#8915: Instance context not respected in pattern match -------------------------------------+------------------------------------ Reporter: heisenbug | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.9 Resolution: invalid | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Changes (by goldfire): * status: new => closed * resolution: => invalid Comment: This is not how instances work, I'm afraid. You've stated that `KnownSymbol a` implies `Foo (Just a)`, but not the other way around. So, pattern matching will bring `Foo (Just a)` into the context, but that says nothing about `KnownSymbol a`. I don't have a good reference of where to point you to explaining this in detail -- sorry. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8915#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler