21 Jan
2009
21 Jan
'09
11:30 a.m.
Gleb Alexeyev:
Sjoerd Visscher wrote:
When I try this bit of code:
class C1 a where type F a :: * x :: F a y :: F a x = y I get this error: Couldn't match expected type `F a1' against inferred type `F a' In the expression: y In the definition of `x': x = y I can't figure out what is going on or how I should fix this.
The discussion [1] seems to be related.
Exactly. See also the related ticket http://hackage.haskell.org/trac/ghc-test/ticket/1897 In short, the signature "F a" is ambiguous (as `a' only occurs as a parameter of a synonym family in the signature). Functions with an ambiguous signature are generally unusable (even if the compiler accepts the definition initially). Manuel