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pinImplementing Type-Classes as OCaml Modules

Overload Journal #142 - December 2017 + Programming Topics   Author: Shayne Fletcher
Type classes achieve overloading in functional paradigms. Shayne Fletcher implements some as OCaml modules.

Modular type classes

In this article, we revisit the idea of type-classes first explored in a previous blog post [Fletcher16a]. This time though, the implementation technique will be by via OCaml modules inspired by the paper ‘Modular Type Classes’ [Dreyer07] by Dreyer et al.

Ad hoc polymorphism

In programming languages, there is a particular kind of polymorphism known formally called ad hoc polymorphism but better known as overloading. For example with overloading, an operator like + may be defined that works for many different kinds of numbers.

In the programming language Haskell, a language construction called type classes provides a structured way to provide for ad hoc polymorphism. The OCaml programming language does not have type classes but rather provides a construction called modules. Ad hoc polymorphism via Haskell-like typeclass style programming can be supported in OCaml by viewing type classes as a particular mode of use of modules. Indeed, the module approach can be argued as better in the sense that programmers can have explicit control over which type class instances are available in a given scope.

Starting with the basics, consider the class of types whose values can be compared for equality. Call this type-class Eq. We represent the class as a module signature.

  module type EQ = sig
    type t
    val eq : t * t → bool
  end

Specific instances of Eq are modules that implement this signature. Here are two examples.

  module Eq_bool : EQ with type t = bool = struct
    type t = bool
    let eq (a, b) = a = b
  end
  module Eq_int : EQ with type t = int = struct
    type t = int
    let eq (a, b) = a = b
  end

Given instances of class Eq (X and Y say,) we realize that products of those instances are also in Eq. This idea can be expressed as a functor with the following type.

  module type EQ_PROD =
    functor (X : EQ) (Y : EQ) → 
      EQ with type t = X.t * Y.t

The implementation of this functor is simply stated as the following.

  module Eq_prod : EQ_PROD =
    functor (X : EQ) (Y : EQ) → struct
      type t = X.t * Y.t
      let eq ((x1, y1), (x2, y2)) 
        = X.eq (x1, x2) && Y.eq(y1, y2)
  end

With this functor we can build concrete instances for products. Here‘s one example.

  module Eq_bool_int : 
    EQ with type t = (bool * int) 
      = Eq_prod (Eq_bool) (Eq_int)

The class Eq can be used as a building block for the construction of new type classes. For example, we might define a new type-class Ord that admits types that are equality comparable and whose values can be ordered with a ‘less-than’ relation. We introduce a new module type to describe this class.

  module type ORD = sig
    include EQ
    val lt : t * t → bool
  end

Here’s an example instance of this class.

  module Ord_int : ORD with type t = int = struct
    include Eq_int
    let lt (x, y) = Pervasives.( < ) x y
  end

As before, given two instances of this class, we observe that products of these instances also reside in the class. Accordingly, we have this functor type

  module type ORD_PROD =
    functor (X : ORD) (Y : ORD) → ORD with type t 
      = X.t * Y.t

with the following implementation.

  module Ord_prod : ORD_PROD =
    functor (X : ORD) (Y : ORD) → struct
      include Eq_prod (X) (Y)
      let lt ((x1, y1), (x2, y2)) =
        X.lt (x1, x2) || X.eq (x1, x2) && 
        Y.lt (y1, y2)
  end

This is the corresponding instance for pairs of integers.

  module Ord_int_int = Ord_prod (Ord_int) (Ord_int)

Here’s a simple usage example.

  let test_ord_int_int = 
    let x = (1, 2) and y = (1, 4) in
    assert ( not (Ord_int_int.eq (x, y)) && 
      Ord_int_int.lt (x, y))

Using type-classes to implement parametric polymorphism

This section begins with the Show type-class.

  module type SHOW = sig
    type t
    val show : t → string
  end

In what follows, it is convenient to make an alias for module values of this type.

  type 'a show_impl = (module SHOW with type t = 'a)

Here are two instances of this class...

  module Show_int : SHOW with type t = int = struct
    type t = int
    let show = Pervasives.string_of_int
  end
  module Show_bool : SHOW with type t = bool 
      = struct
    type t = bool
    let show = function | true → "True" 
      | false → "False"
  end

...and here these instances are ‘packed’ as values:

  let show_int : int show_impl = 
    (module Show_int : SHOW with type t = int)
  let show_bool : bool show_impl = 
    (module Show_bool : SHOW with type t = bool)

The existence of the Show class is all that is required to enable the writing of our first parametrically polymorphic function.

  let print : 'a show_impl → 'a → unit =
    fun (type a) (show : a show_impl) (x : a) →
    let module Show = 
      (val show : SHOW with type t = a) in
    print_endline@@ Show.show x
  let test_print_1 : unit = print show_bool true
  let test_print_2 : unit = print show_int 3

The function print can be used with values of any type 'a as long as the caller can produce evidence of 'a’s membership in Show (in the form of a compatible instance).

Listing 1 begins with the definition of a type-class Num (the class of additive numbers) together with some example instances.

module type NUM = sig
  type t
  val from_int : int → t
  val ( + ) : t → t → t
end

type 'a num_impl = (module NUM with type t = 'a)

module Num_int : NUM with type t = int = struct
  type t = int
  let from_int x = x
  let ( + ) = Pervasives.( + )
end
    
let num_int = (module Num_int : NUM with 
  type t = int)

module Num_bool : NUM with type t = bool = struct
  type t = bool

  let from_int = function | 0 → false 
    | _ → true
  let ( + ) = function | true → fun _ → true 
    | false → fun x → x
end
  
let num_bool = 
  (module Num_bool : NUM with type t = bool)
			
Listing 1

The existence of Num admits writing a polymorphic function sum that will work for any 'a list of values if only 'a can be shown to be in Num.

  let sum : 'a num_impl → 'a list → 'a =
    fun (type a) (num : a num_impl) (ls : a list) →
      let module Num = 
        (val num : NUM with type t = a) in
      List.fold_right Num.( + ) ls (Num.from_int 0)
  let test_sum = sum num_int [1; 2; 3; 4]

This next function requires evidence of membership in two classes.

  let print_incr : ('a show_impl * 'a num_impl)
      → 'a → unit =
    fun (type a) ((show : a show_impl),
      (num : a num_impl)) (x : a) →
      let module Num = 
        (val num : NUM with type t = a) in
      let open Num
      in print show (x + from_int 1)
  (*An instantiation*)
  let print_incr_int (x : int) : unit 
    = print_incr (show_int, num_int) x

If 'a is in Show then we can easily extend Show to include the type 'a list. As we saw earlier, this kind of thing can be done with an appropriate functor. (See Listing 2.)

module type LIST_SHOW =
  functor (X : SHOW) → 
    SHOW with type t = X.t list

  module List_show : LIST_SHOW =
    functor (X : SHOW) → struct
      type t = X.t list
    
      let show =
        fun xs →
          let rec go first = function
            | [] → "]"
            | h :: t →
              (if (first) then "" else ", ") 
               ^ X.show h ^ go false t 
          in "[" ^ go true xs
    end
			
Listing 2

There is also another way: one can write a function to dynamically compute an 'a list show_impl from an 'a show_impl (see Listing 3).

let show_list : 'a show_impl → 'a list show_impl
   = fun (type a) (show : a show_impl) →
    let module Show = 
      (val show : SHOW with type t = a) in
    (module struct
      type t = a list
      let show : t → string =
        fun xs →
          let rec go first = function
            | [] → "]"
            | h :: t →
              (if (first) then "" else ", ") 
                 ^ Show.show h ^ go false t
          in "[" ^ go true xs
    end : SHOW with type t = a list)

let testls : string = let module Show =
  (val (show_list show_int) 
  : SHOW with type t = int list) in
  Show.show (1 :: 2 :: 3 :: [])
			
Listing 3

The type-class Mul is an aggregation of the type-classes Eq and Num together with a function to perform multiplication. (Listing 4.)

module type MUL = sig
  include EQ
  include NUM with type t := t

  val mul : t → t → t
end

type 'a mul_impl = (module MUL with type t = 'a)

module type MUL_F =
   functor (E : EQ) (N : NUM with type t = E.t) 
   → MUL with type t = E.t
			
Listing 4

A default instance of Mul can be provided given compatible instances of Eq and Num. (See Listing 5.)

module Mul_default : MUL_F =
  functor (E : EQ) (N : NUM with type t = E.t)  
      → struct
    include (E : EQ with type t = E.t)
    include (N : NUM with type t := E.t)

    let mul : t → t → t =
      let rec loop x y = begin match () with
         | () when eq (x, (from_int 0)) 
           → from_int 0
         | () when eq (x, (from_int 1)) → y
         | () → y + loop (x + (from_int (-1))) y
       end in loop
end

module Mul_bool : MUL with type t = bool = 
  Mul_default (Eq_bool) (Num_bool)   
			
Listing 5

Specific instances can be constructed as needs demand (Listing 6).

module Mul_int : MUL with type t = int = struct
  include (Eq_int : EQ with type t = int)
  include (Num_int : NUM with type t := int)
  let mul = Pervasives.( * )
end
    
let dot : 'a mul_impl → 'a list → 'a list → 'a
  = fun (type a) (mul : a mul_impl) →
    fun xs ys →
      let module M = 
        (val mul : MUL with type t = a) in
      sum (module M : NUM with type t = a)
        @@ List.map2 M.mul xs ys
let test_dot =
  dot (module Mul_int : MUL with type t = int) 
    [1; 2; 3] [4; 5; 6]
			
Listing 6

Note that in this definition of dot, coercion of the provided Mul instance to its base Num instance is performed.

Listing 7 provides an example of polymorphic recursion utilizing the dynamic production of evidence by way of the show_list function presented earlier.

let rec replicate : int → 'a → 'a list =
  fun n x → if n <= 0 
    then [] else x :: replicate (n - 1) x
let rec print_nested : 'a. 'a show_impl → 
  int → 'a → unit = fun show_mod → function
  | 0 → fun x → print show_mod x
  | n → fun x → print_nested 
    (show_list show_mod) (n - 1) (replicate n x)
let test_nested =
  let n = read_int () in
  print_nested (module Show_int : SHOW 
  with type t = int) n 5
			
Listing 7

This article was previously published as a blog post in 2016. [Fletcher16b] and the source is available at: https://github.com/shayne-fletcher/overload-2017/blob/master/mod.ml

References

[Dreyer07] Derek Dreyer, Robert Harper and Manuel M. T. Chakravarty, ‘Modular Type Classes’, 2007, available online at http://www.cse.unsw.edu.au/~chak/papers/mtc-popl.pdf

[Fletcher16a] Shayne Fletcher, ‘Haskell type-classes in OCaml and C++’, available at http://blog.shaynefletcher.org/2016/10/haskell-type-classes-in-ocaml-and-c.html

[Fletcher16b] Shayne Fletcher, ‘Implementing type-classes as OCaml modules’, available at http://blog.shaynefletcher.org/2016/10/implementing-type-classes-as-ocaml.html

[Kiselyov14] Oleg Kiselyov, ‘Implementing, and Understanding Type Classes’, updated November 2014, available at http://okmij.org/ftp/Computation/typeclass.html

Overload Journal #142 - December 2017 + Programming Topics