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From: Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>
In-Reply-To: <alpine.LFD.2.00.1109271335550.8918@surtur.dico.unimi.it>
Date: Tue, 27 Sep 2011 22:16:36 +0900
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To: Walter Cazzola <cazzola@dico.unimi.it>
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Subject: Re: [Caml-list] still silly issues on polymorphic types

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.

Jacques Garrigue