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

[Christophe Papazian <christophe.papazian@gmail.com>]

Please remember how types can be subtypes of other types :

consider

val f : int -> int
val g : int -> 'a
val h : 'a -> int
val i : 'a -> 'b

then the most general type is the type of g
and most specific type is type of h. In fact we have a diamond  
relation between types :

h<f<g
h<i<g
and i and f are not related.

Why ?

because if you need a function of type like f (int -> int)
you can give h (if h can eat anything, it can eat int), but not g
(i don't now how to handle 'a if I only need a int).

So int is a subtype of 'a,
but int -> int is not a subtype of 'a -> 'b,
and 'a -> 'a is a valid subtype of int-> int.



Le 27 sept. 11 à 13:46, Walter Cazzola a écrit :

> 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
>  # module M1 = Continuation(Sum) ;;
>  Error: Signature mismatch:
>         Modules do not match:
>           sig 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
>
> I'm sure I'm doing something wrong but I can't see what, any help is
> welcome
>
> TIA
>
> Walter
>
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> <sum.ml><OpVarADT.mli><continuation.ml><concat.ml>