#13337: GHC doesn't think a type is of kind *, despite having evidence for it -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): The problem is that `a ~ b` is a ''lifted'' type. That is, it contains bottom. So, when you write `forall k (a :: k). k ~ Type => a -> a` or some such, there's no place to check whether the proof of `k ~ Type` is valid before you use it. When an expression intervenes, then GHC can insert a `case` to evaluate the equality proof box to find the real primitive (`~#`) equality inside. Some notes on this issue (intended solely as notes to self, so YMMV) are [https://ghc.haskell.org/trac/ghc/wiki/DependentHaskell/Internal#Liftedvs.Unl... here]. So I don't agree that solving (2) will help with (1) here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13337#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler