[GHC] #10134: Pattern Matching Causes Infinite Type Error

[GHC]

#10134: Pattern Matching Causes Infinite Type Error
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        Reporter:  dongen            |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.4
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by dongen):

 OK Here's the simplest example. The problem goes away if you don't use
 @Positive (2*(n+d))@ but just @Positive (n+d)@.
 {{{
 > module Dummy( Dummy ) where

 > import CLaSH.Prelude

 > type Positive n
 >     = ( KnownNat n
 >       , KnownNat (n-1)
 >       , ((n-1)+1)~n
 >       )

 > data Dummy n d = Dummy
 >    { vec :: Vec n (Vec d Bool)
 >    } deriving Show

 > nextDummy
 >     :: ( KnownNat n
 >        , KnownNat d
 >        , Positive (2*(n+d))
 >        )
 >     => Dummy n d -> Dummy n d
 > nextDummy d
 >    = Dummy { vec = vec dFst }
 >    where -- d2' = nextDummy' d
 >          -- dFst = fst d2'
 >          -- dSnd = snd d2'
 >          (dFst,dSnd) = nextDummy' d

 > nextDummy'
 >     :: ( KnownNat n
 >        , KnownNat d
 >        , Positive (2*(n+d))
 >        )
 >     => Dummy n d
 >     -> ( Dummy n d, Bool )
 > nextDummy' _ = error []
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10134#comment:7>
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