#7558: Terrible error message when given and wanted are both insoluble ---------------------------------+------------------------------------------ Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.1 Keywords: | Os: Unknown/Multiple Architecture: Unknown/Multiple | Failure: None/Unknown Difficulty: Unknown | Testcase: Blockedby: | Blocking: Related: | ---------------------------------+------------------------------------------ In fixing the ambiguity check I came across this tricky program {{{ data T a b where MkT :: (a~Maybe b) => a -> Maybe b -> T a b

f :: T a a -> Bool f (MkT x y) = [x,y] `seq` True }}} We get an error message {{{ Frozen2.hs:12:18: Could not deduce (a ~ Maybe a) from the context (a ~ Maybe a) bound by a pattern with constructor MkT :: forall a b. a ~ Maybe b => a -> Maybe b -> T a b, in an equation for `f' at Frozen2.hs:12:4-10 `a' is a rigid type variable bound by the type signature for f :: T a a -> Bool at Frozen2.hs:11:6 Relevant bindings include f :: T a a -> Bool (bound at Frozen2.hs:12:1) x :: a (bound at Frozen2.hs:12:8) y :: Maybe a (bound at Frozen2.hs:12:10) In the expression: y In the first argument of `seq', namely `[x, y]' In the expression: [x, y] `seq` True }}} This error message is nonsense! It arises becuase the "insolubles" get both a "given" insoluble `(a~T a)` and a "wanted" insoluble with the same type. This can also arise, rather more easily, with the new ambiguity check, via an inaccessible context {{{ foo :: forall a. (a ~ T a) => a -> a }}} It's a bit obscure, but it needs fixing. -- Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7558> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler _______________________________________________ Glasgow-haskell-bugs mailing list Glasgow-haskell-bugs@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs