Datatype-generic object-oriented transformations for OCaml (a.k.a. GT)

This library implements a framework for datatype-generic programming in Objective Caml language.

The key feature of the approach in question is object-oriented representation of transformations performed over regular algebraic datatypes. Our implementation supports polymorphic variants; in particular, a transformation for a "joined" polymorphic variant type can be acquired via inheritance from the transformations for its counterparts.

See also

visitors

BAP's vistors

Janestreet's PPX Traverse

Installation

opam pin add GT https://github.com/PLTools/GT.git -y

or from the main opam repository

opam update
opam install GT -y

Usage

As PPX

Use findlib package GT.ppx in combination with ppxlib. See ppxlib's manual for full guidance. In short do

# #use "topfind";;
# #require "GT";;
# #require "GT.ppx_all";;
../GT/ppx_all: added to search path
../GT/ppx_all/./ppx.exe --as-ppx: activated

# type 'a list = Nil | Cons of 'a * 'a list [@@deriving gt ~options:{fmt; show}];;
type 'a list = Nil | Cons of 'a * 'a list
class virtual ['ia, 'a, 'sa, 'inh, 'extra, 'syn] list_t :
  object
    method virtual c_Cons : 'inh -> 'extra -> 'a -> 'a list -> 'syn
    method virtual c_Nil : 'inh -> 'extra -> 'syn
  end
val gcata_list :
  ('a, 'typ0__003_, 'b, 'c, 'typ0__003_ list, 'd) #list_t ->
  'c -> 'typ0__003_ list -> 'd = <fun>
class ['a, 'b] show_list_t :
  (unit -> 'a -> string) ->
  (unit -> 'a list -> string) ->
  object
    constraint 'b = 'a list
    method c_Cons : unit -> 'a list -> 'a -> 'a list -> string
    method c_Nil : unit -> 'a list -> string
  end
class ['a, 'b] fmt_list_t :
  (Format.formatter -> 'a -> unit) ->
  (Format.formatter -> 'a list -> unit) ->
  object
    constraint 'b = 'a list
    method c_Cons : Format.formatter -> 'a list -> 'a -> 'a list -> unit
    method c_Nil : Format.formatter -> 'a list -> unit
  end
val fmt_list :
  (Format.formatter -> 'a -> unit) -> Format.formatter -> 'a list -> unit =
  <fun>
val list :
  (('a, 'b, 'c, 'd, 'b list, 'e) #list_t -> 'd -> 'b list -> 'e,
   < fmt : (Format.formatter -> 'f -> unit) ->
           Format.formatter -> 'f list -> unit;
     show : ('g -> string) -> 'g list -> string >,
   (('h -> 'i list -> 'j) -> ('k, 'i, 'l, 'h, 'i list, 'j) #list_t) ->
   'h -> 'i list -> 'j)
  GT.t = {GT.gcata = <fun>; plugins = <obj>; fix = <fun>}
val show_list : ('a -> string) -> 'a list -> string = <fun>

As Camlp5 syntax extension

Use findlib package GT.syntax.all to enable extension and all built-in plugins. To compile and see the generated code use the following command:

ocamlfind opt -syntax camlp5o -package GT.syntax.all regression/test081llist.ml -dsource

To preprocess only the code in this library (for example, a test) use the following shell command:

dune exec camlp5/pp5+gt+plugins+o.exe regression/test005.ml

To use camlp5 (>= 7.12) syntax extension in toplevel try (after installation) this:

# #use "topfind.camlp5";;
# #camlp5o;;
# #require "GT.syntax";;
# #require "GT.syntax.all";;
# @type t = GT.int with gmap,show;; (* for example *)

Directory structure

• The framework for generation is in common/. The generic plugin for adding new transformations is in common/plugin.ml.
• All built-in plugins live in plugins/ and depend on the stuff in common/.
• Camlp5-specific preprocessing plugin lives in camlp5/. Depend on stuff in common/.
• PPX-specific preprocessing plugin lives in ppx/. Depends on stuff in common/.
• Built-in plugins that represent transformations live in plugins/. Depends on common/.
• A library for built-in types and transformations for types from Pervasives live in src/. Depends on syntax extension from camlp5/ and plugins from plugins/.

Dependencies

• ppxlib
• camlp5
• ocamlgraph for topological sorting
• ocamlbuild as build system

Compilation

• make to compile whole library.
• make && make tests to compile regression tests too.

In case some of the tests do not compile use following commands to see generated code:

• with camlp5 use dune exec camlp5/pp5+gt+plugins+o.exe regression/test817logic.ml
• with PPX use dune exec ppx/pp_gt.exe regression/test801mutal.ml

To build documentation set the environment variable GT_WITH_DOCS and run opam install odoc --yes && dune build @doc. The generated HTML files will be located at _build/default/_doc/_html/index.html.

In the following section we describe our approach in a nutshell by a typical example.

Limitations

Known to be not supported or not taken to account:

• non-regular recursive types
• GADTs

TODO

Can be a bug:

• Method on_record_declaration doesn't introduce new pattern names systematically
• For compare and eq plugins in case of ADT with single constructor we generate unreachable pattern matching pattern that gives a warning.

Improvements:

• Documentation for src/GT.ml is not generated (possible because of a macro).
• Better signature for method virtual on_record_constr.
• Custom transformation functions for type parameters has become broken after introducing combinatorial interface for type abbreviations.
• Allow [@@named "..."] attribute to provide a custom name for non-latin constructors (like lists).
• Sometimes we need override class definition for a plugin. It should be possible to specify new custom class inside the attribute.

References

• Dmitry Kosarev, Dmitry Boulytchev. Generic Programming with Combinators and Objects // submitted to ML workshop 2018
• Dmitry Boulytchev. Code Reuse with Object-Encoded Transformers // A talk at the International Symposium on Trends in Functional Programming, 2014.
• Dmitry Boulytchev. Code Reuse with Transformation Objects // unpublished.
• Dmitry Boulytchev. Combinators and Type-Driven Transformers in Objective Caml // submitted to the Science of Computer Programming.