Could not deduce (Controller m v c)

from the context (Controller m v c2)

arising from a use of `MVC' at NM8\GUI\PanZoom.hs:126:32-65

Possible fix:

add (Controller m v c) to the context of the constructor `MVC'

In the expression: MVC m v (PZC s z (unsafeCoerce c))

In the definition of `panZoomedMVC'':

panZoomedMVC' s z (MVC m v c) = MVC m v (PZC s z (unsafeCoerce c))

I got this after adding the type signature of

panZoomedMVC' :: (Controller m v c, PanZoomable z) =>

State -> z -> MVC m v -> MVC m v

But I don't have the problematic code anymore.

Anyway, I've hacking away here, as you can see from the unsafeCoerce call, which is now not needed anymore ;)

On Fri, Feb 13, 2009 at 6:26 PM, Tillmann Rendel <rendel@cs.au.dk> wrote:

Peter Verswyvelen wrote:

Could it be considered a bug when a function compiles fine without type signature, but when you add the type signature that GHCi reports with :type, it fails to compile?

There are such cases where it is not a bug. For example, given

import Data.Map (fromList)

x a = fromList a

ghci will happily report that x has type

Ord k => [(k, a)] -> Data.Map.Map k a

but the name Data.Map.Map is not in scope in the module.

Tillmann