[GHC] #10808: Odd interaction between record update and type families

28 Aug 2015

      #10808: Odd interaction between record update and type families
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              Reporter:  diatchki    |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.2
  (Type checker)                     |
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 Consider the following example
 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}

 type family F a
 type family G a

 data T1
 type instance F T1 = Char
 type instance G T1 = Int

 data T2
 type instance F T2 = Bool
 type instance G T2 = Int

 data R a = R { x :: F a, y :: G a }

 r1 :: R T1
 r1 = R { x = 'a', y = 2 }

 r2 :: R T2
 r2 = r1 { x = True }  -- error: Cannot match T1 with T2

 r3 :: R T2
 r3 = r1 { x = True, y = y r1 } -- OK
 }}}

 The error for `r2` is odd because types `R T1` and `R T2` differ only in
 the type of `x`, so if we start with a value of type `R T1`, and update
 the field where the types differ, we should end up with a valid value of
 type `R T2`.

 The fact that `r2` is OK is demonstrated by `r3` where we update the other
 field with its own value, and now the type checker is happy.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10808
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