Test implicits with pairs.

show_pair.ml

➜  miniKanren-ob git:(ocamlbuild) ✗ cat test_pair.ml | nl -ba
     1  open Printf
     2
     3  let make_buffer () = Buffer.create 20
     4  let bprintf b = Printf.ksprintf (Buffer.add_string b)
     5
     6  module type SHOW = sig
     7      type t
     8      val show : t -> string
     9  end
    10
    11  let show {S : SHOW} x = S.show x;;
    12
    13  implicit module Show_float = struct
    14      type t = float
    15      let show x = sprintf "%f" x
    16  end
    17
    18  module Show_list_impl {X : SHOW} = struct
    19      type t = X.t list
    20      let show xs =
    21        let b = Buffer.create 30 in
    22        bprintf b "Show_list: [";
    23        List.iter (fun x -> bprintf b "%s" @@ X.show x) xs;
    24        bprintf b "]%!"
    25  end
    26
    27  implicit module Show_list = Show_list_impl
    28
    29  implicit module Show_int = struct
    30      type t = int
    31      let show x = sprintf "%d" x
    32  end
    33
    34  implicit module Show_string = struct
    35      type t = string
    36      let show x = x
    37  end
    38
    39  implicit module Show_pair {X : SHOW} {Y : SHOW} = struct
    40      type t = X.t * Y.t
    41      let show (x,y) = sprintf "(%s,%s)" (X.show x) (Y.show y)
    42  end
    43
    44
    45  implicit module Show_string_int = struct
    46      type t = string * int
    47      let show (x,y) = sprintf "(%s,%d)" x y
    48  end
    49
    50
    51  let () = print_endline @@ show ([ ("a", 1)] : (string*int) list )
➜  miniKanren-ob git:(ocamlbuild) ✗ ocaml test_pair.ml       
File "test_pair.ml", line 51, characters 26-30:
Error: No instance found for implicit S.