Am Donnerstag 30 April 2009 15:52:12 schrieb Jan Jakubuv:
Hi,
I have the following problem. Below is the smallest program I have found that shows the problem. That is why the program makes no sense (I have also meaningful but more complicated program). When I run this program in ghci:
class SUBST s where empty :: s
--nonsense :: SUBST s => t -> Maybe s nonsense t = case nonsense t of Nothing -> Just empty
then everything is fine and I can see the type signature of `nonsense` inferred by ghci:
*Main> :t nonsense nonsense :: (SUBST s) => t -> Maybe s
But, when I put this signature into the code (that is, when the commented line above is uncommented) then type checking fails with the following error:
Ambiguous type variable `s' in the constraint: `SUBST s' arising from a use of `nonsense' at problem-type.hs:6:18-27 Probable fix: add a type signature that fixes these type variable(s)
Now, what is the problem here? Why does type checking fail with the signature that the type inference itself inferred?
In nonsense t = case nonsense t of Nothing -> Just empty , which type has the Nothing? It can have the type Maybe s1 for all s1 belonging to SUBST, that is the ambiguous type variable.
BTW, I don't understand why but everything works fine with the following addition:
nonsense' :: SUBST s => t -> Maybe s nonsense' t = case nonsense' t of Nothing -> Just empty x -> x
Here, Nothing must have the same type as x. Since x may be returned, x must have type Maybe s, for the type variable s of the signature, so the types are completely determined.
I am developing kind of generic interface and I don't want to fix the type `s`. I want `nonsense` to work possibly for any instance of SUBST and the concrete instance to be determined by the context where `nonsense` is used.
In my original, meaningful but more complicated example I had the following error:
Couldn't match expected type `STerm s' against inferred type `STerm s1' When generalising the type(s) for `refute'
(this message does not provide any information where `s1` comes from)
Probably something like the above. To fix the error, you could use asTypeOf: nonsense :: SUBST s => t -> Maybe s nonsense t = res where res = case nonsense t `asTypeOf` res of Nothing -> Just empty or {-# LANGUAGE ScopedTypeVariables #-} nonsense :: forall s. SUBST s => t -> Maybe s nonsense t = case nonsense t :: Maybe s of Nothing -> Just empty
The original example shares with the above one the property that the type `s` is not mentioned in types of arguments, just in the type of a result (although, in the original example, some relation between types `t` and `s` is expressed in the type context via equality constrains on associated types (STerm from the error message) ).
I tested this with ghc-6.6.1, ghc-6.10.1, ghc-6.10.2 obtaining the same result.
I'll be grateful for any explanation of this issue.
Sincerely, Jan.