[GHC] #9318: Type error reported in wrong place with repeated type family expressions

15 Jul 2014

      #9318: Type error reported in wrong place with repeated type family expressions
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        Reporter:  goldfire          |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.8.3
  checker)                           |  Differential Revisions:
        Keywords:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  None/Unknown      |              Difficulty:  Unknown
       Test Case:                    |              Blocked By:
        Blocking:                    |         Related Tickets:
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 When I say

 {{{
 {-# LANGUAGE TypeFamilies #-}

 type family F x
 type instance F Int = Bool

 foo :: F Int -> ()
 foo True = ()

 bar :: F Int -> ()
 bar 'x' = ()
 }}}

 I get

 {{{
 /Users/rae/temp/Bug.hs:7:5:
     Couldn't match type ‘Char’ with ‘Bool’
     Expected type: F Int
       Actual type: Bool
     In the pattern: True
     In an equation for ‘foo’: foo True = ()

 /Users/rae/temp/Bug.hs:10:5:
     Couldn't match type ‘Bool’ with ‘Char’
     Expected type: F Int
       Actual type: Char
     In the pattern: 'x'
     In an equation for ‘bar’: bar 'x' = ()
 }}}

 The second error is most certainly correct, but the first one is most
 certainly not. Note that the first error is reported on the definition of
 `foo`, which should type-check. Also, the "Couldn't match ..." bit doesn't
 correspond at all with the expected/actual bit. And, the expected/actual
 bit shows two types that are in fact equal.

 This behavior can be seen in HEAD (as of Jul. 2), 7.8.3, and 7.8.2. This
 is a regression from 7.6.3, where this behavior does not appear.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9318
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