#13490: Ambiguous types with constraints on new variables -------------------------------------+------------------------------------- Reporter: crockeea | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): You are walking on thin ice. Consider {{{ f :: Eq [a] => [a] -> [a] f x = [x] == [x] }}} From the RHS we need `Eq [a]` which is provided by the type signature. But GHC could ''also'' solve `Eq [a]` by using `instance Eq a => Eq [a]` (which is usually the right thing to do). Then it would need `Eq a`, ''which it cannot get from `Eq a`''. To avoid this dead end GHC tries to solve "wanted" constraints from "givens" first. But what if it was {{{ f :: (Eq b, b ~ [a]) => [a] -> [a] f x = [x] == [x] }}} Now it's not quite so obvious. I would have expected this still to work, just, because equalities are always solved first. But the ice is a bit thinner. I would have expected your example to work too, just. And so it does in HEAD, and hence in 8.2. But it'd be much more robust to put `(Typeable z)` in the context. Is there some reason you can't do that? I'll add your example as a regression test and close as 'worksforme'. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13490#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler