[GHC] #14442: InstanceSigs fails

9 Nov 2017


      #14442: InstanceSigs fails
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.3
           Keywords:  InstanceSigs   |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 This may be expected behavior, resulting from instance signatures being
 allowed to be "more polymorphic" than the method `@Bool`. But better safe
 than sorry (can't think of a better title).

 This works fine:

 {{{#!hs
 {-# Language KindSignatures, GADTs, DataKinds, TypeOperators, PolyKinds,
 TypeFamilies, TypeFamilyDependencies, InstanceSigs #-}

 import Data.Kind
 import Data.Type.Equality

 type family Sing = (res :: k -> Type) | res -> k

 type instance Sing = SBool

 data SBool :: Bool -> Type where
   SFalse :: SBool False
   STrue  :: SBool True

 class EQ a where
   eq :: Sing (x::a) -> Sing (y::a) -> Maybe (x :~~: y)

 instance EQ Bool where
   eq :: Sing (x :: Bool) -> Sing (y :: Bool) -> Maybe (x :~~: y)
   eq SFalse SFalse = Just HRefl
 }}}

 Removing the kind annotation from `x :: Bool` and/or `y :: Bool` causes it
 to fail: Here I have removed it from `x`: `eq :: Sing x -> Sing (y ::
 Bool) -> Maybe (x :~~: y)`

 {{{
 $ ghci -ignore-dot-ghci B.hs
 GHCi, version 8.3.20170920: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( B.hs, interpreted )

 B.hs:19:6: error:
     • Couldn't match type ‘SBool’ with ‘Sing’
       Expected type: Sing x
         Actual type: SBool a0
     • In the pattern: SFalse
       In an equation for ‘eq’: eq SFalse SFalse = Just HRefl
       In the instance declaration for ‘EQ Bool’
     • Relevant bindings include
         eq :: Sing x -> Sing y -> Maybe (x :~~: y) (bound at B.hs:19:3)
    |
 19 |   eq SFalse SFalse = Just HRefl
    |      ^^^^^^

 B.hs:19:22: error:
     • Could not deduce: (x :: k1) ~~ ('False :: Bool)
       from the context: a0 ~ 'False
         bound by a pattern with constructor: SFalse :: SBool 'False,
                  in an equation for ‘eq’
         at B.hs:19:6-11
       or from: y ~ 'False
         bound by a pattern with constructor: SFalse :: SBool 'False,
                  in an equation for ‘eq’
         at B.hs:19:13-18
       ‘x’ is a rigid type variable bound by
         the type signature for:
           eq :: forall k1 (x :: k1) (y :: Bool).
                 Sing x -> Sing y -> Maybe (x :~~: y)
         at B.hs:18:9-54
       Expected type: Maybe (x :~~: y)
         Actual type: Maybe (x :~~: x)
     • In the expression: Just HRefl
       In an equation for ‘eq’: eq SFalse SFalse = Just HRefl
       In the instance declaration for ‘EQ Bool’
     • Relevant bindings include
         eq :: Sing x -> Sing y -> Maybe (x :~~: y) (bound at B.hs:19:3)
    |
 19 |   eq SFalse SFalse = Just HRefl
    |                      ^^^^^^^^^^
 Failed, 0 modules loaded.
 Prelude>
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14442
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