Re: [GHC] #10770: Typeable solver has strange effects

12 Aug 2015


      #10770: Typeable solver has strange effects
-------------------------------------+-------------------------------------
        Reporter:  NeilMitchell      |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.10.1
  checker)                           |
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
-------------------------------------+-------------------------------------
Changes (by thomie):

 * component:  Compiler => Compiler (Type checker)


Comment:

 I get the same error with HEAD (ghc-7.11.20150809), and also notice the
 following:
 {{{#!haskell
 {-# LANGUAGE FlexibleContexts #-}
 module Test where
 import Data.Typeable
 f :: (Typeable a, Typeable (Maybe a)) => Maybe a -> TypeRep
 f x = let k = typeOf x in k
 }}}
 shows the warning:
 {{{
 Redundant constraint: Typeable (Maybe a)
 }}}
 Whereas the same program, with the last line changed to:
 {{{#!haskell
 f x = typeOf x
 }}}
 shows
 {{{
 Redundant constraint: Typeable a
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10770#comment:2
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