[GHC] #7887: panic when playing with pipes

4 May 2013


      #7887: panic when playing with pipes
-----------------------------+----------------------------------------------
Reporter:  bfr               |          Owner:                  
    Type:  bug               |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  7.6.3             |       Keywords:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:                  
Blocking:                    |        Related:                  
-----------------------------+----------------------------------------------
 I am playing around with pipes-3.2.0 using ghc-6.7.3. I load this code in
 ghci:

 """
 import Control.Proxy

 fork :: (Monad m, Proxy p1, Proxy p2, Proxy p3)
  => () -> Consumer p1 a (Producer p2 a (Producer p3 a m)) r
 fork () = runIdentityP . hoist (runIdentityP . hoist runIdentityP) $
 forever $ do
   a <- request ()          -- Request from the 'Consumer'
   lift $ respond a         -- Send to the outer 'Producer'
   lift $ lift $ respond a  -- Send to the inner 'Producer'

 test :: (Proxy p) => () -> Producer p Int IO r
 test () = runIdentityP $ forever $ do
   respond 1
 """

 then I type

 *Main> :t hoist (hoist lift) $ test

 <interactive>:1:1: Warning:
     Could not deduce (Monad ()) arising from a use of `hoist'
     from the context (Monad m, MonadTrans t1, Proxy p, MFunctor t)
       bound by the inferred type of
                it :: (Monad m, MonadTrans t1, Proxy p, MFunctor t) =>
                      t (t1 m) -> Producer p Int IO r
       at Top level
     Possible fix: add an instance declaration for (Monad ())
     In the expression: hoist (hoist lift)
     In the expression: hoist (hoist lift) $ test

 <interactive>:1:1: Warning:
     Could not deduce (MFunctor (->)) arising from a use of `hoist'
     from the context (Monad m, MonadTrans t1, Proxy p, MFunctor t)
       bound by the inferred type of
                it :: (Monad m, MonadTrans t1, Proxy p, MFunctor t) =>
                      t (t1 m) -> Producer p Int IO r
       at Top level
     Possible fix: add an instance declaration for (MFunctor (->))
     In the expression: hoist (hoist lift)
     In the expression: hoist (hoist lift) $ test

 <interactive>:1:8: Warning:ghc: panic! (the 'impossible' happened)
   (GHC version 7.6.3 for x86_64-unknown-linux):
         kindFunResult
 <<details unavailable>>

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7887
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