oleg | 1 May 21:01 2003
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Typing problems with polymorphic recursion and typeclasses


[Commenting of the problem in the message "GHC doesn't like its own
type?", Iavor Diatchki wrote]
> i remeber some time ago levent erkok run intro a simillar problem.  take 
> a look at this post:
> http://www.mail-archive.com/haskell <at> haskell.org/msg06754.html

The problems seem different. However, Levent Erkok's problem as well
as the problem in the follow-up message to categorical maxima point
out that there is something else goes on besides the explicit
typing. Let us use Levent Erkok's problem as it is simpler:

> class X a where
>   u :: a b -> a b

> f :: X a => b -> a b
> f = f

> g :: X a => a b -> b
> g = g

> -- h :: X a => b -> a c
> h b = f (g (h b))

This file loads and checks both in GHC and Hugs. The problem is that if
we uncomment the explicit type declaration of h (which is identical to
the inferred type reported by the compiler), typechecker complains.

Let us keep the function h without the explicit type declaration. Let
us apply what I thought as an equivalent transformation:
(Continue reading)

Derek Elkins | 1 May 21:29 2003

Re: Typing problems with polymorphic recursion and typeclasses

[...]
> Let us keep the function h without the explicit type declaration. Let
> us apply what I thought as an equivalent transformation:
> 
> > h = \b -> f (g (h b))
> 
> Hugs now reports
> 	ERROR "/tmp/b.hs":12 - Unresolved top-level overloading
> 	*** Binding             : h
> 	*** Outstanding context : X b
> and GHCi reports
>     /tmp/b.hs:12:
>     Ambiguous type variable(s) `a' in the constraint `X a'
>     arising from use of `f' at /tmp/b.hs:12
>     In a lambda abstraction: f (g (h b))
> 
> It is interesting that the error message is exactly the same as when
> we used the explicit type declaration for the old "h b=..."
> definition.
> 
> Something seems to be going on...

Isn't this just the monomorphism restriction.

In GHCi,
Prelude> let k = (<)
<interactive>:1:
    Ambiguous type variable(s) `a' in the constraint `Ord a'
    arising from use of `<' at <interactive>:1
    In the definition of `k': (<)
(Continue reading)


Gmane