Ouch, how silly of me, I did not make a backup and have been fiddling with the code and now it works. Next time I make sure I'll make backup, sorry about that. On Fri, Feb 13, 2009 at 5:45 PM, Martijn van Steenbergen < martijn@van.steenbergen.nl> 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?

Can you share with us your function and the compiler error, or a small test case and the compiler error?

Groetjes,

Martijn.

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 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

Thanks Ryan. I'm always struggling with functional dependencies since to be honest - I don't really understand how the type inferer figures out all the types and I didn't take the time to study it yet. Your email will help me a bit further with this. My functional dependency was c -> m v. It can't be m v -> c since for the same model and view type , you can have many controllers types. On Sat, Feb 14, 2009 at 11:38 PM, Ryan Ingram wrote:

2009/2/13 Peter Verswyvelen :

...

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

No function with the type signature of panZoomedMVC' can be called (unless there is a functional dependency that uniquely determines c from m and v). It's ambiguous; there's no way to know which instance to call.

GHC allows such a function to get an inferred type, but then when it comes time to call it (and provide the Controller instance) or type check it against a provided signature, it cannot resolve the ambiguity and you get that error.

What is happening in this case is something along these lines:

1) Infer a type and constraints for panZoomedMVC':

Constraints: Controller t1 t2 t3 PanZoomable t4

Type: State -> t4 -> MVC t1 t2 -> MVC t1 t2

2) Unify the inferred type signature with your provided signature

Constraints: Controller m v t3 PanZoomable z

Type: State -> z -> MVC m v -> MVC m v

3) Verify that constraints are sufficient. This fails, because the use of Controller in the function (Controller m v t3) doesn't match the use provided by your constraint (Controller m v c).

However, leaving out the type signature doesn't help you; it just delays your problem. Because of the ambiguity, panZoomedMVC' cannot be called; you'll get the error at the callsite instead.

To solve this problem, either add a dummy argument that fixes "c", or add a functional dependency or associated type to Controller that fixes c based on m and v. For example:

...

data Proxy a = Proxy panZoomedMVC' :: (Controller m v c, PanZoomable z) => Proxy c -> State -> z -> MVC m v -> MVC m v panZoomedMVC' _ s z mvc = ...

Then you can pass the proper "Proxy" when calling the function to make the typechecker happy.

or

...

class Controller m v c | m v -> c where ...

or

...

class Controller m v where type Control m v ...

-- ryan