Re: higher-order functors

The syntax you want is something like

   funsig COROUTINE (type result) = sig ... end

   functor Generator (functor Coroutine : COROUTINE) : GENERATOR =  
struct ... end

see http://smlnj.org/doc/features.html for details.

Of course, this syntax is SML/NJ specific.

	- John

On Jan 22, 2007, at 6:16 PM, Dave Herman wrote:

> I found the need for a higher-order functor like the following:
>
>      functor Generator(functor Coroutine(type result) : COROUTINE)
>                    : GENERATOR =
>      struct
>          datatype signal = ...
>          structure C = Coroutine(type result = signal)
>          ...
>      end
>
> In other words, it takes an implementation of COROUTINE that's itself
> parameterized over the result type, and instantiates that type at
> `signal'. So a use would look like:
>
>      structure G = Generator(functor Coroutine = MyCoroutine)
>
> What's a good workaround for this kind of use case? I'd like the
> Coroutine module to be abstract in the type of values passed between
> coroutines, and I'd like the Generator module to be abstract in the
> implementation of Coroutines.
>
> (Also, it turns out I can't just change the COROUTINE signature to  
> use a
> polymorphic type 'a coroutine for implementation reasons; one of the
> implementations uses continuations in a way that has to be  
> monomorphic.)
>
> Thanks,
> Dave
>
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