[GHC] #9258: Type inference fails with closed type families

2 Jul 2014

      #9258: Type inference fails with closed type families
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       Reporter:  augustss          |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.8.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 Consider the following module:
 {{{
 {-# LANGUAGE TypeFamilies #-}
 module M where

 type family D d a where
     D () a = Bool

 data Descr d = Descr {
     fld :: D d Double
     }

 --descrIn :: (D d Double ~ Bool) => Descr d
 descrIn = Descr { fld = True }
 }}}
 I expected ghc to infer the commented out type signature, but instead I
 get an error:
 {{{
 $ ghc -Wall -c ./test/M.hs

 test\M.hs:12:25:
     Couldn't match expected type `D d0 Double' with actual type `Bool'
     The type variable `d0' is ambiguous
     Relevant bindings include
       descrIn :: Descr d0 (bound at test\M.hs:12:1)
     In the `fld' field of a record
     In the expression: Descr {fld = True}
 }}}
 Uncommenting the type signature makes the module compile.

 As an aside, the signature I really want ghc to deduce is
 {{{
 descrIn :: Descr ()
 }}}
 But since ghc doesn't (yet) use the full information provided by the
 closed type family equations this doesn't happen.  Still, it should be
 able to figure out the commented out one.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9258
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