#9893: Switching on TypeFamilies extension stops code from typechecking -------------------------------------+------------------------------------- Reporter: phischu | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Resolution: invalid | Keywords: Operating System: | Architecture: x86_64 (amd64) Unknown/Multiple | Difficulty: Unknown Type of failure: GHC | Blocked By: rejects valid program | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Changes (by simonpj): * status: new => closed * resolution: => invalid Comment: This is confusing, but it's by design. With `TypeFamilies` we also get `MonoLocalBinds`, so that `f`'s definition (which is local) is not generalised. But `runST` requires a polymorphic `f`; and a monomorphic `f` isn't good enough. Hence the error. Giving a type signature to `f` is enough. (Giving a type signature to its right hand side, on the other hand, is not enough Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9893#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler