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.
[1] http://hackage.haskell.org/trac/ghc-test/ticket/2855
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
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