# The Module System of GF

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`
- `transfer`

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.

## The principal module types

### Abstract syntax

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 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 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.

#### Compilation of 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.

### Concrete syntax

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` which it is claimed to be. The validity requires that there has to be

- a `lincat` judgement for each `cat` judgement, telling what the linearization types of categories are
- a `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 linearization types of the arguments and value of the function.

There can also be `lindef` and `printname` judgements in a concrete syntax.

### Top-level grammar

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

- linearization: take an abstract syntax tree and find the corresponding string
- parsing: take a string and find the corresponding abstract syntax trees (which can be zero, one, or many)

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

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.

#### Compiling top-level grammars

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 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.)

#### Using top-level grammars

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:

- `-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)

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. (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").

### Multilingual grammar

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 concrete syntax.)

#### Using multilingual grammars

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 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 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

#### Exercise

Write yet another concrete syntax of `Logic`, for a language or symbolic notation of your choice.

### Resource modules

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 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` 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:

- `oper` defining auxiliary operations
- `param` defining parameter types

A `resource` can be used in a `concrete` (or another `resource`) by `open`ing 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 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 `open`ed 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`.

#### Compiling resource modules

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 `oper`s 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` file. This file is written in the GF source code notation, but it is type checked and type annotated, and `oper`s are computed as far as possible.

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.

#### Using resource modules

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 `oper`s 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`) can now be used for evaluating expressions that may contain operations defined in `Util`:

      > cc ss (paren "foo")
      {s = "(" ++ "foo" ++ ")"}

To find out what `oper`s 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 ;

#### Exercise

Rewrite the modules `LogicEng` and `LogicFre` by making use of the resource.

### Inheritance

The most characteristic modularity of GF lies in the division of 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, 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.

#### Compiling inheritance

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

all refer to the same category, declared in the module `Logic`.

#### Inspecting grammar hierarchies

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`).

### Reuse of top-level grammars as resources

Top-level grammars have a straightforward translation to `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 `open`ed 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 : A = \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.

## Additional module types

### Interfaces, instances, and incomplete grammars

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` 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, 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 `instance`s 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 `interface`s and `instance`s may `open` `resource`s (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.

#### Using an interface

An `interface` or an `instance` can be `open`ed 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, 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 `open`s 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.

#### Instantiating an interface

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 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 `open`ed 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.

#### Compiling interfaces, instances, and parametrized modules

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 nevertheless to keep track of their dependencies and modification times. Here is a summary of how they are compiled:

- an `interface` is compiled into a `resource` with an empty body
- an `instance` is compiled into a `resource` in union with its `interface`
- an `incomplete` module (`concrete` or `resource`) is compiled into a module of the same type with an empty body
- a completion module (`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

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`.

### Transfer modules

Translation by transfer means that syntax trees are manipulated by non-compositional functions (transfer rules) between the source and target languages. They are being introduce to GF as a module type of its own, but their development is still in progress. What will be available are at least `fun` and `def` judgements, but more is needed. It has not yet been defined how transfer modules are integrated in multilingual grammars, i.e.\ where in the grammar it is specified what transfer to use. (Both GF and GFC have a syntax for transfer modules and multilingual headers, but their compilation further than parsing has not been implemented.)

## Summary of module syntax and semantics

#### Abstract syntax modules

Syntax:

`abstract` A `=` (A1,...,An `**`)? `{`J1 `;` ... `;` Jm `; }`

where

- i >= 0
- each Ai is itself an abstract module
- each Ji is a judgement of one of the forms `cat, fun, def, data`

Semantic conditions:

- all names declared in each Ai and A must be distinct

#### Concrete syntax modules

Syntax:

`incomplete`? `concrete` C `of` A `=` (C1,...,Cn `**`)? (`open` O1,...,Ok `in`)? `{`J1 `;` ... `;` Jm `; }`

where

- i >= 0
- A is an abstract module
- each Ci is a concrete module
- each Oi is an open specification, of one of the forms
  - R
  - `(`Q`=`R`)`
  where R is a resource, instance, or concrete, and Q is any identifier
- each Ji is a judgement of one of the forms `lincat, lin, lindef, printname`

If the modifier `incomplete` appears, then any R in an open specification may also be an interface.

Semantic conditions:

- each `cat` judgement in A must have a corresponding, unique `lincat` judgement in C
- each `fun` judgement in A must have a corresponding, unique `lin` judgement in C

#### Resource modules

Syntax:

`resource` R `=` (R1,...,Rn `**`)? (`open` O1,...,Ok `in`)? `{`J1 `;` ... `;` Jm `; }`

where

- i >= 0
- each Ri is a resource module
- each Oi is an open specification, of one of the forms
  - P
  - `(`Q`=`R`)`
  where P is a resource, instance, or concrete, and Q is any identifier
- each Ji is a judgement of one of the forms `oper, param`

Semantic conditions:

- all names declared in each Ri and R must be distinct
- all constants declared must have a definition

#### Interface modules

Syntax:

`interface` R `=` (R1,...,Rn `**`)? (`open` O1,...,Ok `in`)? `{`J1 `;` ... `;` Jm `; }`

where

- i >= 0
- each Ri is an interface module
- each Oi is an open specification, of one of the forms
  - P
  - `(`Q`=`R`)`
  where P is a resource, instance, or concrete, and Q is any identifier
- each Ji is a judgement of one of the forms `oper, param`

Semantic conditions:

- all names declared in each Ri and R must be distinct

#### Instance modules

Syntax:

`instance` R `of` I `=` (R1,...,Rn `**`)? (`open` O1,...,Ok `in`)? `{`J1 `;` ... `;` Jm `; }`

where

- i >= 0
- I is an interface module
- each Ri is an instance module
- each Oi is an open specification, of one of the forms
  - P
  - `(`Q`=`R`)`
  where P is a resource, instance, or concrete, and Q is any identifier
- each Ji is a judgement of one of the forms `oper, param`

Semantic conditions:

- all names declared in each Ri, I, and R must be distinct
- all constants declared in I must have a definition either in I or R

#### Instantiated concrete syntax modules

Syntax:

`concrete` C `of` A `=` (C1,...,Cn `**`)? B `with` `(`I1 `=`J1`),` ... `, (`Im `=`Jm`) ;`

where

- i >= 0
- A is an abstract module
- each Ci is a concrete module
- B is an incomplete concrete syntax of A
- each Ii is an interface
- each Ji is an instance of Ii