ; Jm ; } - -
- +
+
+incomplete? concrete C of A =
+(C1,...,Cn **)?
+(open O1,...,Ok in)?
+{J1 ; ... ; Jm ; }
+
where -
-[f,..,g]
+(Q=R)
+ - -If the modifier incomplete appears, then any R in -an open specification may also be an interface. - -
+
+ where R is a resource, instance, or concrete, and Q is any identifier +
+lincat, lin, lindef, printname; also the forms oper, param are
+ allowed, but they cannot be inherited.
+
+If the modifier incomplete appears, then any R in
+an open specification may also be an interface or an abstract.
+
Semantic conditions: -
cat judgement in A
must have a corresponding, unique
- lincat judgement in C
-lincat judgement in C
+fun judgement in A
must have a corresponding, unique
- lin judgement in C
-lin judgement in C
+Syntax: -
-resource R = -(R1,...,Rn **)? -(open O1,...,Ok in)? -{J1 ; ... ; Jm ; } - -
+
+
+resource R =
+(R1,...,Rn **)?
+(open O1,...,Ok in)?
+{J1 ; ... ; Jm ; }
+
where -
-[f,..,g]
+(Q=R)
+ +
+ where P is a resource, instance, or concrete, and Q is any identifier +
+oper, param
+Semantic conditions: -
Syntax: -
-interface R = -(R1,...,Rn **)? -(open O1,...,Ok in)? -{J1 ; ... ; Jm ; } - -
+
+
+interface R =
+(R1,...,Rn **)?
+(open O1,...,Ok in)?
+{J1 ; ... ; Jm ; }
+
where -
-[f,..,g]
+(Q=R)
+ +
+ where P is a resource, instance, or concrete, and Q is any identifier +
+oper, param
+Semantic conditions: -
Syntax: -
-instance R of I = -(R1,...,Rn **)? -(open O1,...,Ok in)? -{J1 ; ... ; Jm ; } - -
+
+
+instance R of I =
+(R1,...,Rn **)?
+(open O1,...,Ok in)?
+{J1 ; ... ; Jm ; }
+
where -
-[f,..,g]
+
+(Q=R)
+ +
+ where P is a resource, instance, or concrete, and Q is any identifier +
+oper, param
+Semantic conditions: -
Syntax: -
-concrete C of A = -(C1,...,Cn **)? +
+
+concrete C of A =
+(C1,...,Cn **)?
B
-with
-(I1 =J1), ...
-, (Im =Jm) ;
-
-
-
+with
+(I1 =J1), ...
+, (Ip =Jp)
+(-? [c1,...,cq ])?
+(**?
+(open O1,...,Ok in)?
+{J1 ; ... ; Jm ; })? ;
+
where -
-[f,..,g]
+(Q=R)
+ + where R is a resource, instance, or concrete, and Q is any identifier +
+lincat, lin, lindef, printname; also the forms oper, param are
+ allowed, but they cannot be inherited.
+
+
+
++ +8/4/2005 - 10/4 + +
+ +Aarne Ranta + +
+ +This document is meant as an appendix to the GF tutorial, and +presupposes knowledge of GF judgements and expressions. It aims +just to tell what module system adds to the old functionality; +some information is repeated to give understanding on how the +module system relates to the already familiar uses of GF grammars. + + + +
+ abstract Logic = {
+ cat Prop ;
+ fun Conj : Prop -> Prop -> Prop ;
+ fun Disj : Prop -> Prop -> Prop ;
+ fun Impl : Prop -> Prop -> Prop ;
+ fun Falsum : Prop ;
+ }
+
+The name of this module is Logic.
+
++ +An abstract module defines an abstract syntax, which +is a language-independent representation of a fragment of language. +It consists of two kinds of judgements: +
+ concrete LogicEng of Logic = {
+ lincat Prop = {s : Str} ;
+ lin Conj a b = {s = a.s ++ "and" ++ b.s} ;
+ lin Disj a b = {s = a.s ++ "or" ++ b.s} ;
+ lin Impl a b = {s = "if" ++ a.s ++ "then" ++ b.s} ;
+ lin Falsum = {s = ["we have a contradiction"]} ;
+ }
+
+The module LogicEng is a concrete syntax of the
+abstract syntax Logic. The GF grammar compiler checks that
+the concrete is valid with respect to the abstract syntax of
+which it is claimed to be. The validity requires that there has to be
++ +There can also be lindef and printname judgements in a +concrete syntax. + + +
+ Impl (Disj Falsum Falsum) Falsum ++has the linearization +
+ if we have a contradiction or we have a contradiction then we have a contradiction ++which in turn can be parsed uniquely as that tree. + + +
+ i LogicEng.gf ++It is also possible to specify the imported grammar(s) on the command +line when invoking GF: +
+ gf LogicEng.gf ++Various compilation flags can be added to both ways of compiling a module: +
+ +Grammar modules can reside in different directories. They can then be found +by means of a search path, which is a flag such as +
+ -path=.:../prelude ++given to the import command or the shell command invoking GF. +(It can also be defined in the grammar file; see below.) The compiler +writes every gfc file in the same directory as the corresponding +gf file. + +
+ +Parsing and linearization can be performed with the parse +(p) and linearize (l) commands, respectively. +For instance, +
+ > l Impl (Disj Falsum Falsum) Falsum + if we have a contradiction or we have a contradiction then we have a contradiction + + > p -cat=Prop "we have a contradiction" + Falsum ++Notice that the parse command needs the parsing category +as a flag. This necessary since a grammar can have several +possible parsing categories ("entry points"). + + + +
+ concrete LogicFre of Logic = {
+ lincat Prop = {s : Str} ;
+ lin Conj a b = {s = a.s ++ "et" ++ b.s} ;
+ lin Disj a b = {s = a.s ++ "ou" ++ b.s} ;
+ lin Impl a b = {s = "si" ++ a.s ++ "alors" ++ b.s} ;
+ lin Falsum = {s = ["nous avons une contradiction"]} ;
+ }
+
+ concrete LogicSymb of Logic = {
+ lincat Prop = {s : Str} ;
+ lin Conj a b = {s = "(" ++ a.s ++ "&" ++ b.s ++ ")"} ;
+ lin Disj a b = {s = "(" ++ a.s ++ "v" ++ b.s ++ ")"} ;
+ lin Impl a b = {s = "(" ++ a.s ++ "->" ++ b.s ++ ")"} ;
+ lin Falsum = {s = "_|_"} ;
+ }
+
+The four modules Logic, LogicEng, LogicFre, and
+LogicSymb together form a multilingual grammar, in which
+it is possible to perform parsing and linearization with respect to any
+of the concrete syntaxes. As a combination of parsing and linearization,
+one can also perform translation from one language to another.
+(By language we mean the set of expressions generated by one
+concrete syntax.)
+
+
++ i LogicEng.gf + i LogicFre.gf + i LogicSymb.gf + pm | wf logic.gfcm ++The "end user" of the grammar only needs the file logic.gfcm to +access all the functionality of the multilingual grammar. It can be +imported in the GF system in the same way as .gf files. But +it can also be used in the Embedded Java Interpreter for GF to +build Java programs of which the multilingual grammar functionalities +(linearization, parsing, translation) form a part. + +
+ +In a multilingual grammar, the concrete syntax module names work as +names of languages that can be selected for linearization and parsing: +
+ > l -lang=LogicFre Impl Falsum Falsum + si nous avons une contradiction alors nous avons une contradiction + + > l -lang=LogicSymb Impl Falsum Falsum + ( _|_ -> _|_ ) + + > p -cat=Prop -lang=LogicSymb "( _|_ & _|_ )" + Conj Falsum Falsum ++The option -multi gives linearization to all languages: +
+ > l -multi Impl Falsum Falsum + if we have a contradiction then we have a contradiction + si nous avons une contradiction alors nous avons une contradiction + ( _|_ -> _|_ ) ++Translation can be obtained by using a pipe from a parser +to a linearizer: +
+ > p -cat=Prop -lang=LogicSymb "( _|_ & _|_ )" | l -lang=LogicEng + if we have a contradiction then we have a contradiction ++ + +
+ resource Util = {
+ oper SS : Type = {s : Str} ;
+ oper ss : Str -> SS = \s -> {s = s} ;
+ oper paren : Str -> Str = \s -> "(" ++ s ++ ")" ;
+ oper infix : Str -> SS -> SS -> SS = \h,x,y ->
+ ss (x.s ++ h ++ y.s) ;
+ oper infixp : Str -> SS -> SS -> SS = \h,x,y ->
+ ss (paren (infix h x y)) ;
+ }
+
+Modules of resource type have two forms of judgement:
+
+ concrete LogicSymb of Logic = open Util in {
+ lincat Prop = SS ;
+ lin Conj = infixp "&" ;
+ lin Disj = infixp "v" ;
+ lin Impl = infixp "->" ;
+ lin Falsum = ss "_|_" ;
+ }
+
+What happens when this variant of LogicSymb is
+compiled is that the oper-defined constants
+of Util are inlined in the
+right-hand-sides of the judgements of LogicSymb,
+and these expressions are partially evaluated, i.e.
+computed as far as possible. The generated gfc file
+will look just like the file generated for the first version
+of LogicSymb - at least, it will do the same job.
+
++ +Several resource modules can be opened +at the same time. If the modules contain same names, the +conflict can be resolved by qualified opening and +reference. For instance, +
+ concrete LogicSymb of Logic = open Util, Prelude in { ...
+ } ;
+
+(where Prelude is a standard library of GF) brings
+into scope two definitions of the constant SS.
+To specify which one is used, you can write
+Util.SS or Prelude.SS instead of just SS.
+You can also introduce abbreviations to avoid long qualifiers, e.g.
+
+ concrete LogicSymb of Logic = open (U=Util), (P=Prelude) in { ...
+ } ;
+
+which means that you can write U.SS and P.SS.
+
+
++ +If you look at any gfc or gfr file generated +by the GF compiler, you see that all names have been replaced by +their qualified variants. This is an important first step (after parsing) +the compiler does. As for the commands in the GF shell, some output +qualified names and some not. The difference does not always result +from firm principles. + + +
+ > i -retain Util.gf ++The command compute_concrete (cc) +can now be used for evaluating expressions that may contain +operations defined in Util: +
+ > cc ss (paren "foo")
+ {s = "(" ++ "foo" ++ ")"}
+
+To find out what opers are available for a given type,
+the command show_operations (so) can be used:
++ > so SS + Util.ss : Str -> SS ; + Util.infix : Str -> SS -> SS -> SS ; + Util.infixp : Str -> SS -> SS -> SS ; ++ + +
+ +In addition to this special kind of modularity, GF provides inheritance, +which is familiar from other programming languages (in particular, +object-oriented ones). Inheritance means that a module inherits all +judgements from another module; we also say that it extends +the other module. Inheritance is useful to divide big grammars into +smaller units, and also to reuse the same units in different bigger +grammars. + +
+ +The first example of inheritance is for abstract syntax. Let us +extend the module Logic to Arithmetic: +
+ abstract Arithmetic = Logic ** {
+ cat Nat ;
+ fun Even : Nat -> Prop ;
+ fun Odd : Nat -> Prop ;
+ fun Zero : Nat ;
+ fun Succ : Nat -> Nat ;
+ }
+
+In parallel with the extension of the abstract syntax
+Logic to Arithmetic, we can extend
+the concrete syntax LogicEng to ArithmeticEng:
+
+ concrete ArithmeticEng of Arithmetic = LogicEng ** open Util in {
+ lincat Nat = SS ;
+ lin Even x = ss (x.s ++ "is" ++ "even") ;
+ lin Odd x = ss (x.s ++ "is" ++ "odd") ;
+ lin Zero = ss "zero" ;
+ lin Succ x = ss ("the" ++ "successor" ++ "of" ++ x.s) ;
+ }
+
+Another extension of Logic is Geometry,
+
+ abstract Geometry = Logic ** {
+ cat Point ;
+ cat Line ;
+ fun Incident : Point -> Line -> Prop ;
+ }
+
+The corresponding concrete syntax is left as exercise.
+
++ +Inheritance can be multiple, which means that a module +may extend many modules at the same time. Suppose, for instance, +that we want to build a module for mathematics covering both +arithmetic and geometry, and the underlying logic. We then write +
+ abstract Mathematics = Arithmetic, Geometry ** {
+ } ;
+
+We could of course add some new judgements in this module, but
+it is not necessary to do so.
+
++ +The module Mathematics also shows that it is possibe +to extend a module already built by extension. The correctness +criterion for extensions is that the same name +(cat, fun, oper, or param) +may not be defined twice in the resulting union of names. +That the names defined in Logic are "inherited twice" +by Mathematics (via both Arithmetic and +Geometry) is no violation of this rule; the usual +problems of multiple inheritance do not arise, since +the definitions of inherited constants cannot be changed. + + +
+ Mathematics.Prop Arithmetic.Prop Geometry.Prop Logic.Prop ++all refer to the same category, declared in the module +Logic. + + + +
+ cat C ; ===> oper C : Type = T ; + lincat C = T ; + + fun f : A ; ===> oper f : A = t ; + lin f = t ; ++Due to this translation, a concrete module +can be opened in the same way as a +resource module; the translation is done +on the fly (it is computationally very cheap). + +
+ +Modular grammar engineering often means that some grammarians +focus on the semantics of the domain whereas others take care +of linguistic details. Thus a typical reuse opens a +linguistically oriented resource grammar, +
+ abstract Resource = {
+ cat S ; NP ; A ;
+ fun PredA : NP -> A -> S ;
+ }
+ concrete ResourceEng of Resource = {
+ lincat S = ... ;
+ lin PredA = ... ;
+ }
+
+The application grammar, instead of giving linearizations
+explicitly, just reduces them to categories and functions in the
+resource grammar:
+
+ concrete ArithmeticEng of Arithmetic = LogicEng ** open ResourceEng in {
+ lincat Nat = NP ;
+ lin Even x = PredA x (regA "even") ;
+ }
+
+If the resource grammar is only capable of generating grammatically
+correct expressions, then the grammaticality of the application
+grammar is also guaranteed: the type checker of GF is used as
+grammar checker.
+To guarantee distinctions between categories that have
+the same linearization type, the actual translation used
+in GF adds to every linearization type and linearization
+a lock field,
+
+ cat C ; ===> oper C : Type = T ** {lock_C : {}} ;
+ lincat C = T ;
+
+ fun f : C_1 ... C_n -> C ; ===> oper f : C_1 ... C_n -> C = \x_1,...,x_n ->
+ lin f = t ; t x_1 ... x_n ** {lock_C = <>};
+
+(Notice that the latter translation is type-correct because of
+record subtyping, which means that t can ignore the
+lock fields of its arguments.) An application grammarian who
+only uses resource grammar categories and functions never
+needs to write these lock fields herself. Having to do so
+serves as a warning that the grammaticality guarantee given
+by the resource grammar no longer holds.
+
+
++ +However, the separation of declarations and definitions is so +useful a notion that GF also has specific modules types that +resource modules into two parts. In this splitting, +an interface module corresponds to an abstract syntax, +in giving the declarations of operations (and parameter types). +For instance, a generic markup interface would look as follows: +
+ interface Markup = open Util in {
+ oper Boldface : Str -> Str ;
+ oper Heading : Str -> Str ;
+ oper markupSS : (Str -> Str) -> SS -> SS = \f,r ->
+ ss (f r.s) ;
+ }
+
+The definitions of the constants declared in an interface
+are given in an instance module (which is always of
+an interface, in the same way as a concrete is always
+of an abstract). The following instances
+define markup in HTML and latex.
+
+ instance MarkupHTML of Markup = open Util in {
+ oper Boldface s = "<b>" ++ s ++ "</b>" ;
+ oper Heading s = "<h2>" ++ s ++ "</h2>" ;
+ }
+
+ instance MarkupLatex of Markup = open Util in {
+ oper Boldface s = "\\textbf{" ++ s ++ "}" ;
+ oper Heading s = "\\section{" ++ s ++ "}" ;
+ }
+
+Notice that both interfaces and instances may
+open resources (and also reused top-level grammars).
+An interface may moreover define some of the operations it
+declares; these definitions are inherited by all instances and cannot
+be changed in them. Inheritance by module extension
+is possible, as always, between modules of the same type.
+
+
++ +A module that opens an interface is therefore +incomplete, and has to be completed with an +instance of the interface to become complete. To make +this situation clear, GF requires any module that opens an +interface to be marked as incomplete. Thus +the module +
+ incomplete concrete DocMarkup of Doc = open Markup in {
+ ...
+ }
+
+uses the interface Markup to place markup in
+chosen places in its linearization rules, but the
+implementation of markup - whether in HTML or in LaTeX - is
+left unspecified. This is a powerful way of sharing
+the code of a whole module with just differences in
+the definitions of some constants.
+
++ +Another terminology for incomplete modules is +parametrized modules or functors. +The interface gives the list of parameters +that the functor depends on. + + +
+ concrete DocHTML of Doc = DocMarkup with (Markup = MarkupHTML) ; ++Instantiation of Markup with MarkupLatex is +another one-liner. + +
+ +If more interfaces than one are instantiated, a comma-separated +list of equations in parentheses is used, e.g. +
+ concrete RulesIta = CategoriesIta ** RulesRomance with + (TypesRomance = TypesIta), (SyntaxRomance = SyntaxIta) ; ++(an example from the GF resource grammar library, where languages for +Romance languages share two interfaces). +All interfaces that are opened in the completed model +must be completed. + +
+ +Notice that the completion of an incomplete module +may at the same time extend modules of the same type (which need +not be completions). But it cannot add new judgements. + + +
+abstract A = (A1,...,An **)? +{J1 ; ... ; Jm ; } + +
+ +where +
+incomplete? concrete C of A = +(C1,...,Cn **)? +(open O1,...,Ok in)? +{J1 ; ... ; Jm ; } + +
+ +where +
+ +If the modifier incomplete appears, then any R in +an open specification may also be an interface. + +
+ +Semantic conditions: +
+resource R = +(R1,...,Rn **)? +(open O1,...,Ok in)? +{J1 ; ... ; Jm ; } + +
+where +
+ +Semantic conditions: +
+interface R = +(R1,...,Rn **)? +(open O1,...,Ok in)? +{J1 ; ... ; Jm ; } + +
+where +
+ +Semantic conditions: +
+instance R of I = +(R1,...,Rn **)? +(open O1,...,Ok in)? +{J1 ; ... ; Jm ; } + +
+where +
+ +Semantic conditions: +
+concrete C of A = +(C1,...,Cn **)? +B +with +(I1 =J1), ... +, (Im =Jm) ; + +
+ +where +