Re: higher-order functors

Sorry, I'm still struggling with this. I'm afraid I'm still new to the 
advanced module system features of SML.

>   funsig COROUTINE (type result) = sig ... end
> 
>   functor Generator (functor Coroutine : COROUTINE) : GENERATOR = struct 
> ... end

I tried the following:

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

     functor Generator (functor MkCoroutine : MK_COROUTINE) : GENERATOR =
     struct
         datatype signal = ...
         structure Coroutine : COROUTINE =
             MkCoroutine (type result = signal)
         ...
     end

But COROUTINE was too abstract since it didn't know that 
Coroutine.result = result. So I changed the declaration of structure 
Coroutine to:

     structure Coroutine : COROUTINE where type result = signal =
         MkCoroutine (type result = signal)

But I got the error "type result does not match definitional 
specification". I couldn't figure out what this error means.

At this point I was stabbing in the dark, but I tried moving the 
where-clause instead to the declaration of MK_COROUTINE:

     funsig MK_COROUTINE (type result) = COROUTINE
         where type result = result

But then I got the compiler error:

     Error: Compiler bug: LtyExtern: wrong TCs in tc_select

     I

Thanks again,
Dave

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