#13530: Horrible error message due to TypeInType -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider this {{{ {-# LANGUAGE MagicHash, UnboxedTuples #-} module Foo where import GHC.Exts g :: Int -> (# Int#, a #) g (I# y) = (# y, undefined #) f :: Int -> (# Int#, Int# #) f x = g x }}} With GHC 8 we get {{{ Foo.hs:11:7: error: • Couldn't match a lifted type with an unlifted type Expected type: (# Int#, Int# #) Actual type: (# Int#, Int# #) }}} What a terrible error message!! It was much better in GHC 7.10: {{{ Foo.hs:11:7: Couldn't match kind ‘*’ with ‘#’ When matching types a0 :: * Int# :: # Expected type: (# Int#, Int# #) Actual type: (# Int#, a0 #) }}} What's going on? The constraint solver sees {{{ [W] alpha::TYPE LiftedRep ~ Int#::TYPE IntRep }}} So it homogenises the kinds, ''and unifies alpha'' (this did not happen in GHC 7.10), thus {{{ alpha := Int# |> TYPE co [W] co :: LiftedRep ~ IntRep }}} Of course the new constraint fails. But since we have unified alpha, when we print out the types are are unifying they both look like `(# Int#, Int# #)` (there's a suppressed cast in the second component). I'm not sure what to do here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13530 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler