Re: [Caml-list] still silly issues on polymorphic types

[Walter Cazzola <cazzola@dico.unimi.it>]

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On Tue, 27 Sep 2011, Jacques Garrigue wrote:

> On 2011/09/27, at 20:46, Walter Cazzola wrote:
>
>> Dear all,
>> I'm still playing around with functors, modules and polymorphism but
>> I've some problems with this last concept.
>>
>> In the attached files I have tried to implement a sort of function with
>> a variable number of arguments (based on continuation) and to generalize
>> the approach I've used a functor (OpVarADT) where I defined the
>> operation of type 'a -> 'b -> 'c but seems that it is not general enough
>> to contain int->int->int or 'a -> 'a list -> 'a list
>>
>> This is the functor instantiation with the errors I get:
>>
>>  # module M0 = Continuation(StringConcat) ;;
>>  Error: Signature mismatch:
>>         Modules do not match:
>>           sig val op : 'a -> 'a list -> 'a list val init : 'a list end
>>         is not included in
>>           OpVarADT.OpVarADT
>>         Values do not match:
>>           val op : 'a -> 'a list -> 'a list
>>         is not included in
>>           val op : 'a -> 'b -> 'c

> It seems that your continuation functor has not the right specification.
> Namely, there is no way you can provide a meaningful function of
> type 'a -> 'b -> 'c (i.e. a -> b -> c for all possible a, b, and c)
> other than raising an exception or going into an infinite loop.
> I think that you meant something else, like

> type a and b and c
> val op : a -> b -> c

> which would let you give a specific function for op.

uhm, I've tried to follow your advice but the problem persists (attached
the new version) when I try to apply the functor I get the following
error:

# module M1 = Continuation(Sum) ;;
Error: Signature mismatch:
        Modules do not match:
          sig
            type a = int
            and b = int
            and c = int
            val op : int -> int -> int
            val init : int
          end
        is not included in
          OpVarADT.OpVarADT
        Values do not match:
          val op : int -> int -> int
        is not included in
          val op : 'a -> 'b -> 'c

But probably I misinterpreted your advice.

Walter

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module Continuation (OP : OpVarADT.OpVarADT) = 
  struct
    let arg x = fun y continuation -> continuation (OP.op x y) ;;
    let stop x = x;;
    let f g = g OP.init;;
  end

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module type OpVarADT = 
 sig
   type a and b and c
   val op: a -> b -> c
   val init : 'c
 end

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module Sum = struct
  type a=int and b=int and c=int
  let op = fun x y -> x+y ;;
  let init = 0 ;;
end