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example.ml

  1.  
  2. open Batteries
  4. module type N = sig
  5. type t
  6. val make: unit -> t
  7. val add_to_self: t -> t -> unit
  8. val as_float: t -> float
  9. end
  11. module Int: N = struct
  12. type t = int ref
  13. let make () = ref 1
  14. let add_to_self x y = x := !x + !y
  15. let as_float = (!) |- float_of_int
  16. end
  18. module Float: N = struct
  19. type t = float ref
  20. let make () = ref 0.5
  21. let add_to_self x y = x := !x +. !y
  22. let as_float = (!) |- identity
  23. end
  25. let which_module = function
  26. | "float" -> (module Float: N)
  27. | "int" -> (module Int: N)
  28. | _ -> invalid_arg "which_module"
  30. let do_repetitive_work m x =
  31. let module C = (val m: N) in
  32. C.make () |> C.add_to_self x;
  33. C.add_to_self x x
  35. let () =
  36. let module C = (val (which_module Sys.argv.(1)): N) in
  37. let x = C.make () in
  38. do_repetitive_work (module C: N) x;
  39. C.as_float x |> Printf.printf "%g\n"
  40.  

error.txt

  1.  
  2. File "example.ml", line 31, characters 29-30:
  3. Error: This expression has type 'a but an expression was expected of type C.t
  4. The type constructor C.t would escape its scope
  5.  
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