[Haskell-cafe] Problem type checking class-method implementation

[Stefan Holdermans]

Ralf,

> The problem is the type-scheme polymorphic result type of
> castItem which is consumed by a type-variable polymorphic
> but type-class bounded argument type of process.

Thanks for your explanation: I hope it's still safe to say that I did 
not miss something entirely obvious. :) Anyway, indeed, I do see 
bubbling that one type-class constraint to the top-level.

So, playing around, rewriting things just a little, I end up with

   instance IsProcessor p (Processor p) where
     process = Processor . unwrap . processItem

which is disappointingly the most obvious implementation anyway. 
Disappointingly, however, because it equivalent to what I had written 
in the original program, i.e., the one that brought me to consider all 
this in the first place. And there, for some reason, it did not work 
and led me to the deviation via (something equivalent to) castItem. 
Well, that's what you get from simplifying stuff. Anyway, it seems time 
to have another look at the original program and maybe come with 
another type-checking puzzle later. ;)

Thanks,

Stefan

http://www.cs.uu.nl/~stefan/