GF Language Reference Manual

Aarne Ranta, Krasimir Angelov
November 2013



This document is a reference manual to the GF programming language. GF, Grammatical Framework, is a special-purpose programming language, designed to support definitions of grammars.

This document is not an introduction to GF; such introduction can be found in the GF tutorial available on line on the GF web page,

grammaticalframework.org

This manual covers only the language, not the GF compiler or interactive system. We will however make some references to different compiler versions, if they involve changes of behaviour having to do with the language specification.

This manual is meant to be fully compatible with GF version 3.0. Main discrepancies with version 2.8 are indicated, as well as with the reference article on GF,

A. Ranta, "Grammatical Framework. A Type Theoretical Grammar Formalism", The Journal of Functional Programming 14(2), 2004, pp. 145-189.

This article will referred to as "the JFP article".

As metalinguistic notation, we will use the symbols

Overview of GF

GF is a typed functional language, borrowing many of its constructs from ML and Haskell: algebraic datatypes, higher-order functions, pattern matching. The module system bears resemblance to ML (functors) but also to object-oriented languages (inheritance). The type theory used in the abstract syntax part of GF is inherited from logical frameworks, in particular ALF ("Another Logical Framework"; in a sense, GF is Yet Another ALF). From ALF comes also the use of dependent types, including the use of explicit type variables instead of Hindley-Milner polymorphism.

The look and feel of GF is close to Java and C, due to the use of curly brackets and semicolons in structuring the code; the expression syntax, however, follows Haskell in using juxtaposition for function application and parentheses only for grouping.

To understand the constructs of GF, and especially their limitations in comparison to general-purpose programming languages, it is essential to keep in mind that GF is a special-purpose and non-turing-complete language. Every GF program is ultimately compiled to a multilingual grammar, which consists of an abstract syntax and a set of concrete syntaxes. The abstract syntax defines a system of syntax trees, and each concrete syntax defines a mapping from those syntax trees to nested tuples of strings and integers. This mapping is compositional, i.e. homomorphic, and moreover reversible: given a nested tuple, there exists an effective way of finding the set of syntax trees that map to this tuple. The procedure of applying the mapping to a tree to produce a tuple is called linearization, and the reverse search procedure is called parsing. It is ultimately the requirement of reversibility that restricts GF to be less than turing-complete. This is reflected in restrictions to recursion in concrete syntax. Tree formation in abstract syntax, however, is fully recursive.

Even though run-time GF grammars manipulate just nested tuples, at compile time these are represented by by the more fine-grained labelled records and finite functions over algebraic datatypes. This enables the programmer to write on a higher abstraction level, and also adds type distinctions and hence raises the level of checking of programs.

The module system

Top-level and supplementary module structure

The big picture of GF as a programming language for multilingual grammars explains its principal module structure. Any GF grammar must have an abstract syntax module; it can in addition have any number of concrete syntax modules matching that abstract syntax. Before going to details, we give a simple example: a module defining the category A of adjectives and one adjective-forming function, the zero-place function Even. We give the module the name Adj. The GF code for the module looks as follows:

    abstract Adj = {
      cat A ;
      fun Even : A ;
    }

Here are two concrete syntax modules, one intended for mapping the trees to English, the other to Swedish. The mappling is defined by lincat definitions assigning a linearization type to each category, and lin definitions assigning a linearization to each function.

    concrete AdjEng of Adj = {
      lincat A = {s : Str} ;
      lin Even = {s = "even"} ;
    }
  
    concrete AdjSwe of Adj = {
      lincat A = {s : AForm => Str} ;
      lin Even = {s = table {
        ASg Utr   => "jämn" ;
        ASg Neutr => "jämnt" ;
        APl       => "jämna"
        }
      } ;
      param AForm = ASg Gender | APl ;
      param Gender = Utr | Neutr ;
    }

These examples illustrate the main ideas of multilingual grammars:

The first two ideas form the core of the static checking of GF grammars, eliminating the possibility of run-time errors in linearization and parsing. The third idea gives GF the expressive power needed to map abstract syntax to vastly different languages.

Abstract and concrete modules are called top-level grammar modules, since they are the ones that remain in grammar systems at run time. However, in order to support modular grammar engineering, GF provides much more module structure than strictly required in top-level grammars.

Inheritance, also known as extension, means that a module can inherit the contents of one or more other modules to which new judgements are added, e.g.

    abstract MoreAdj = Adj ** {
      fun Odd : A ;
    }

Resource modules define parameter types and operations usable in several concrete syntaxes,

    resource MorphoFre = {
      param Number = Sg | Pl ;
      param Gender = Masc | Fem ;
      oper regA : Str -> {s : Gender => Number => Str} = 
        \fin -> {
          s = table {
            Masc => table {Sg => fin ; Pl => fin + "s"} ;
            Fem  => table {Sg => fin + "e" ; Pl => fin + "es"}
          }
        } ;
    }

By opening, a module can use the contents of a resource module without inheriting them, e.g.

    concrete AdjFre of Adj = open MorphoFre in {
      lincat A = {s : Gender => Number => Str} ;
      lin Even = regA "pair" ;
    }

Interfaces and instances separate the contents of a resource module to type signatures and definitions, in a way analogous to abstract vs. concrete modules, e.g.

    interface Lexicon = {
      oper Adjective : Type ;
      oper even_A : Adjective ;
    }
  
    instance LexiconEng of Lexicon = {
      oper Adjective = {s : Str} ;
      oper even_A = {s = "even"} ;
    }

Functors i.e. parametrized modules i.e. incomplete modules, defining a concrete syntax in terms of an interface.

    incomplete concrete AdjI of Adj = open Lexicon in {
      lincat A = Adjective ;
      lin Even = even_A ;
    }

A functor can be instantiated by providing instances of its open interfaces.

    concrete AdjEng of Adj = AdjI with (Lexicon = LexiconEng) ;

Compilation units

The compilation unit of GF source code is a file that contains a module. Judgements outside modules are supported only for backward compatibility, as explained here. Every source file, suffixed .gf, is compiled to a "GF object file", suffixed .gfo (as of GF Version 3.0 and later). For runtime grammar objects used for parsing and linearization, a set of .gfo files is linked to a single file suffixed .gfcc. While .gf and .gfo files may contain modules of any kinds, a .gfcc file always contains a multilingual grammar with one abstract and a set of concrete syntaxes.

The following diagram summarizes the files involved in the compilation process.

module1.gf module2.gf ... modulen.gf

==>

module1.gfo module2.gfo ... modulen.gfo

==>

grammar.gfcc

Both .gf and .gfo files are written in the GF source language; .gfcc files are written in a lower-level format. The process of translating .gf to .gfo consists of name resolution, type annotation, partial evaluation, and optimization. There is a great advantage in the possibility to do this separately for GF modules and saving the result in .gfo files. The partial evaluation phase, in particular, is time and memory consuming, and GF libraries are therefore distributed in .gfo to make their use less arduous.

In GF before version 3.0, the object files are in a format called .gfc, and the multilingual runtime grammar is in a format called .gfcm.

The standard compiler has a built-in make facility, which finds out what other modules are needed when compiling an explicitly given module. This facility builds a dependency graph and decides which of the involved modules need recompilation (from .gf to .gfo), and for which the GF object can be used directly.

Names

Each module M defines a set of names, which are visible in M itself, in all modules extending M (unless excluded, as explained here), and all modules opening M. These names can stand for abstract syntax categories and functions, parameter types and parameter constructors, and operations. All these names live in the same name space, which means that a name entering a module more than once due to inheritance or opening can lead to a conflict. It is specified here how these conflicts are resolved.

The names of modules live in a name space separate from the other names. Even here, all names must be distinct in a set of files compiled to a multilingual grammar. In particular, even files residing in different directories must have different names, since GF has no notion of hierarchic module names.

Lexically, names belong to the class of identifiers. An idenfifier is a letter followed by any number of letters, digits, undercores (_) and primes ('). Upper- and lower-case letters are treated as distinct. Nothing dictates the choice of upper or lower-case initials, but the standard libraries follow conventions similar to Haskell:

"Letters" as mentioned in the identifier syntax include all 7-bit ASCII letters. Iso-latin-1 and Unicode letters are supported in varying degrees by different tools and platforms, and are hence not recommended in identifiers.

The structure of a module

Modules of all types have the following structure:

moduletype name = extends opens body
The part of the module preceding the body is its header. The header defines the type of the module and tells what other modules it inherits and opens. The body consists of the judgements that introduce all the new names defined by the module.

Any of the parts extends, opens, and body may be empty. If they are all filled, delimiters and keywords separate the parts in the following way:

moduletype name = extends ** open opens in { body }
The part moduletype name looks slightly different if the type is concrete or instance: the name intrudes between the type keyword and the name of the module being implemented and which really belongs to the type of the module:
concrete name of abstractname
The only exception to the schema of functor syntax is functor instantiations: the instantiation list is given in a special way between extends and opens:
incomplete concrete name of abstractname = extends ** functorname with instantiations ** open opens in { body }
Logically, the part "functorname with instantiations" should really be one of the extends. This is also shown by the fact that it can have restricted inheritance (concept defined here).

Module types, headers, and bodies

The extends and opens parts of a module header are lists of module names (with possible qualifications, as defined below here). The first step of type checking a module consists of verifying that these names stand for modules of approptiate module types. As a rule of thumb,

However, the precise rules are a little more fine-grained, because of the presence of interfaces and their instances, and the possibility to reuse abstract and concrete modules as resources. The following table gives, for all module types, the possible module types of their extends and opens, as well as the forms of judgement legal in that module type.

module type extends opens body
abstract abstract - cat, fun, def, data
concrete of abstract concrete resource* lincat, cat, oper, param
resource resource* resource* oper, param
interface resource+ resource* oper, param
instance of interface resource* resource* oper, param
incomplete concrete concrete+ resource+ lincat, cat, oper, param

The table uses the following shorthands for lists of module types:

The legality of judgements in the body is checked before the judgements themselves are checked.

The forms of judgement are explained here.

Digression: the logic of module types

Why are the legality conditions of opens and extends so complicated? The best way to grasp them is probably to consider a simplified logical model of the module system, replacing modules by types and functions. This model could actually be developed towards treating modules in GF as first-class objects; so far, however, this step has not been motivated by any practical needs.

module object and type
abstract A = B A = B : type
concrete C of A = B C = B : A -> S
interface I = B I = B : type
instance J of I = B J = B : I
incomplete concrete C of A = open I in B C = B : I -> A -> S
concrete K of A = C with (I=J) K = B(J) : A -> S
resource R = B R = B : I
concrete C of A = open R in B C = B(R) : A -> S

A further step of defining modules as first-class objects would use GADTs and record types:

Slightly unexpectedly, interfaces and instances are easier to understand in this way than resources - a resource is, indeed, more complex, since it fuses together an interface and an instance.

When an abstract is used as an interface and a concrete as its instance, they are actually reinterpreted so that they match the model. Then the abstract is no longer a GADT, but a system of abstract datatypes, with a record field of type Type for each category, and a function among these types for each abstract syntax function. A concrete syntax instantiates this record with linearization types and linearizations.

Inheritance

After checking that the extends of a module are of appropriate module types, the compiler adds the inherited judgements to the judgements included in the body. The inherited judgements are not copied entirely, but their names with links to the inherited module. Conflicts may arise in this process: a name can have two definitions in the combined pool of inherited and added judgements. Such a conflict is always an error: GF provides no way to redefine an inherited constant.

Simple as the definition of a conflict may sound, it has to take care of the inheritance hierarchy. A very common pattern of inheritance is the diamond: inheritance from two modules which themselves inherit a common base module. Assume that the base module defines a name f:

            N
          /   \
        M1     M2
          \   /
          Base {f}

Now, N inherits f from both M1 and M2, so is there a conflict? The answer in GF is no, because the "two" f's are in the end the same: the one defined in Base. The situation is thus simpler than in multiple inheritance in languages like C++, because definitions in GF are immutable: neither M1 nor M2 can possibly have changed the definition of f given in Base. In practice, the compiler manages inheritance through hierarchy in a very simple way, by just always creating a link not to the immediate parent, but the original ancestor; this ancestor can be read from the link provided by the immediate parent. Here is how links are created from source modules by the compiler:

    Base {f}
    M1 {m1}   ===>  M1 {Base.f, m1}
    M2 {m2}   ===>  M2 {Base.f, m2}
    N  {n}    ===>  N  {Base.f, M1.m1, M2.m2, n}

Inheritance can be restricted. This means that a module can be specified as inheriting only explicitly listed constants, or all constants except ones explicitly listed. The syntax uses constant names in brackets, prefixed by a minus sign in the case of an exclusion list. In the following configuration, N inherits a,b,c from M1, and all names but d from M2

    N = M1 {a,b,c}, M2-{d}

Restrictions are performed as a part of inheritance linking, module by module: the link is created for a constant if and only if it is both included in the module and compatible with the restriction. Thus, for instance, an inadvertent usage can exclude a constant from one module but inherit it from another one. In the following configuration, f is inherited via M1, if M1 inherits it.

    N = M1 [a,b,c], M2-[f]

Unintended inheritance may cause problems later in compilation, in the judgement-level dependency analysis phase. For instance, suppose a function f has category C as its type in M, and we only include f. The exclusion has the effect of creating an ill-formed module:

    abstract M = {cat C ; fun f : C ;}
    M [f]   ===> {fun f : C ;}

One might expect inheritance restriction to be transitive: if an included constant b depends on some other constant a, then a should be included automatically. However, this rule would leave to hard-to-detect inheritances. And it could only be applied later in the compilation phase, when the compiler has not only collected the names defined, but also resolved the names used in definitions.

Yet another pitfall with restricted inheritance is that it must be stated for each module separately. For instance, a concrete syntax of an abstract must exclude all those names that the abstract does, and a functor instantiation must replicate all restrictions of the functor.

Opening

Opening makes constants from other modules usable in judgements, without inheriting them. This means that, unlike inheritance, opening is not transitive.

Opening cannot be restricted as inheritance can, but it can be qualified. This means that the names from the opened modules cannot be used as such, but only as prefixed by a qualifier and a dot (.). The qualifier can be any identifier, including the name of the module. Here is an example of an opens list:

    open A, (X = XSLTS), (Y = XSLTS), B

If A defines the constant a, it can be accessed by the names

    a  A.a

If XSLTS defines the constant x, it can be accessed by the names

    X.x  Y.x  XSLTS.x

Thus qualification by real module name is always possible, and one and the same module can be qualified in different ways at the same time (the latter can be useful if you want to be able to change the implementations of some constants to a different resource later). Since the qualification with real module name is always possible, it is not possible to "swap" the names of modules locally:

    open (A=B), (B=A) -- NOT POSSIBLE!

The list of qualifiers names and module names in a module header may thus not contain any duplicates.

Name resolution

Name resolution is the compiler phase taking place after inheritance linking. It qualifies all names occurring in the definition parts of judgements (that is, just excluding the defined names themselves) with the names of the modules they come from. If a name can come from different modules (that is, not from their common ancestor), a conflict is reported; this decision is hence not dependent on e.g. types, which are known only at a later phase.

Qualification of names is the main device for avoiding conflicts in name resolution. No other information is used, such as priorities between modules. However, if a name is defined in different opened modules but never used in the module body, a conflict does not arise: conflicts arise only when names are used. Also in this respect, opening is thus different from inheritance, where conflicts are checked independently of use.

As usual, inner scope has priority in name resolution. This means that if an identifier is in scope as a bound variable, it will not be interpreted as a constant, unless qualified by a module name (variable bindings are explained here).

Functor instantiations

We have dealt with the principles of module headers, inheritance, and names in a general way that applies to all module types. The exception is functor instantiations, that have an extra part of the instantiating equations, assigning an instance to every interface. Here is a typical example, displaying the full generality:

    concrete FoodsEng of Foods = PhrasesEng ** 
      FoodsI-[Pizza] with 
        (Syntax = SyntaxEng),
        (LexFoods = LexFoodsEng) ** 
      open SyntaxEng, ParadigmsEng in {
        lin Pizza = mkCN (mkA "Italian") (mkN "pie") ;
    }

(The example is modified from Section 5.9 in the GF Tutorial.)

The instantiation syntax is similar to qualified opens. The left-hand-side names must be interfaces, the right-hand-side names their instances. (Recall that abstract can be use as interface and concrete as its instance.) Inheritance from the functor can be restricted, typically in the purpose of defining some excluded functions in language-specific ways in the module body.

Completeness

(This section refers to the forms of judgement introduced here.)

A concrete is complete with respect to an abstract, if it contains a lincat definition for every cat declaration, and a lin definition for every fun declaration.

The same completeness criterion applies to functor instantiations. It is not possible to use a partial functor instantiation, leading to another functor.

Functors do not need to be complete in the sense concrete modules need. The missing definitions can then be provided in the body of each functor instantiation.

A resource is complete, if all its oper and param judgements have a definition part. While a resource must be complete, an interface need not. For an interface, it is the definition parts of judgements are optional.

An instance is complete with respect to an interface, if it gives the definition parts of all oper and param judgements that are omitted in the interface. Giving definitions to judgements that have already been defined in the interface is illegal. Type signatures, on the other hand, can be repeated if the same types are used.

In addition to completing the definitions in an interface, its instance may contain other judgements, but these must all be complete with definitions.

Here is an example of an instance and its interface showing the above variations:

    interface Pos = {
      param Case ;                 -- no definition
      param Number = Sg | Pl ;     -- definition given
      oper Noun : Type = {         -- relative definition given
        s : Number => Case => Str
      } ;
      oper regNoun : Str -> Noun ; -- no definition
    }
  
    instance PosEng of Pos = {
      param Case = Nom | Gen ;     -- definition of Case
                                   -- Number and Noun inherited
      oper regNoun = \dog -> {     -- type of regNoun inherited
        s = table {                -- definition of regNoun
          Sg => table {
            Nom => dog
            -- etc
          }
        } ;
      oper house_N : Noun =        -- new definition
        regNoun "house" ;
    }

Judgements

Overview of the forms of judgement

A module body in GF is a set of judgements. Judgements are definitions or declarations, sometimes combinations of the two; the common feature is that every judgement introduces a name, which is available in the module and whenever the module is extended or opened.

There are several different forms of judgement, identified by different judgement keywords. Here is a list of all these forms, together with syntax descriptions and the types of modules in which each form can occur. The table moreover indicates whether the judgement has a default value, and whether it contributes to the name base, i.e. introduces a new name to the scope.

judgement where module default base
cat C G G context abstract N/A yes
fun f : A A type abstract N/A yes
def f ps = t f fun, ps patterns, t term abstract yes no
data C = f | ... | g C cat, f...g fun abstract yes no
lincat C = T C cat, T type concrete* yes yes
lin f = t f fun, t term concrete* no yes
lindef C = t C cat, t term concrete* yes no
linref C = t C cat, t term concrete* yes no
printname cat C = t C cat, t term concrete* yes no
printname fun f = t f fun, t term concrete* yes no
param P = C| ... | D C...D constructors resource* N/A yes
oper f : T = t T type, t term resource* N/A yes
flags o = v o flag, v value all yes N/A

Judgements that have default values are rarely used, except lincat and flags, which often need values different from the defaults.

Introducing a name twice in the same module is an error. In other words, all judgements that have a "yes" in the name base column, must have distinct identifiers on their left-hand sides.

All judgement end with semicolons (;).

In addition to the syntax given in the table, many of the forms have syntactic sugar. This sugar will be explained below in connection to each form. There are moreover two kinds of syntactic sugar common to all forms:

These conventions, like all syntactic sugar, are performed at an early compilation phase, directly after parsing. This means that e.g.

    lin f,g = \x -> x ;

can be correct even though f and g required different function types.

Within a module, judgements can occur in any order. In particular, a name can be used before it is introduced.

The explanations of judgement forms refer to the notions of type and term (the latter also called expression). These notions will be explained in detail here.

Category declarations, cat

Category declarations

cat C G
define the basic types of abstract syntax. A basic type is formed from a category by giving values to all variables in the context G. If the context is empty, the basic type looks the same as the category itself. Otherwise, application syntax is used:
C a1...an

Hypotheses and contexts

A context is a sequence of hypotheses, i.e. variable-type pairs. A hypothesis is written

( x : T )
and a sequence does not have any separator symbols. As syntactic sugar,

An abstract syntax has dependent types, if any of its categories has a non-empty context.

Function declarations, fun

Function declarations,

fun f : T
define the syntactic constructors of abstract syntax. The type T of f is built built from basic types (formed from categories) by using the function type constructor ->. Thus its form is
(x1 : A1) -> ... -> (xn : An) -> B
where Ai are types, called the argument types, and B is a basic type, called the value type of f. The value category of f is the category that forms the type B.

A syntax tree is formed from f by applying it to a full list of arguments, so that the result is of a basic type.

A higher-order function is one that has a function type as an argument. The concrete syntax of GF does not support displaying the bound variables of functions of higher than second order, but they are legal in abstract syntax.

An abstract syntax is context-free, if it has neither dependent types nor higher-order functions. Grammars with context-free abstract syntax are an important subclass of GF, with more limited complexity than full GF. Whether the concrete syntax is context-free in the sense of the Chomsky hierarchy is independent of the context-freeness of the abstract syntax.

Function definitions, def

Function definitions,

def f p1 ... pn = t
where f is a fun function and pi# are patterns, impose a relation of definitional equality on abstract syntax trees. They form the basis of computation, which is used when comparing whether two types are equal; this notion is relevant only if the types are dependent. Computation can also be used for the normalization of syntax trees, which applies even in context-free abstract syntax.

The set of def definitions for f can be scattered around the module in which f is introduced as a function. The compiler builds the set of pattern equations in the order in which the equations appear; this order is significant in the case of overlapping patterns. All equations must appear in the same module in which f itself declared.

The syntax of patterns will be specified here, commonly for abstract and concrete syntax. In abstract syntax, constructor patterns are those of the form

C p1 ... pn
where C is declared as data for some abstract syntax category (see next section). A variable pattern is either an identifier or a wildcard.

A common pitfall is to forget to declare a constructor as data, which causes it to be interpreted as a variable pattern in definitions.

Computation is performed by applying definitions and beta conversions, and in general by using pattern matching. Computation and pattern matching are explained commonly for abstract and concrete syntax here.

In contrast to concrete syntax, abstract syntax computation is completely symbolic: it does not produce a value, but just another term. Hence it is not an error to have incomplete systems of pattern equations for a function. In addition, the definitions can be recursive, which means that computation can fail to terminate; this can never happen in concrete syntax.

Data constructor definitions, data

A data constructor definition,

data C = f1 | ... | fn
defines the functions f1...fn to be constructors of the category C. This means that they are recognized as constructor patterns when used in function definitions.

In order for the data constructor definition to be correct, f1...fn must be functions with C as their value category.

The complete set of constructors for a category C is the union of all its data constructor definitions. Thus a category can be "extended" by new constructors afterwards. However, all these constructor definitions must appear in the same module in which the category is itself defined.

There is syntactic sugar for declaring a function as a constructor at the same time as introducing it:

data f : A1 -> ... -> An -> C t1 ... tm

===

fun f : A1 -> ... -> An -> C t1 ... tm ; data C = f

The semantic status of an abstract syntax function

There are three possible statuses for a function declared in a fun judgement:

The "constructor" and "defined" statuses are in contradiction with each other, whereas the primitive notion status is overridden by any of the two others.

This distinction is relevant for the semantics of abstract syntax, not for concrete syntax. It shows in the way patterns are treated in equations in def definitions: a constructor in a pattern matches only itself, whereas any other name is treated as a variable pattern, which matches anything.

Linearization type definitions, lincat

A linearization type definition,

lincat C = T
defines the type of linearizations of trees whose type has category C. Type dependences have no effect on the linearization type.

The type T must be a legal linearization type, which means that it is a record type whose fields have either parameter types, the type Str of strings, or table or record types of these. In particular, function types may not appear in T. A detailed explanation of types in concrete syntax will be given here.

If K is the concrete syntax of an abstract syntax A, then K must define the linearization type of all categories declared in A. However, the definition can be omitted from the source code, in which case the default type {s : Str} is used.

Linearization definitions, lin

A linearization definition,

lin f = t
defines the linearizations function of function f, i.e. the function used for linearizing trees formed by f.

The type of t must be the homomorphic image of the type of f. In other words, if

fun f : A1 -> ... -> An -> A
then
lin f : A1* -> ... -> An* -> A*
where the type T* is defined as follows depending on T:

The second case is relevant for higher-order functions only. It says that the linearization type of the value type is extended by adding a string field for each argument types; these fields store the variable symbol used for the binding of each variable.

Since the arguments of a function argument are treated as bare strings, orders higher than the second are irrelevant for concrete syntax.

There is syntactic sugar for binding the variables of the linearization of a function on the left-hand side:

lin f p = t === lin f = \p -> t
The pattern p must be either a variable or a wildcard (_); this is what the syntax of lambda abstracts (\p -> t) requires.

Linearization default definitions, lindef

A linearization default definition,

lindef C = t
defines the default linearization of category C, i.e. the function applicable to a string to make it into an object of the linearization type of C.

Linearization defaults are invoked when linearizing variable bindings in higher-order abstract syntax. A variable symbol is then presented as a string, which must be converted to correct type in order for the linearization not to fail with an error.

The other use of the defaults is for linearizing metavariables and abstract functions without linearization in the concrete syntax. In the first case the default linearization is applied to the string "?X" where X is the unique index of the metavariable, and in the second case the string is "[f]" where f is the name of the abstract function with missing linearization.

Usually, linearization defaults are generated by using the default rule that "uses the symbol itself for every string, and the first value of the parameter type for every parameter". The precise definition is by structural recursion on the type:

The notion of the first value of a parameter type (#1(P)) is defined below.

Linearization reference definitions, linref

A linearization reference definition,

linref C = t
defines the reference linearization of category C, i.e. the function applicable to an object of the linearization type of C to make it into a string.

The reference linearization is always applied to the top-level node of the abstract syntax tree. For example when we linearize the tree f x1 x2 .. xn, then we first apply f to its arguments which gives us an object of the linearization type of its category. After that we apply the reference linearization for the same category to get a string out of the object. This is particularly useful when the linearization type of C contains discontious constituents. In this case usually the reference linearization glues the constituents together to produce an intuitive linearization string.

The reference linearization is also used for linearizing metavariables which stand in function position. For example the tree f (? x1 x2 .. xn) is linearized as follows. Each of the arguments x1 x2 .. xn is linearized, and after that the reference linearization of the its category is applied to the output of the linearization. The result is a sequence of n strings which are concatenated into a single string. The final string is the input to the default linearization of the category for the argument of f. After applying the default linearization we get an object that we could safely pass to f.

Usually, linearization references are generated by using the rule that "picks the first string in the linearization type". The precise definition is by structural recursion on the type:

Here each call to reference returns either (Just o) or Nothing. When we compute the reference for a table or a record then we pick the reference for the first expression for which the recursive call gives us Just. If we get Nothing for all of them then the final result is Nothing too.

Printname definitions, printname cat and printname fun

A category printname definition,

printname cat C = s
defines the printname of category C, i.e. the name used in some abstract syntax information shown to the user.

Likewise, a function printname definition,

printname fun f = s
defines the printname of function f, i.e. the name used in some abstract syntax information shown to the user.

The most common use of printnames is in the interactive syntax editor, where printnames are displayed in menus. It is possible e.g. to adapt them to each language, or to embed HTML tooltips in them (as is used in some HTML-based editor GUIs).

Usually, printnames are generated automatically from the symbol and/or concrete syntax information.

Parameter type definitions, param

A parameter type definition,

param P = C1 G1 | ... | Cn Gn
defines a parameter type P with the parameter constructors C1...Cn, with their respective contexts G1...Gn.

Contexts have the same syntax as in cat judgements, explained here. Since dependent types are not available in parameter type definitions, the use of variables is never necessary. The types in the context must themselves be parameter types, which are defined as follows:

The names defined by a parameter type definition include both the type name P and the constructor names Ci. Therefore all these names must be distinct in a module.

A parameter type may not be recursive, i.e. P itself may not occur in the contexts of its constructors. This restriction extends to mutual recursion: we say that P depends on the types that occur in the contexts of its constructors and on all types that those types depend on, and state that P may not depend on itself.

In an interface module, it is possible to declare a parameter type without defining it,

param P ;

Parameter values

All parameter types are finite, and the GF compiler will internally compute them to lists of parameter values. These lists are formed by traversing the param definitions, usually respecting the order of constructors in the source code. For records, bibliographical sorting is applied. However, both the order of traversal of param definitions and the order of fields in a record are specified in a compiler-internal way, which means that the programmer should not rely on any particular order.

The order of the list of parameter values can affect the program in two cases:

The first usage implies that, if lindef definitions are essential for the application, they should be given manually. The second usage implies that course-of-value tables should be avoided in hand-written GF code.

In run-time grammar generation, all parameter values are translated to integers denotions positions in these parameter lists.

Operation definitions, oper

An operation definition,

oper h : T = t
defines an operation h of type T, with the computation rule
h ==> t
The type T can be any concrete syntax type, including function types of any order. The term t must have the type T, as defined here.

As syntactic sugar, the type can be omitted,

oper h = t
which works in two cases

It is also possible to give the type and the definition separately:

oper h : T ; oper h = t === oper h : T = t
The order of the type part and the definition part is free, and there can be other judgements in between. However, they must occur in the same resource module for it to be complete (as defined here). In an interface module, it is enough to give the type.

When only the definition is given, it is possible to use a shorthand similar to lin judgements:

oper h p = t === oper h = \p -> t
The pattern p is either a variable or a wildcard (_).

Operation definitions may not be recursive, not even mutually recursive. This condition ensures that functions can in the end be eliminated from concrete syntax code (as explained here).

Operation overloading

One and the same operation name h can be used for different operations, which have to have different types. For each call of h, the type checker selects one of these operations depending on what type is expected in the context of the call. The syntax of overloaded operation definitions is

oper h = overload {h : T1 = t1 ; ... ; h : Tn = tn}
Notice that h must be the same in all cases. This format can be used to give the complete implementation; to give just the types, e.g. in an interface, one can use the form
oper h : overload {h : T1 ; ... ; h : Tn}
The implementation of this operation typing is given by a judgement of the first form. The order of branches need not be the same.

Flag definitions, flags

A flag definition,

flags o = v
sets the value of the flag o, to be used when compiling or using the module.

The flag o is an identifier, and the value v is either an identifier or a quoted string.

Flags are a kind of metadata, which do not strictly belong to the GF language. For instance, compilers do not necessarily check the consistency of flags, or the meaningfulness of their values. The inheritance of flags is not well-defined; the only certain rule is that flags set in the module body override the settings from inherited modules.

Here are some flags commonly included in grammars.

flag value description module
coding character encoding encoding used in string literals concrete
lexer predefined lexer lexer before parsing concrete
startcat category default target of parsing abstract
unlexer predefined unlexer unlexer after linearization concrete

The possible values of these flags are specified here.

Types and expressions

Overview of expression forms

Like many dependently typed languages, GF makes no syntactic distinction between expressions and types. An illegal use of a type as an expression or vice versa comes out as a type error. Whether a variable, for instance, stands for a type or an expression value, can only be resolved from its context of use.

One practical consequence of the common syntax is that global and local definitions (oper judgements and let expressions, respectively) work in the same way for types and expressions. Thus it is possible to abbreviate a type occurring in a type expression:

    let A = {s : Str ; b : Bool} in A -> A -> A

Type and other expressions have a system of precedences. The following table summarizes all expression forms, from the highest to the lowest precedence. Some expressions are moreover left- or right-associative.

prec expression example explanation
7 c constant or variable
7 Type the type of types
7 PType the type of parameter types
7 Str the type of strings/token lists
7 "foo" string literal
7 123 integer literal
7 0.123 floating point literal
7 ? metavariable
7 [] empty token list
7 [C a b] list category
7 ["foo bar"] token list
7 {"s : Str ; n : Num} record type
7 {"s = "foo" ; n = Sg} record
7 <Sg,Fem,Gen> tuple
7 <n : Num> type-annotated expression
6 left t.r projection or qualification
5 left f a function application
5 table {Sg => [] ; _ => "xs"} table
5 table P [a ; b ; c] course-of-values table
5 case n of {Sg => [] ; _ => "xs"} case expression
5 variants {"color" ; "colour"} free variation
5 pre {"a" ; "an"/vowel} prefix-dependent choice
4 left t ! v table selection
4 left A * B tuple type
4 left R ** {b : Bool} record (type) extension
3 left t + s token gluing
2 left t ++ s token list concatenation
1 right \x,y -> t function abstraction ("lambda")
1 right \\x,y => t table abstraction
1 right (x : A) -> B dependent function type
1 right A -> B function type
1 right P => T table type
1 right let x = v in t local definition
1 t where {x = v} local definition
1 in M.C "foo" rule by example

Any expression in parentheses ((exp)) is in the highest precedence class.

The functional fragment: expressions in abstract syntax

The expression syntax is the same in abstract and concrete syntax, although only a part of the syntax is actually usable in well-typed expressions in abstract syntax. An abstract syntax is essentially used for defining a set of types and a set of functions between those types. Therefore it needs essentially the functional fragment of the syntax. This fragment comprises two kinds of types:

When defining basic types, we used the notation t{x1 = t1,...,xn=tn} for the substitution of values to variables. This is a metalevel notation, which denotes a term that is formed by replacing the free occurrences of each variable xi by ti.

These types have six kinds of expressions:

The notion of binding is defined for occurrences of variables in subexpressions as follows:

As syntactic sugar, function types have sharing of types and suppression of variables, in the same way as contexts (defined here):

There is analogous syntactic sugar for constant functions,

\_ -> t === \x -> t
where x does not occur in t, and for multiple lambda abstractions:
\p,q -> t === \p -> \q -> t
where p and q are variables or wild cards (_).

Conversions

Among expressions, there is a relation of definitional equality defined by four conversion rules:

Pattern matching substitution used in delta conversion is defined here.

An expression is in beta-eta-normal form if

Notice that the iteration of eta expansion would lead to an expression not in beta-normal form.

Syntax trees

The syntax trees defined by an abstract syntax are well-typed expressions of basic types in beta-eta normal form. Linearization defined in concrete syntax applies to all and only these expressions.

There is also a direct definition of syntax trees, which does not refer to beta and eta conversions: keeping in mind that a type always has the form

(x1 : A1) -> ... -> (xn : An) -> B
where Ai are types and B is a basic type, a syntax tree is an expression
b t1 ... tn : B'
where

Predefined types in abstract syntax

GF provides three predefined categories for abstract syntax, with predefined expressions:

category expressions
Int integer literals, e.g. 123
Float floating point literals, e.g. 12.34
String string literals, e.g. "foo"

These categories take no arguments, and they can be used as basic types in the same way as if they were introduced in cat judgements. However, it is not legal to define fun functions that have any of these types as value type: their only well-typed expressions are literals as defined in the above table.

Overview of expressions in concrete syntax

Concrete syntax is about defining mappings from abstract syntax trees to concrete syntax objects. These objects comprise

Thus functions are not concrete syntax objects; however, the mappings themselves are expressed as functions, and the source code of a concrete syntax can use functions under the condition that they can be eliminated from the final compiled grammar (which they can; this is one of the fundamental properties of compilation, as explained in more detail in the JFP article).

Concrete syntax thus has the same function types and expression forms as abstract syntax, specified here. The basic types defined by categories (cat judgements) are available via grammar reuse explained here; this also comprises the predefined categories Float and String.

Values, canonical forms, and run-time variables

In abstract syntax, the conversion rules fiven here define a computational relation among expressions, but there is no separate notion of a value of computation: the value (the end point) of a computation chain is simply an expression to which no more conversions apply. In general, we are interested in expressions that satisfy the conditions of being syntax trees (as defined here), but there can be many computationally equivalent syntax trees which nonetheless are distinct syntax trees and hence have different linearizations. The main use of computation in abstract syntax is to compare types in dependent type checking.

In concrete syntax, the notion of values is central. At run time, we want to compute the values of linearizations; at compile time, we want to perform partial evaluation, which computes expressions as far as possible. To specify what happens in computation we therefore have to distinguish between canonical forms and other forms of expressions. The canonical forms are defined separately for each form of type, whereas the other forms may usually produce expressions of any type.

What is done at compile time is the elimination of any noncanonical forms, except for those depending on run-time variables. Run-time variables are the same as the argument variables of linearization rules, i.e. the variables x1,...,xn in

lin f = \ x1,...,xn -> t
where
fun f : (x1 : A1) -> ... -> (xn : An) -> B
Notice that this definition refers to the eta-expanded linearization term, which has one abstracted variable for each argument type of f. These variables are not necessarily explicit in GF source code, but introduced by the compiler.

Since certain expression forms should be eliminated in compilation but cannot be eliminated if run-time variables appear in them, errors can appear late in compilation. This is an issue with the following expression forms:

Token lists, tokens, and strings

The most prominent basic type is Str, the type of token lists. This type is often sloppily referred to as the type of strings; but it should be kept in mind that the objects of Str are lists of strings rather than single strings.

Expressions of type Str have the following canonical forms:

For convenience, the notation is overloaded so that tokens are identified with singleton token lists, and there is no separate type of tokens (this is a change from the JFP article). The notion of a token is still important for compilation: all tokens introduced by the grammar must be known at compile time. This, in turn, is required by the parsing algorithms used for parsing with GF grammars.

In addition to string literals, tokens can be formed by a specific non-canonical operator:

Being noncanonical, gluing is equipped with a computation rule: string literals are glued by forming a new string literal, and empty token lists can be ignored:

Since tokens must be known at compile time, the operands of gluing may not depend on run-time variables, as defined here.

As syntactic sugar, token lists can be given as bracketed string literals, where spaces separate tokens:

Notice that there are no empty tokens, but the expression [] can be used in a context requiring a token, in particular in gluing expression below. Since [] denotes an empty token list, the following computation laws are valid:

Moreover, concatenation and gluing are associative:

For the programmer, associativity and the empty token laws mean that the compiler can use them to simplify string expressions. It also means that these laws are respected in pattern matching on strings.

A prime example of prefix-dependent choice operation is the following approximative expression for the English indefinite article:

    pre {"a" ; "an" / variants {"a" ; "e" ; "i" ; "o"}}

This expression can be computed in the context of a subsequent token:

The matching prefix is defined by comparing the string with the prefix of the token. If the prefix is a variant list of strings, then it matches the token if any of the strings in the list matches it.

The computation rule can sometimes be applied at compile time, but it general, prefix-dependent choices need to be passed to the run-time grammar, because they are not given a subsequent token to compare with, or because the subsequent token depends on a run-time variable.

The prefix-dependent choice expression itself may not depend on run-time variables.

In GF prior to 3.0, a specific type Strs is used for defining prefixes, instead of just variants of Str.

Records and record types

A record is a collection of objects of possibly different types, accessible by projections from the record with labels pointing to these objects. A record is also itself an object, whose type is a record type. Record types have the form

{ r1 : A1 ; ... ; rn : An }
where n >= 0, each Ai is a type, and the labels ri are distinct. A record of this type has the form
{ r1 = a1 ; ... ; rn = an }
where each #aii : "Aii. A limiting case is the empty record type {}, which has the object {}, the empty record.

The fields of a record type are its parts of the form r : A, also called typings. The fields of a record are of the form r = a, also called value assignments. Value assignments may optionally indicate the type, as in r : A = a.

The order of fields in record types and records is insignificant: two record types (or records) are equal if they have the same fields, in any order, and a record is an object of a record type, if it has type-correct value assignments for all fields of the record type. The latter definition implies the even stronger principle of record subtyping: a record can have any type that has some subset of its fields. This principle is explained further here.

All fields in a record must have distinct labels. Thus it is not possible e.g. to "redefine" a field "later" in a record.

Lexically, labels are identifiers (defined here). This is with the exception of the labels selecting bound variables in the linearization of higher-order abstract syntax, which have the form $i for an integer i, as specified here. In source code, these labels should not appear in records fields, but only in selections.

Labels occur only in syntactic positions where they cannot be confused with constants or variables. Therefore it is safe to write, as in Prelude,

    ss : Str -> {s : Str} = \s -> {s = s} ;

A projection is an expression of the form

t.r
where t must be a record and r must be a label defined in it. The type of the projection is the type of that field. The computation rule for projection returns the value assigned to that field:
{ ... ; r = a ; ... }.r ==> a
Notice that the dot notation t.r is also used for qualified names as specified here. This ambiguity follows tradition and convenience. It is resolved by the following rules (before type checking):

  1. if t is a bound variable or a constant in scope, t.r is type-checked as a projection
  2. otherwise, t.r is type-checked as a qualified name

As syntactic sugar, types and values can be shared:

Another syntactic sugar are tuple types and tuples, which are translated by endowing their unlabelled fields by the labels p1, p2,... in the order of appearance of the fields:

A record extension is formed by adding fields to a record or a record type. The general syntax involves two expressions,

R ** S
The result is a record type or a record with a union of the fields of R and S. It is therefore well-formed if

Subtyping

The possibility of having superfluous fields in a record forms the basis of the subtyping relation. That A is a subtype of B means that a : A implies a : B. This is clearly satisfied for records with superfluous fields:

The GF grammar compiler extends subtyping to function types by covariance and contravariance:

The logic of these rules is natural: if a function is returns a value in a subtype, then this value is a fortiori in the supertype. If a function is defined for some type, then it is a fortiori defined for any subtype.

In addition to the well-known principles of record subtyping and co- and contravariance, GF implements subtyping for initial segments of integers:

As the last rule, subtyping is transitive:

Tables and table types

One of the most characteristic constructs of GF is tables, also called finite functions. That these functions are finite means that it is possible to finitely enumerate all argument-value pairs; this, in turn, is possible because the argument types are finite.

A table type has the form

P => T
where P must be a parameter type in the sense defined here, whereas T can be any type.

Canonical expressions of table types are tables, of the form

table { V1 => t1 ; ... ; Vn => tn }
where V1,...,Vn is the complete list of the parameter values of the argument type P (defined here), and each ti is an expression of the value type T.

In addition to explicit enumerations, tables can be given by pattern matching,

table {p1 => t1 ; ... ; pm => tm}
where p1,....,pm is a list of patterns that covers all values of type P. Each pattern pi may bind some variables, on which the expression ti may depend. A complete account of patterns and pattern matching is given here.

A course-of-values table omits the patterns and just lists all values. It uses the enumeration of all values of the argument type P to pair the values with arguments:

table P [t1 ; ... ; tn]
This format is not recommended for GF source code, since the ordering of parameter values is not specified and therefore a compiler-internal decision.

The argument type can be indicated in ordinary tables as well, which is sometimes helpful for type inference:

table P { ... }

The selection operator !, applied to a table t and to an expression v of its argument type

t ! v
returns the first pattern matching result from t with v, as defined here. The order of patterns is thus significant as long as the patterns contain variables or wildcards. When the compiler reorders the patterns following the enumeration of all values of the argument type, this order no longer matters, because no overlap remains between patterns.

The GF compiler performs table expansion, i.e. an analogue of eta expansion defined here, where a table is applied to all values to its argument type:

t : P => T ==> table P [t ! V1 ; ... ; t ! Vn]
As syntactic sugar, one-branch tables can be written in a way similar to lambda abstractions:
\\p => t === table {p => t }
where p is either a variable or a wildcard (_). Multiple bindings can be abbreviated:
\\p,q => t === \\p => \\q => t
Case expressions are syntactic sugar for selections:
case e of {...} === table {...} ! e

Pattern matching

We will list all forms of patterns that can be used in table branches. We define their variable bindings and matching substitutions.

We start with the patterns available for all parameter types, as well as for the types Integer and Str.

The following patterns are only available for the type Str:

The following pattern is only available for the types Integer and Ints n:

All patterns must be linear: the same pattern variable may occur only once in them. This is what makes it straightforward to speak about unions of binding sets and substitutions.

Pattern matching is performed in the order in which the branches appear in the source code: the branch of the first matching pattern is followed. In concrete syntax, the type checker reject sets of patterns that are not exhaustive, and warns for completely overshadowed patterns. It also checks the type correctness of patterns with respect to the argument type. In abstract syntax, only type correctness is checked, no exhaustiveness or overshadowing.

It follows from the definition of record pattern matching that it can utilize partial records: the branch

    {g = Fem} => t

in a table of type {g : Gender ; n : Number} => T means the same as

    {g = Fem ; n = _} => t

Variables in regular expression patterns are always bound to the first match, which is the first in the sequence of binding lists. For example:

Free variation

An expressions of the form

variants {t1 ; ... ; tn}
where all ti are of the same type T, has itseld type T. This expression presents ti,...,tn as being in free variation: the choice between them is not determined by semantics or parameters. A limiting case is
variants {}
which encodes a rule saying that there is no way to express a certain thing, e.g. that a certain inflectional form does not exist.

A common wisdom in linguistics is that "there is no free variation", which refers to the situation where all aspects are taken into account. For instance, the English negation contraction could be expressed as free variation,

    variants {"don't" ; "do" ++ "not"}

if only semantics is taken into account, but if stylistic aspects are included, then the proper formulation might be with a parameter distinguishing between informal and formal style:

    case style of {Informal => "don't" ; Formal => "do" ++ "not"}

Since there is not way to choose a particular element from a ``variants` list, free variants is normally not adequate in libraries, nor in grammars meant for natural language generation. In application grammars meant to parse user input, free variation is a way to avoid cluttering the abstract syntax with semantically insignificant distinctions and even to tolerate some grammatical errors.

Permitting variants in all types involves a major modification of the semantics of GF expressions. All computation rules have to be lifted to deal with lists of expressions and values. For instance,

t ! variants {t1 ; ... ; tn} ==> variants {t ! t1 ; ... ; t ! tn}
This is done in such a way that variation does not distribute to records (or other product-like structures). For instance, variants of records,

    variants {{s = "Auto" ; g = Neutr} ; {s = "Wagen" ; g = Masc}}

is not the same as a record of variants,

    {s = variants {"Auto" ; "Wagen"} ; g = variants {Neutr ; Masc}}

Variants of variants are flattened,

variants {...; variants {t1 ;...; tn} ;...} ==> variants {...; t1 ;...; tn ;...}
and singleton variants are eliminated,
variants {t} ==> t

Local definitions

A local definition, i.e. a let expression has the form

let x : T = t in e
The type of x must be T, which also has to be the type of t. Computation is performed by substituting t for x in e:
let x : T = t in e ==> e {x = t}
As syntactic sugar, the type can be omitted if the type checker is able to infer it:
let x = t in e
It is possible to compress several local definitions into one block:
let x : T = t ; y : U = u in e === let x : T = t in let y : U = u in e
Another notational variant is a definition block appearing after the main expression:
e where {...} === let {...} in e
Curly brackets are obligatory in the where form, and can also be optionally used in the let form.

Since a block of definitions is treated as syntactic sugar for a nested let expression, a constant must be defined before it is used: the scope is not mutual, as in a module body. Furthermore, unlike in lin and oper definitions, it is not possible to bind variables on the left of the equality sign.

Function applications in concrete syntax

Fully compiled concrete syntax may not include expressions of function types except on the outermost level of lin rules, as defined here. However, in the source code, and especially in oper definitions, functions are the main vehicle of code reuse and abstraction. Thus function types and functions follow the same rules as in abstract syntax, as specified here. In particular, the application of a lambda abstract is computed by beta conversion.

To ensure the elimination of functions, GF uses a special computation rule for pushing function applications inside tables, since otherwise run-time variables could block their applications:

(table {p1 => f1 ; ... ; pn => fn } ! e) a ==> table {p1 => f1 a ; ... ; pn => fn a} ! e
Also parameter constructors with non-empty contexts, as defined here, result in expressions in application form. These expressions are never a problem if their arguments are just constructors, because they can then be translated to integers corresponding to the position of the expression in the enumaration of the values of its type. However, a constructor applied to a run-time variable may need to be converted as follows:
C...x... ==> case x of {_ => C...x}
The resulting expression, when processed by table expansion as explained here, results in C being applied to just values of the type of x, and the application thereby disappears.

Reusing top-level grammars as resources

This section is valid for GF 3.0, which abandons the "lock field" discipline of GF 2.8.

As explained here, abstract syntax modules can be opened as interfaces and concrete syntaxes as their instances. This means that judgements are, as it were, translated in the following way:

Notice that the value T of lincat definitions is not disclosed in the translation. This means that the type C remains abstract: the only ways of building an object of type C are the operations f obtained from fun and lin rules.

The purpose of keeping linearization types abstract is to enforce grammar checking via type checking. This means that any well-typed operation application is also well-typed in the sense of the original grammar. If the types were disclosed, then we could for instance easily confuse all categories that have the linearization type {s : Str}. Yet another reason is that revealing the types makes it impossible for the library programmers to change their type definitions afterwards.

Library writers may occasionally want to have access to the values of linearization types. The way to make it possible is to add an extra construction operation to a module in which the linearization type is available:

    oper MkC : T -> C = \x -> x

In object-oriented terms, the type C itself is protected, whereas MkC is a public constructor of C. Of course, it is possible to make these constructors overloaded (concept explained here), to enable easy access to special cases.

Predefined concrete syntax types

The following concrete syntax types are predefined:

The last two types are, in a way, extended by user-written grammars, since new parameter types can be defined in the way shown here, and every paramater type is also a type. From the point of view of the values of expressions, however, a param declaration does not extend PType, since all parameter types get compiled to initial segments of integers.

Notice the difference between the concrete syntax types Str and Integer on the one hand, and the abstract syntax categories String and Int, on the other. As concrete syntax types, the latter are treated in the same way as any reused categories: their objects can be formed by using syntax trees (string and integer literals).

The type name Integer replaces in GF 3.0 the name Int, to avoid confusion with the abstract syntax type and to be analogous with the Str vs. String distinction.

Predefined concrete syntax operations

The following predefined operations are defined in the resource module prelude/Predef.gf. Their implementations are defined as a part of the GF grammar compiler.

operation type explanation
PBool PType PTrue | PFalse
Error Type the empty type
Int Type the type of integers
Ints Integer -> Type the type of integers from 0 to n
error Str -> Error forms error message
length Str -> Int length of string
drop Integer -> Str -> Str drop prefix of length
take Integer -> Str -> Str take prefix of length
tk Integer -> Str -> Str drop suffix of length
dp Integer -> Str -> Str take suffix of length
eqInt Integer -> Integer -> PBool test if equal integers
lessInt Integer -> Integer -> PBool test order of integers
plus Integer -> Integer -> Integer add integers
eqStr Str -> Str -> PBool test if equal strings
occur Str -> Str -> PBool test if occurs as substring
occurs Str -> Str -> PBool test if any char occurs
show (P : Type) -> P -> Str convert param to string
read (P : Type) -> Str -> P convert string to param
toStr (L : Type) -> L -> Str find the "first" string
nonExist Str this is a special token marking
non-existing morphological forms
BIND Str this is a special token marking
that the surrounding tokens should not
be separated by space
SOFT_BIND Str this is a special token marking
that the surrounding tokens may not
be separated by space

Compilation eliminates these operations, and they may therefore not take arguments that depend on run-time variables.

The module Predef is included in the opens list of all modules, and therefore does not need to be opened explicitly.

Flags and pragmas

Some flags and their values

The flag coding in concrete syntax sets the character encoding used in the grammar. Internally, GF uses unicode, and .gfcc files are always written in UTF8 encoding. The presence of the flag coding=utf8 prevents GF from encoding an already encoded file.

The flag lexer in concrete syntax sets the lexer, i.e. the processor that turns strings into token lists sent to the parser. Some GF implementations support the following lexers.

lexer description
words (default) tokens are separated by spaces or newlines
literals like words, but integer and string literals recognized
chars each character is a token
code program code conventions (uses Haskell's lex)
text with conventions on punctuation and capital letters
codelit like code, but recognize literals (unknown words as strings)
textlit like text, but recognize literals (unknown words as strings)

The flag startcat in abstract syntax sets the default start category for parsing, random generation, and any other grammar operation that depends on category. Its legal values are the categories defined or inherited in the abstract syntax.

The flag unlexer in concrete syntax sets the lexer, i.e. the processor that turns token lists obrained from the linearizer to strings. Some GF implementations support the following unlexers.

unlexer description
unwords (default) space-separated token list
text format as text: punctuation, capitals, paragraph <p>
code format as code (spacing, indentation)
textlit like text, but remove string literal quotes
codelit like code, but remove string literal quotes
concat remove all spaces

Compiler pragmas

Compiler pragmas are a special form of comments prefixed with --#. Currently GF interprets the following pragmas.

pragma explanation
-path=PATH path list for searching modules

For instance, the line

    --# -path=.:present:prelude:/home/aarne/GF/tmp

in the top of FILE.gf causes the GF compiler, when invoked on FILE.gf, to search through the current directory (.) and the directories present, prelude, and /home/aarne/GF/tmp, in this order. If a directory DIR is not found relative to the working directory, also $(GF_LIB_PATH)/DIR is searched.

Alternative grammar input formats

While the GF language as specified in this document is the most versatile and powerful way of writing GF grammars, there are several other formats that a GF compiler may make available for users, either to get started with small grammars or to semiautomatically convert grammars from other formats to GF. Here are the ones supported by GF 2.8 and 3.0.

Old GF without modules

Before GF compiler version 2.0, there was no module system, and all kinds of judgement could be written in all files, without any headers. This format is still available, and the compiler (version 2.8) detects automatically if a file is in the current or the old format. However, the old format is not recommended because of pure modularity and missing separate compilation, and also because libraries are not available, since the old and the new format cannot be mixed. With version 2.8, grammars in the old format can be converted to modular grammar with the command

    > import -o FILE.gf

which rewrites the grammar divided into three files: an abstract, a concrete, and a resource module.

Context-free grammars

A quick way to write a GF grammar is to use the context-free format, also known as BNF. Files of this form are recognized by the suffix .cf. Rules in these files have the form

Label . Cat ::= (String | Cat)* ;
where Label and Cat are identifiers and String quoted strings.

There is a shortcut form generating labels automatically,

Cat ::= (String | Cat)* ;
In the shortcut form, vertical bars (|) can be used to give several right-hand-sides at a time. An empty right-hand side means the singleton of an empty sequence, and not an empty union.

Just like old-style GF files (previous section), contex-free grammar files can be converted to modular GF by using the -o option to the compiler in GF 2.8.

Extended BNF grammars

Extended BNF (FILE.ebnf) goes one step further from the shortcut notation of previous section. The rules have the form

Cat ::= RHS ;
where an RHS can be any regular expression built from quoted strings and category symbols, in the following ways:

RHS item explanation
Cat nonterminal
String terminal
RHS RHS sequence
RHS | RHS alternatives
RHS ? optional
RHS * repetition
RHS + non-empty repetition|

Parentheses are used to override standard precedences, where | binds weaker than sequencing, which binds weaker than the unary operations.

The compiler generates not only labels, but also new categories corresponding to the regular expression combinations actually in use.

Just like .cf files (previous section), .ebnf files can be converted to modular GF by using the -o option to the compiler in GF 2.8.

Example-based grammars

Example-based grammars (.gfe) provide a way to use resource grammar libraries without having to know the names of functions in them. The compiler works as a preprocessor, saving the result in a (.gf) file, which can be compiled as usual.

If a library is implemented as an abstract and concrete syntax, it can be used for parsing. Calls of library functions can therefore be formed by parsing strings in the library. GF has an expression format for this,

in C String
where C is the category in which to parse (it can be qualified by the module name) and the string is the input to parser. Expressions of this form are replaced by the syntax trees that result. These trees are always type-correct. If several parses are found, all but the first one are given in comments.

Here is an example, from GF/examples/animal/:

    --# -resource=../../lib/present/LangEng.gfc
    --# -path=.:present:prelude
  
    incomplete concrete QuestionsI of Questions = open Lang in {
      lincat
        Phrase = Phr ;
        Entity = N ;
        Action = V2 ;
      lin
        Who  love_V2 man_N           = in Phr "who loves men" ;
        Whom man_N love_V2           = in Phr "whom does the man love" ;
        Answer woman_N love_V2 man_N = in Phr "the woman loves men" ;
    }

The resource pragma shows the grammar that is used for parsing the examples.

Notice that the variables love_V2, man_N, etc, are actually constants in the library. In the resulting rules, such as

    lin Whom = \man_N -> \love_V2 -> 
      PhrUtt NoPConj (UttQS (UseQCl TPres ASimul PPos 
        (QuestSlash whoPl_IP (SlashV2 (DetCN (DetSg (SgQuant 
          DefArt)NoOrd)(UseN man_N)) love_V2)))) NoVoc ;

those constants are nonetheless treated as variables, following the normal binding conventions, as stated here.

The grammar of GF

The following grammar is actually used in the parser of GF, although we have omitted some obsolete rules still included in the parser for backward compatibility reasons.

This document was automatically generated by the BNF-Converter. It was generated together with the lexer, the parser, and the abstract syntax module, which guarantees that the document matches with the implementation of the language (provided no hand-hacking has taken place).

The lexical structure of GF

Identifiers

Identifiers Ident are unquoted strings beginning with a letter, followed by any combination of letters, digits, and the characters _ ' reserved words excluded.

Literals

Integer literals Integer are nonempty sequences of digits.

String literals String have the form "x"}, where x is any sequence of any characters except " unless preceded by \.

Double-precision float literals Double have the structure indicated by the regular expression digit+ '.' digit+ ('e' ('-')? digit+)? i.e.\ two sequences of digits separated by a decimal point, optionally followed by an unsigned or negative exponent.

Reserved words and symbols

The set of reserved words is the set of terminals appearing in the grammar. Those reserved words that consist of non-letter characters are called symbols, and they are treated in a different way from those that are similar to identifiers. The lexer follows rules familiar from languages like Haskell, C, and Java, including longest match and spacing conventions.

The reserved words used in GF are the following:

PType Str Strs Type
abstract case cat concrete
data def flags fun
in incomplete instance interface
let lin lincat lindef
linref of open oper
param pre printname resource
strs table transfer variants
where with

The symbols used in GF are the following:

; = : ->
{ } ** ,
( ) [ ]
- . | ?
< > @ !
* + ++ \
=> _ $ /

Comments

Single-line comments begin with --.Multiple-line comments are enclosed with {- and -}.

The syntactic structure of GF

Non-terminals are enclosed between < and >. The symbols -> (production), | (union) and eps (empty rule) belong to the BNF notation. All other symbols are terminals.

Grammar -> [ModDef]
[ModDef] -> eps
| ModDef [ModDef]
ModDef -> ModDef ;
| ComplMod ModType = ModBody
ModType -> abstract Ident
| resource Ident
| interface Ident
| concrete Ident of Ident
| instance Ident of Ident
| transfer Ident : Open -> Open
ModBody -> Extend Opens { [TopDef] }
| [Included]
| Included with [Open]
| Included with [Open] ** Opens { [TopDef] }
| [Included] ** Included with [Open]
| [Included] ** Included with [Open] ** Opens { [TopDef] }
[TopDef] -> eps
| TopDef [TopDef]
Extend -> [Included] **
| eps
[Open] -> eps
| Open
| Open , [Open]
Opens -> eps
| open [Open] in
Open -> Ident
| ( QualOpen Ident )
| ( QualOpen Ident = Ident )
ComplMod -> eps
| incomplete
QualOpen -> eps
[Included] -> eps
| Included
| Included , [Included]
Included -> Ident
| Ident [ [Ident] ]
| Ident - [ [Ident] ]
Def -> [Name] : Exp
| [Name] = Exp
| Name [Patt] = Exp
| [Name] : Exp = Exp
TopDef -> cat [CatDef]
| fun [FunDef]
| data [FunDef]
| def [Def]
| data [DataDef]
| param [ParDef]
| oper [Def]
| lincat [PrintDef]
| lindef [Def]
| linref [Def]
| lin [Def]
| printname cat [PrintDef]
| printname fun [PrintDef]
| flags [FlagDef]
CatDef -> Ident [DDecl]
| [ Ident [DDecl] ]
| [ Ident [DDecl] ] { Integer }
FunDef -> [Ident] : Exp
DataDef -> Ident = [DataConstr]
DataConstr -> Ident
| Ident . Ident
[DataConstr] -> eps
| DataConstr
| DataConstr | [DataConstr]
ParDef -> Ident = [ParConstr]
| Ident = ( in Ident )
| Ident
ParConstr -> Ident [DDecl]
PrintDef -> [Name] = Exp
FlagDef -> Ident = Ident
[Def] -> Def ;
| Def ; [Def]
[CatDef] -> CatDef ;
| CatDef ; [CatDef]
[FunDef] -> FunDef ;
| FunDef ; [FunDef]
[DataDef] -> DataDef ;
| DataDef ; [DataDef]
[ParDef] -> ParDef ;
| ParDef ; [ParDef]
[PrintDef] -> PrintDef ;
| PrintDef ; [PrintDef]
[FlagDef] -> FlagDef ;
| FlagDef ; [FlagDef]
[ParConstr] -> eps
| ParConstr
| ParConstr | [ParConstr]
[Ident] -> Ident
| Ident , [Ident]
Name -> Ident
| [ Ident ]
[Name] -> Name
| Name , [Name]
LocDef -> [Ident] : Exp
| [Ident] = Exp
| [Ident] : Exp = Exp
[LocDef] -> eps
| LocDef
| LocDef ; [LocDef]
Exp6 -> Ident
| Sort
| String
| Integer
| Double
| ?
| [ ]
| data
| [ Ident Exps ]
| [ String ]
| { [LocDef] }
| < [TupleComp] >
| < Exp : Exp >
| ( Exp )
Exp5 -> Exp5 . Label
| Exp6
Exp4 -> Exp4 Exp5
| table { [Case] }
| table Exp6 { [Case] }
| table Exp6 [ [Exp] ]
| case Exp of { [Case] }
| variants { [Exp] }
| pre { Exp ; [Altern] }
| strs { [Exp] }
| Ident @ Exp6
| Exp5
Exp3 -> Exp3 ! Exp4
| Exp3 * Exp4
| Exp3 ** Exp4
| Exp4
Exp1 -> Exp2 + Exp1
| Exp2
Exp -> Exp1 ++ Exp
| \ [Bind] -> Exp
| \ \ [Bind] => Exp
| Decl -> Exp
| Exp3 => Exp
| let { [LocDef] } in Exp
| let [LocDef] in Exp
| Exp3 where { [LocDef] }
| in Exp5 String
| Exp1
Exp2 -> Exp3
[Exp] -> eps
| Exp
| Exp ; [Exp]
Exps -> eps
| Exp6 Exps
Patt2 -> _
| Ident
| Ident . Ident
| Integer
| Double
| String
| { [PattAss] }
| < [PattTupleComp] >
| ( Patt )
Patt1 -> Ident [Patt]
| Ident . Ident [Patt]
| Patt2 *
| Ident @ Patt2
| - Patt2
| Patt2
Patt -> Patt | Patt1
| Patt + Patt1
| Patt1
PattAss -> [Ident] = Patt
Label -> Ident
| $ Integer
Sort -> Type
| PType
| Str
| Strs
[PattAss] -> eps
| PattAss
| PattAss ; [PattAss]
[Patt] -> Patt2
| Patt2 [Patt]
Bind -> Ident
| _
[Bind] -> eps
| Bind
| Bind , [Bind]
Decl -> ( [Bind] : Exp )
| Exp4
TupleComp -> Exp
PattTupleComp -> Patt
[TupleComp] -> eps
| TupleComp
| TupleComp , [TupleComp]
[PattTupleComp] -> eps
| PattTupleComp
| PattTupleComp , [PattTupleComp]
Case -> Patt => Exp
[Case] -> Case
| Case ; [Case]
Altern -> Exp / Exp
[Altern] -> eps
| Altern
| Altern ; [Altern]
DDecl -> ( [Bind] : Exp )
| Exp6
[DDecl] -> eps
| DDecl [DDecl]