[GHC] #10082: Church Booleans - xor

[GHC]

#10082: Church Booleans - xor
-------------------------------------+-------------------------------------
              Reporter:  honza889    |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.9
              Keywords:              |  Operating System:  Linux
          Architecture:  x86_64      |   Type of failure:  Incorrect result
  (amd64)                            |  at runtime
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
-------------------------------------+-------------------------------------
 When I use following implementation of Church Booleans, all is ok:
 {{{#!hs
 lTrue = \x y -> x
 lFalse = \x y -> y
 lNot = \t -> t lFalse lTrue
 lXor = \u v -> u (lNot v) (lNot lNot v)
 lXor' = \u v -> u (v lFalse lTrue) (v lTrue lFalse)
 }}}

 But this simplified versions of lXor fails, if first parameter is lFalse:
 {{{#!hs
 lXor'' = \u v -> u (lNot v) (v)  -- not work, bug in ghc?
 lXor''' = \u v -> u (lNot v) v  -- not work, bug in ghc?
 }}}

 {{{#!hs
 GHCi, version 7.10.0.20150123: http://www.haskell.org/ghc/  :? for help
 Prelude> :l lambda.hs
 [1 of 1] Compiling Main             ( lambda.hs, interpreted )
 Ok, modules loaded: Main.
 *Main> (lXor'' lFalse lFalse) 1 0

 <interactive>:3:1:
     Non type-variable argument in the constraint: Num (t5 -> t4 -> t5)
     (Use FlexibleContexts to permit this)
     When checking that ‘it’ has the inferred type
       it :: forall t4 t5. Num (t5 -> t4 -> t5) => t5 -> t4 -> t5

 <interactive>:3:24:
     Could not deduce (Num (t20 -> t30 -> t30))
       arising from the literal ‘1’
     from the context (Num (t5 -> t4 -> t5))
       bound by the inferred type of
                it :: Num (t5 -> t4 -> t5) => t5 -> t4 -> t5
       at <interactive>:3:1-26
     The type variables ‘t20’, ‘t30’ are ambiguous
     In the third argument of ‘lXor''’, namely ‘1’
     In the expression: (lXor'' lFalse lFalse) 1 0
     In an equation for ‘it’: it = (lXor'' lFalse lFalse) 1 0
 }}}

 Because both lXor implementations in first example works ok, I believe
 this is bug in ghc. But similar problem occure in older versions of ghc:

 {{{#!hs
 GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
 Loading package ghc-prim ... linking ... done.
 Loading package integer-gmp ... linking ... done.
 Loading package base ... linking ... done.
 Prelude> :l lambda.hs
 [1 of 1] Compiling Main             ( lambda.hs, interpreted )
 Ok, modules loaded: Main.
 *Main> (lXor'' lFalse lFalse) 1 0

 <interactive>:3:24:
     No instance for (Num (t20 -> t30 -> t30))
       arising from the literal `1'
     Possible fix:
       add an instance declaration for (Num (t20 -> t30 -> t30))
     In the third argument of lXor'', namely `1'
     In the expression: (lXor'' lFalse lFalse) 1 0
     In an equation for `it': it = (lXor'' lFalse lFalse) 1 0

 <interactive>:3:26:
     No instance for (Num (t50 -> t40 -> t50))
       arising from the literal `0'
     Possible fix:
       add an instance declaration for (Num (t50 -> t40 -> t50))
     In the fourth argument of lXor'', namely `0'
     In the expression: (lXor'' lFalse lFalse) 1 0
     In an equation for `it': it = (lXor'' lFalse lFalse) 1 0
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10082>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler