[Haskell-cafe] Displaying infered type signature of 'offside'functions

[Stefan O'Rear]

On Sun, Apr 29, 2007 at 07:03:32PM -0500, Antoine Latter wrote:
> This looks like a good place to ask a question that's been bugging me for a
> bit:
> 
> I've had cases in my own code where I can't seem to create a type annotation
> for an "inner" declaration that the type-checker likes.  Here's a toy
> example:
> 
> In the following code:
> 
> >applyfunc :: (a -> b -> c) -> a -> b -> c
> >applyfunc f x y = doit y where
> >                  doit = f x
> 
> What type annotation can I successfully apply to the function "doit"?

There isn't one, plain and simple.  However, if you allow GHC
extensions, you can:

{-# Language ScopedTypeVariables #-}

applyfunc :: forall a b c . (a -> b -> c) -> a -> b -> c
applyfunc f x y = doit y where
                  doit :: b -> c
                  doit = f x

The 'forall' serves no other purpose than to deliberately break
haskell 98 compatibility. 

Stefan