GHC bug,or Hugs feature?

[Mark Tullsen]

I believe the incompatibilities are explained thus:

  In section 4.5.1 of the Haskell Report it only states that
   "A dependency analysis transformation is first performed to increase
    polymorphism"

  But hugs appears to be using a more refined version of the dependency
  analysis as explained in section 11.6.3 of Mark Jones' paper Typing
  Haskell in Haskell.  Read that section.

- Mark


Arthur Baars wrote:
> In Mark Jones' paper Typing Haskell in Haskell, I found the following
> example(in the section on binding-groups):
> 
> f   :: Eq a => a -> Bool
> f x = x==x || g True
> g y = y<=y || f True
> 
> According to the paper the inferred type of g should be:
>  g::Ord a => a -> Bool
> 
> Hugs infers this type but GHC infers the following *ambiguous* type:
> *Main> :i g
> -- g is a variable, defined at Test.hs:25
> g :: forall a. (Eq a) => Bool -> Bool
> 
> When adding an explicit type signature for g, Hugs happily accepts the code,
> but GHC gives the following error:
> 
> f   :: Eq a => a -> Bool
> f x = x==x || g True
> g   :: Ord a => a -> Bool
> g y = y<=y || f True
> 
> Test.hs:24:
>     Couldn't match `{Ord a}' against `{Eq a1}'
>     When matching the contexts of the signatures for
>       g :: forall a. (Ord a) => a -> Bool
>       f :: forall a. (Eq a) => a -> Bool
>     The signature contexts in a mutually recursive group should all be
> identical
>     When generalising the type(s) for g, f
> Failed, modules loaded: none.
> 
> I think the problems are caused by differences in the binding group analysis
> in Hugs and GHC. 
> 
> Malcolm, could you check what NHC says about the examples above?
> 
> Cheers, 
>  Arthur
> 
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