No the error I got was
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
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