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4.9. THE TYPE LIBRARY: TYPE VALUES AND DATATYPES
type α ab = A of α| B of int
module Poly_ab =
Datatype.Polymorphic
(struct
type α t = α ab
let name ty = Type.name ty ^ " ab"
let module_name = "Ab"
let reprs ty = [ A ty ]
let structural_descr d =
Structural_descr.Structure
(Structural_descr.Sum
[| [| Structural_descr.pack d |]; [| Structural_descr.p_int |] |]
let mk_equal f x y = match x, y with
| A x, A y → f x y
| B x, B y → x = y
| A _, B _ | B _, A _ → false
let mk_compare f x y = match x, y with
| A x, A y → f x y
| B x, B y → Pervasives.compare x y
| A _, B _ → 1
| B _, A _ → -1
let mk_hash f = function A x → f x | B x → 257 * x
let map f = function A x → A (f x) | B x → B x
let mk_internal_pretty_code f prec_caller fmt = function
| A x →
Type.par
prec_caller
Type.Basic
fmt
(fun fmt → Format.fprintf fmt "A %a" (f Type.Call) x)
| B n →
Type.par
prec_caller
Type.Call
fmt
(fun fmt → Format.fprintf fmt "B %d" n)
let mk_pretty f fmt x =
mk_internal_pretty_code (fun _ → f) Type.Basic fmt x
let mk_varname _ = "ab"
let mk_mem_project mem f = function
| A x → mem f x
| B _ → false
end)
module Ab = Poly_AB.Make
(∗ datatype corresponding to the type [int ab] ∗)
module Ab_int = Ab(Datatype.Int)
(∗ datatype corresponding to the type [int list ab] ∗)
module Ab_Ab_string = Ab(Datatype.List(Datatype.Int))
(∗ datatype corresponding to the type [(string, int) Hashtbl.t ab] ∗)
module HAb = Ab(Datatype.String.Hashtbl.Make(Datatype.Int))
Clearly it is a bit painful. However you probably will never apply this functor yourself. It is
already applied for the standard OCaml polymorphic types like list and function (respectively
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