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

4 Mar 2015

      #10134: Pattern Matching Causes Infinite Type Error
-------------------------------------+-------------------------------------
        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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