#12933: Wrong class instance selection with Data.Kind.Type -------------------------------------+------------------------------------- Reporter: julm | Owner: Type: bug | Status: new Priority: highest | Milestone: Component: Compiler | Version: 8.0.1 Keywords: TypeInType | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC accepts Unknown/Multiple | invalid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- If you consider the following code: {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE FlexibleInstances #-} module Bug where import GHC.Exts (Constraint) import Data.Kind -- | Partial singleton for a kind type. data SKind k where SKiTy :: SKind Type SKiCo :: SKind Constraint instance Show (SKind k) where show SKiTy = "*" show SKiCo = "Constraint" class IKind k where kind :: SKind k instance IKind Constraint where kind = SKiCo }}} Then, the main below will compile even though there is no (IKind Type) instance, and it will print "Constraint" two times, instead of an expected "Constraint" then "*": {{{#!hs main :: IO () main = do print (kind::SKind Constraint) print (kind::SKind Type) }}} And, the main below will print "*" two times, instead of an expected "*" then "Constraint": {{{#!hs instance IKind Type where kind = SKiTy main :: IO () main = do print (kind::SKind Type) print (kind::SKind Constraint) }}} This can be worked around by replacing Type with a new data type Ty to select the right class instances, using two type families Ty_of_Type and Type_of_Ty, as done in the attached Workaround.hs. Sorry if this bug has already been fixed in HEAD: I was unable to find neither a bug report similar, nor a Linux x86_64 build of HEAD for me to test. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12933 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler