# Grammatical Framework Version 2

Highlights, versions 2.0, 2.1, and 2.2 (2.2 coming soon)

13/10/2003 - 25/11 - 2/4/2004 - 18/6 - 13/10 - 16/2/2005

Aarne Ranta

## Syntax of GF

An accurate language specification is now available.

## Summary of novelties in Versions 2.0 to 2.2

#### Module system

- Separate modules for `abstract`, `concrete`, and `resource`.
- Replaces the file-based `include` system
- Name space handling with qualified names
- Hierarchic structure (single inheritance `**`) + cross-cutting reuse (`open`)
- Separate compilation, one module per file
- Reuse of `abstract`+`concrete` as `resource`
  Version 2.2: separate `reuse` modules no longer needed
- Parametrized modules: `interface`, `instance`, `incomplete`.
- New experimental module types: `transfer`, `union`.
- Version 2.1: multiple inheritance in module extension.

  #### Canonical format GFC

- The target of GF compiler; to reuse, just read in.
- Readable by Haskell/Java/C++/C applications.
- Version 2.1: Java interpreter available for GFC (by Björn Bringert).
- Version 2.2: new optimizations to reduce the size of GFC files

  #### New features in expression language

- Disjunctive patterns `P | ... | Q`.
- String patterns `"foo"`.
- Binding token `&+` to glue separate tokens at unlexing phase, and unlexer to resolve this.
- New syntax alternatives for local definitions: `let` without braces and `where`.
- Pattern variables can be used on lhs's of `oper` definitions.
- New Unicode transliterations (by Harad Hammarström).
- Version 2.1: Initial segments of integers (`Ints` n) available as parameter types.

  #### New shell commands and command functionalities

- `pi` = `print_info`: information on an identifier in scope.
- `h` = `help` now in long or short form, and on individual commands.
- `gt` = `generate_trees`: all trees of a given category or instantiations of a given incomplete term, up to a given depth.
- `gr` = `generate_random` can now be given an incomplete term as an argument, to constrain generation.
- `so` = `show_opers` shows all `ope` operations with a given value type.
- `pm` = `print_multi` prints the multilingual grammar resident in the current state to a ready-compiles `.gfcm` file.
- Version 2.2: several new command options
- Version 2.2: `vg` visializes the module dependency graph
- All commands have both long and short names (see help). Short names are easier to type, whereas long names make scripts more readable.
- Meaningless command options generate warnings.

  #### New editor features

- Active text field: click the middle button in the focus to send in refinement through the parser.
- Clipboard: copy complex terms into the refine menu.
- Version 2.2: text corresponding to subtrees with constraints marked with red colour

  #### Improved implementation

- Haskell source code is organized into subdirectories.
- BNF Converter is used for defining the languages GF and GFC, which also give reliable LaTeX documentation.
- Lexical rules sorted out by option `-cflexer` for efficient parsing with large lexica.
- GHC optimizations and strictness flags are used for improving performance.
- Version 2.2: started haddock documentation by using uniform module headers

  #### New parser (work in progress)

- By Peter Ljunglöf, based on MCFG.
- Much more efficient for morphology and discontinuous constituents.
- Treatment of cyclic rules.
- Version 2.1: improved generation of speech recognition grammars (by Björn Bringert).
- Version 2.1: output of Labelled BNF files readable by the BNF Converter.

  ## Abstract, concrete, and resource modules

  Judgement forms are sorted as follows:
  - abstract: `cat`, `fun`, `def`, `data`, `flags`
  - concrete: `lincat`, `cat`, `printname`, `flags`
  - resource: `param`, `oper`, `flags`
  Example:

        abstract Sums = {
          cat
            Exp ;
          fun
            One : Exp ;
            plus : Exp -> Exp -> Exp ;
        }

        concrete EnglishSums of Sums = open ResEng in {
          lincat
            Exp = {s : Str ; n : Number} ;
          lin
            One = expSg "one" ;
            sum x y = expSg ("the" ++ "sum" ++ "of" ++ x.s ++ "and" ++ y.s) ;
        }

        resource ResEng = {
          param
            Number = Sg | Pl ;
          oper
            expSG : Str -> {s : Str ; n : Number} = \s -> {s = s ; n = Sg} ;
        }

  ## Opening and extending modules

  A `concrete` or `resource` can open a `resource`. This means that
  - the names defined in `resource` can be used ("become visible")
  - but: these names are not included in ("exported from") the opening module
  A module of any type can moreover extend a module of the same type. This means that
  - the names defined in the extended module can be used ("become visible")
  - and also: these names are included in ("exported from") the extending module
  Examples of extension:

        abstract Products = Sums ** {
          fun times : Exp -> Exp -> Exp ;
        }
        -- names exported: Exp, plus, times

        concrete English of Products = EnglishSums ** open ResEng in {
          lin times x y = expSg ("the" ++ "product" ++ "of" ++ x.s ++ "and" ++ y.s) ;
        }

  Opening, but not extension, can be qualified:

        concrete NumberSystems of Systems = open (Bin = Binary), (Dec = Decimal) in {
          lin
            BZero = Bin.Zero ;
            DZero = Dec.Zero
        }

  Version 2.1 introduces `multiple inheritance`: a module can extend several modules at the same time, for instance,

        abstract Dialogue = User, System ** { ...}

  may be used to put together "User's moves" and "System's moves" into one Dialogue System grammar.

  ## Compiling modules

  Separate compilation assumes there is one module per file.

  The module header is the beginning of the module code up to the first left bracket (`{`). The header gives

  - the module type: `abstract`, `concrete` (`of` A), or `resource`
  - the name of the module (next to the module type keyword)
  - the names of extended modules (between `=` and `**`)
  - the names of opened modules

  filename = modulename `.` extension

  File name extensions:

  - `gf`: GF source file (uses GF syntax, is type checked and compiled)
  - `gfc`: canonical GF file (uses GFC syntax, is simply read in instead of compiled; produced from all kinds of modules)
  - `gfr`: GF resource file (uses GF syntax, is only read in; produced from `resource` modules)
  - `gfcm`: canonical multilingual GF file (uses GFC syntax, is only read in; produced from a set of `abstract` and `conctrete` modules)
  Only `gf` files should ever be written/edited manually!

  What the make facility does when compiling `Foo.gf`

  1. read the module header of `Foo.gf`, and recursively all headers from the modules it depends on (i.e. extends or opens)
  2. build a dependency graph of these modules, and do topological sorting
  3. starting from the first module in topological order, compare the modification times of each `gf` and `gfc` file:
     - if `gf` is later, compile the module and all modules depending on it
     - if `gfc` is later, just read in the module
  Inside the GF shell, also time stamps of modules read into memory are taken into account. Thus a module need not be read from a file if the module is in the memory and the file has not been modified.

  If the compilation of a grammar fails at some module, the state of the GF shell contains all modules read up to that point. This makes it faster to compile the faulty module again after fixing it.

  Use the command `po` = `print_options` to see what modules are in the state.

  To force compilation:

  - The flag -src in the import command forces compilation from source even if more recent object files exist. This is useful when testing new versions of GF.
  - The flag -retain in the import command forces reading in `gfr` files in addition to `gfc` files. This is useful when testing operations with the `cc` command.

  ### Compiler optimizations

  Version 2.2

  The sometimes exploding size of generated `gfc` and `gfr` files has made it urgent to find optimizations that reduce the size of the code. There are five combinations optimizations that can be chosen, as the value of the `optimize` flag:

  - `share`: group tables so that common branch values are shared by the use of disjunctive patterns.
  - `parametrize`: if table branches differ at most at the occurrence of the pattern, replace the expanded table by a one-branch table with a variable. If this fails, perform `share`.
  - `values`: only show the values of table branches, not the patterns.
  - `all`: try `parametrize`; if this fails, do `values`.
  - `none`: don't do any optimizations
  The `share` and `parametrize` optimizations are always just good, whereas the `values` optimization may slow down the use of the table. However, it is very good for grammars mostly consisting of the inflection tables of lexical items: it can reduce the file size by the factor of 4.

  An optimization can be selected individually for each `resource` and `concrete` module by including the judgement

        flags optimize=(share|parametrize|values|all|none) ;

  in the module body. These flags can be overridden by a flag given in the `i` command, e.g.

        i -src -optimize=none Foo.gf

  Notice that the option `-src` is needed if there already are generated files created with other optimization flags.

  ## Module search paths

  Modules can reside in different directories. Use the `path` flag to extend the directory search path. For instance,

        -path=.:../resource/russian:../prelude

  enables files to be found in three different directories. By default, only the current directory is included. If a `path` flag is given, the current directory `.` must be explicitly included if it is wanted.

  The `path` flag can be set in any of the following places:

  - when invoking GF: `gf -path=xxx`
  - when importing a module: `i -path=xxx Foo.gf`
  - as a pragma in a topmost file: `--# -path=xxx`
  A flag set on a command line overrides ones set in files.

  ## How to use GF 1.* files

  Backward compatibility with respect to old GF grammars has been a central goal. All GF grammars, from version 0.9, should work in the old way in GF2. The main exceptions are some features that are rarely used.
  - The `package` system introduced in GF 1.2, cannot be interpreted in the module system of GF 2.0, since packages are in mutual scope with the top level.
  - `tokenizer` pragmas are cannot be parsed any more. In GF 1.2, they are already replaced by `lexer` flags.
  - `var` pragmas cannot be parsed any more.

  Very old GF grammars (from versions before 0.9), with the completely different notation, do not work. They should be first converted to GF1 by using GF version 1.2.

  The import command `i` can be given the option `-old`. E.g.

        i -old tut1.Eng.g2

  But this is no more necessary: GF2 detects automatically if a grammar is in the GF1 format.

  Importing a set of GF2 files generates, internally, three modules:

        abstract tut1 = ...
        resource ResEng = ...
        concrete Eng of tut1 = open ResEng in ...

  (The names are different if the file name has fewer parts.)

  The option `-o` causes GF2 to write these modules into files.

  The flags `-abs`, `-cnc`, and `-res` can be used to give custom names to the modules. In particular, it is good to use the `-abs` flag to guarantee that the abstract syntax module has the same name for all grammars in a multilingual environmens:

        i -old -abs=Numerals hungarian.gf
        i -old -abs=Numerals tamil.gf
        i -old -abs=Numerals sanskrit.gf

  The same flags as in the import command can be used when invoking GF2 from the system shell. Many grammars can be imported on the same command line, e.g.

        % gf2 -old -abs=Tutorial tut1.Eng.gf tut1.Fin.gf tut1.Fra.gf

  To write a GF2 grammar back to GF1 (as one big file), use the command

        > pg -old

  GF2 has more reserved words than GF 1.2. When old files are read, a preprocessor replaces every identifier that has the shape of a new reserved word with a variant where the last letter is replaced by `Z`, e.g. `instance` is replaced by `instancZ`. This method is of course unsafe and should be replaced by something better.

  ## Missing features of GF 1.2 (13/10/2004)

  Generally, GF1 grammars can be automatically translated to GF2, although the result is not as good as manual, since indentation and comments are destroyed. The results can be saved in GF2 files, but this is not necessary. Some rarely used GF1 features are no longer supported (see next section). It is also possible to write a GF2 grammar back to GF1, with the command `pg -printer=old`.

  Resource libraries and some example grammars have been converted. Most old example grammars work without any changes. However, there is a new resource API with many new constructions, and which is recommended.

  Soundness checking of module depencencies and completeness is not complete. This means that some errors may show up too late.

  Latex and XML printing of grammars do not work yet.