package GT

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Generic programming with extensible transformations


Dune Dependency






Yet another library for generic programming. Provides syntax extensions both for camlp5 and PPX which allow decoration of type declarations with following compile-time code generation. Provides the way for creating plugins (compiled separately from the library) for enchancing supported type transformations.

Strongly reminds the visitors library from François Pottier. During desing of a library of these kind there many possible design decision and in many cases we decided to implement the decision opposite to the one used in visitors.

P.S. Since 2023 development team is no longer associated with JetBrains Research

Published: 21 Aug 2023


Branch master

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


BAP's vistors

Janestreet's PPX Traverse


opam pin add GT -y

or from the main opam repository

opam update
opam install GT -y



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

~ ocaml
OCaml version 4.14.1
Enter #help;; for help.

# #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}];;

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/ -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/

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

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

Directory structure

  • The framework for generation is in common/. The generic plugin for adding new transformations is in common/

  • 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/.


  • ppxlib

  • `camlp5

  • ocamlgraph for topological sorting

  • ocamlbuild as build system


  • 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/

  • with PPX use dune exec ppx/pp_gt.exe regression/

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.

Example: Processing Expressions

Let us have the following type for simple arithmetic expressions:

 type expr =
   Add of expr * expr
 | Mul of expr * expr
 | Int of int
 | Var of string

One of the first typical "boilerplate" tasks is printing; much like other available generic frameworks this simple goal can be achieved with our library by a little decoration of the original declaration:

 type expr =
 | Add of expr * expr
 | Mul of expr * expr
 | Int of
 | Var of GT.string [@@deriving gt ~options:{show}]

For mutually recursive type declarations add decoration only to the last type

type t = ....
and heap = t [@@deriving gt ~options:{ show }]

We replaced here int and string with and GT.string respectively, and added [@@deriving gt ~options:{show}] to the end of type declaration to make the framework generate all "boilerplate" code for us. and GT.string are two synonyms for regular standard types, equipped with some additional generic features; alternatively, we could just add open GT to the beginning of the code snippet and use short names. Further we will continue to explicitly mention features of the framework in a fully-qualified form.

Having made this, we can instantly print expressions with the following (a bit cryptic) construct:

 GT.transform(expr) (new show_expr_t) () (Mul (Var "a", Add (Int 1, Var "b")))


  • GT.transform(expr) - type-indexed function, applied to the type expr; in our framework all computations are performed by this single function;

  • new show_expr_t - an expression, which creates a transformation object, encapsulating the "show" functionality for type expr;

  • we provide unit value as additional parameter, which in fact is not used; think of it as an initial value for fold-like transformations;

  • the rest is the expression tree we're going to show.

The result of this expression evaluation, as expected, is

   Mul (Var (a), Add (Int (1), Var (b)))

In our framework (at least by now) all transformations are expressed by the following common pattern:

  GT.transform(t) tr_obj init value

or more precisely

GT.fix (fun fself init value ->
    GT.transform tree (new tr_class f_1 ... f_n fself) init value
  ) init value


  * `t` is a polymorphic type with _n_ parameters;
  * `tr_obj` - transformation object for some transformation;
  * `f_1`, ..., `f_n`  - transformation functions for type parameters;
  * `init` - some initial value (additional parameter);
  * `value` - the value to transform of type `(a_1, a_2, ..., a_n) t`.

Transformations function `f_j` usually have type `inh_j -> a_j -> syn_j`.
Types `inh_j` and `syn_j` may be arbitrary; they can be interpreted as _inherited_ and _synthesized_ attributes for type parameter transformations, if we interpret catamorphisms in  attribute-grammar fashion. For example, for "show" `inh_j` = `unit` and `s_j` = `string`.

Transformation object is an object which performs the actual transformation on a per-constructor basis; we can think of it as a collection of methods, one per data type constructor. Transformation objects can be given either implicitly by object expressions or created as instances of
transformation classes. Each class, in turn, can be generated by a system, hand-written from scratch or inherited from an existing ones.

In our example the phrase "`with show`" makes the framework to invoke a used-defined plugin, called "`show`". The architecture of the framework is developed to encourage the end-users to provide their own plugins; writing plugins is considered as an easy task.

The key feature of the approach we advocate here is that object-oriented representation of transformations makes them quite easy to modify.
For example, if we are not satisfied by the "default" behavior of "show", we can adjust it only for the "cases of interest":

 class show' fself = object
   inherit show_expr_t fself
   method c_Var _ _ s = s

GT.fix (fun fself ->
    GT.transform tree (new show' fself) ()
  (Mul (Var "a", Add (Int 1, Var "b")))

Now the result is

 Mul (a, Add (Int (1), b))

We fixed only the "case of interest"; method "c_Var" takes three arguments - the inherited attribute (which is always unit here), the original value (actually, augmented original value, see below), and immediate arguments of corresponding constructor (actually, their augmented versions). In this case "s" is just a string argument of the constructor "Var".

If we still not satisfied with the result, we can further proceed with fixing things up:

 class show'' =
   object inherit show'
     method c_Int _ _ i = string_of_int i

 GT.transform(expr) (new show'') () (Mul (Var "a", Add (Int 1, Var "b")))

The result now is

 Mul (a, Add (1, b))

In the next step we're going to switch to infix representation of operators; this case is interesting since we have to adjust the behavior of the transformation not only for the single node, but to all its sub-trees as well. Fortunately, this is easy:

 class show''' =
   object inherit show''
     method c_Add _ _ x y = x.GT.fx () ^ " + " ^ y.GT.fx ()
     method c_Mul _ _ x y = x.GT.fx () ^ " * " ^ y.GT.fx ()

 GT.transform(expr) (new show''') () (Mul (Var "a", Add (Int 1, Var "b")))

Method "c_Add" takes four arguments:

  • inherited attribute (here unit);

  • augmented original node;

  • augmented parameters of the constructor ("x" and "y").

Augmentation attaches to a value a transformation for the type of that value. Augmented value is represented as a structure with the following fields:

  • GT.x is the original value;

  • GT.f is current transformation function for the type of original value;

  • GT.fx is a (partial) application of "GT.f" to "GT.x".

In other word, the construct x.GT.fx here means "the same transformation we're dealing with right now, applied to the node x"; note that due to late binding this transformation is not necessarily that defined by the class show'''.

Only values of types, corresponding to type variables and the "root type" are augmented; in our example the only augmented values are those of type expr.

Finally, we may want to provide a complete infix representation (including a minimal amount of necessary brackets):

 class show'''' =
   let enclose op p x y =
     let prio = function
       | Add (_, _) -> 1
       | Mul (_, _) -> 2
       | _ -> 3
     let bracket f x = if f then "(" ^ x ^ ")" else x in
     bracket (p >  prio x.GT.x) (x.GT.fx ()) ^ op ^
     bracket (p >= prio y.GT.x) (y.GT.fx ())
   object inherit show'''
     method c_Mul _ _ x y = enclose "*" 2 x y
     method c_Add _ _ x y = enclose "+" 1 x y

On the final note for this example we point out that all these flavors of "show" transformation coexist simultaneously; any of them can be used as a starting point for further adjustments.

Our next example is variable-collecting function. For this purpose we add "foldl" the the list of user-defined plugins:

 @type expr =
   Add of expr * expr
 | Mul of expr * expr
 | Int of
 | Var of GT.string with show, foldl

With this plugin enabled we can easily express what we want:

 module S = Set.Make (String)
 class vars =
   object inherit [S.t] @expr[foldl]
     method c_Var s _ x = S.add x s

 let vars e = S.elements (GT.transform(expr) (new vars) S.empty e

In the default version, "@expr[foldl]" is generated in such a way that inherited attribute value (in out case of type "S.t") is simply threaded through all nodes of the data structure. This behavior as such gives us nothing; however we can redefine the "interesting case" (variable occurrence) to take this occurrence into account.

The next example - expression evaluator - demonstrates the case when we implement transformation class "from scratch". The appropriate class type is rather cumbersome; fortunately, the framework provides us some empty virtual class to inherit from:

 class eval =
   object inherit [string -> int, int] @expr
     method c_Var s _ x = s x
     method c_Int _ _ i = i
     method c_Add s _ x y = x.GT.fx s + y.GT.fx s
     method c_Mul s _ x y = x.GT.fx s * y.GT.fx s

Since we develop a new transformation, we have to take care of types for inherited and synthesized attributes (when we're extending the existing classes these types are already taken care of). Since our evaluator needs a state to bind variables, the type of inherited attribute is "string -> int" and the type of synthesized attribute is just "int". The implementations of methods are straightforward.

As a final example we consider expression simplification. This time we can make use of plugin "map", which in default implementation just copies the data structure (beware: multiplying shared substructures):

 @type expr =
   Add of expr * expr
 | Mul of expr * expr
 | Int of
 | Var of GT.string with show, foldl, map

In the first iteration we simplify additions by performing constant calculations; we also "normalize" additions in such a way, that if it has one constant operand, then this operand occupies "left" position. The normalization makes it possible to take into account the associativity of addition:

 class simplify_add =
   let (+) a b =
     match a, b with
     | Int a, Int b -> Int (a+b)
     | Int a, Add (Int b, c)
     | Add (Int a, c), Int b -> Add (Int (a+b), c)
     | Add (Int a, c), Add (Int b, d) -> Add (Int (a+b), Add (c, d))
     | _, Int _ -> Add (b, a)
     | _ -> Add (a, b)
     object inherit @expr[map]
       method c_Add _ _ x y = x.GT.fx () + y.GT.fx ()

As we can see, we again concentrated only on the "interesting case"; the implementation of infix "+" may look cumbersome, but this is an essential part of the transformation.

Equally, we can handle the simplification of multiplication:

 class simplify_mul =
   let ( * ) a b =
     match a, b with
     | Int a, Int b -> Int (a*b)
     | Int a, Mul (Int b, c)
     | Mul (Int a, c), Int b -> Mul (Int (a*b), c)
     | Mul (Int a, c), Mul (Int b, d) -> Mul (Int (a*b), Add (c, d))
     | _, Int _ -> Mul (b, a)
     | _ -> Mul (a, b)
     inherit simplify_add
     method c_Mul _ _ x y = x.GT.fx () * y.GT.fx ()

The class "simplify_mul" implements a decent simplifier; however, it overlooks the following equalities: "0x=0", "0+x=x", and "1x=x". These cases can be easily integrated into existing implementation:

 class simplify_all =
   object inherit simplify_mul as super
     method c_Add i it x y =
       match super#c_Add i it x y with
       | Add (Int 0, a) -> a
       | x -> x
     method c_Mul i it x y =
       match super#c_Mul i it x y with
       | Mul (Int 1, a) -> a
       | Mul (Int 0, _) -> Int 0
       | x -> x

The interesting part of this implementation is an explicit utilization of a superclass' methods. It may looks at first glance that we handle only top-level case; however, due to late binding, for example, "x.GT.fx ()" in "simplify_mul" implementation is bound to the overriden transformation, which is (in this particular case) is "simplify_all".

The complete example can be found in file sample/


Known to be not supported or not taken to account:

  • non-regular recursive types

  • GADTs


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.


  • Documentation for src/ 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.


Dependencies (10)

  1. ppx_inline_test
  2. ocamlgraph
  3. conf-m4 build
  4. bisect_ppx build
  5. logger-p5 build
  6. ocamlfind build
  7. dune >= "2.8"
  8. camlp5 >= "8.00.05"
  9. ocaml >= "4.13"
  10. ppxlib <= "0.28.0"

Dev Dependencies (1)

  1. odoc with-doc

Used by (4)

  1. noCanren
  2. OCanren >= "0.3.0~alpha1"
  3. OCanren-ppx = "0.3.0~alpha1"
  4. ostap




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