#8634: Code valid in GHC 7.6 is impossible to move over GHC 7.7 (because of liberal coverage condition) -----------------------------------+--------------------------------------- Reporter: danilo2 | Owner: Type: bug | Status: new Priority: high | Milestone: Component: Compiler | Version: 7.7 Resolution: | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: None/Unknown | Blocked By: Test Case: | Related Tickets: #1241, #2247, #8356 Blocking: | -----------------------------------+--------------------------------------- Changes (by rwbarton): * related: => #1241, #2247, #8356 Comment: {{{#!haskell class Method1 cls m func | cls -> m, cls -> func where ... }}} This means "for any type `cls`, there must be at most one type `func` for which there is an instance `Method1 cls m func`". (And the same for `m`.) {{{#!haskell instance (out ~ (t1->t1)) => Method1 (Vector a) Vector_testid out where ... }}} This defines instances like `Method1 (Vector Bool) Vector_testid (Int -> Int)`, `Method1 (Vector Bool) Vector_testid (Char -> Char)`, etc., so it violates the functional dependency. So, it was a (long-standing) bug that GHC 7.6 allowed this instance declaration. See the related tickets for further discussion. As for how to fix your program: it's hard to see what's going on with the `Call` type class, but can you try dropping both functional dependencies and writing {{{#!haskell instance (m ~ Vector_testid, out ~ (t1->t1)) => Method1 (Vector a) m out where ... }}} I'll leave this ticket open as several people have asked for an option to relax this functional dependency sanity condition, but I don't think it's a very good idea myself; the condition seems to usually catch real bugs. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8634#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler