#8968: Pattern synonyms and GADTs ----------------------------------------------+---------------------------- Reporter: kosmikus | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type checker) | Version: Resolution: | 7.8.1-rc2 Operating System: Unknown/Multiple | Keywords: Type of failure: GHC rejects valid program | Architecture: Test Case: | Unknown/Multiple Blocking: | Difficulty: Unknown | Blocked By: | Related Tickets: ----------------------------------------------+---------------------------- Comment (by kosmikus): Thanks. This is actually helping. However, it doesn't seem to work all the time. Consider this small variation of my program above: {{{ {-# LANGUAGE GADTs, KindSignatures, PatternSynonyms, ScopedTypeVariables #-} data X :: (* -> *) -> * -> * where Y :: f a -> X f (Maybe a) -- pattern C :: a -> X Maybe (Maybe a) pattern C x = Y (Just x) :: X Maybe (Maybe a) }}} The `a` variable in the type signature cannot be quantified, because it's the type of `x`. I don't seem to be able to give a type signature to `x` on the LHS. The attempt as given above results in an internal error: {{{ PatKind.hs:7:44: GHC internal error: ‘a’ is not in scope during type checking, but it passed the renamer tcl_env of environment: [(a5PN, Identifier[x::a, <NotTopLevel>])] In an expression type signature: X Maybe (Maybe a) In the expression: Y (Just x) :: X Maybe (Maybe a) In an equation for ‘$WC’: ($WC) x = Y (Just x) :: X Maybe (Maybe a) }}} Any further ideas? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8968#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler