#15459: Wredundant-constraints does not work when constraint synonym is used -------------------------------------+------------------------------------- Reporter: flip101 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Yes, this is a little awkward * `withWarning` has two separate constraints; its type is really curried, like this: {{{ withWarning :: Show a => Ord a => [a] -> [a] }}} * `withoutWarnig` only has one constraint, a tuple. It is uncurried. This is so even though it's only a synonym, so that if you ask say `:t withoutWarning` it'll display the type with the synonym. But the result is that the compound constraint for `withoutWarning` is in fact used (by `sort`). I'm sure it'd be possible to fix this, but there's a bit of a grey zone. What about {{{ type family F a :: Constraint type instance F [a] = (Show a, Ord a) foo :: F [a] => [a] -> [a] }}} Do you expect a warning for that? A type system is a degenerate form of this. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15459#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler