Dan Bensen <danben...@att.net> writes:
> I'm trying to write a functor that extends a user-supplied polymorphic
> variant type (PVT). How do you declare the user's type in the signature
> for the argument to the functor without locking in the individual variant
> definitions?
> The code below (in revised syntax) generates an error message that says
> the type isn't a PVT.
>
> code:
>
>> module type Reader = sig type ast; end;
>>
>> module Make (Read: Reader) = struct
>> type ast = [= Read.ast | `Lid of string];
>> end;
>
> message:
>
>> Error: The type Read.ast is not a polymorphic variant type
>
> How do you make ast a PVT while allowing the user to
> specify the variants?
Even if that did work how usefull would that be? You couldn't write
let foo = function
| `Lid s -> ()
| x -> Read.foo x
unless Read.foo allows [> Read.ast]. In which case you don't need a
functor but can just implement foo straight up.
What is your use case? What do you want to do?
MfG
Goswin
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