Impossible happened when using ViewPattern in Arrows

[uzytkownik]

When I try to load file in ghci impossible happened:

ghc: panic! (the 'impossible' happened)
  (GHC version 6.12.1 for x86_64-unknown-linux):
	collectl/go
    ((=={v aOp} [lid]
      u{v avw} [lid]) -> {229:56-59}{229:56-59}ghc-prim:GHC.Bool.True{(w) d 6u})

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

Problematic function:

isOn :: ArrowChoice a => DSignal a c -> SF a b (Maybe c)                        
isOn (DSignal _ f u) = let sf = SF $! \_ -> proc (_, s, e) ->                   
                             case fromDynamic (_payload e) of                   
                                 Just (Event ((== u) -> True) v) ->             
                                     returnA -< (Just (f v), s, sf)             
                                 Nothing -> returnA -< (Nothing, s, sf)         
                       in sf

(Full sources will be attached. As it was found in-development so style may not be perfect)

[uzytkownik]

Current minimal file in which problem was detected

[uzytkownik]

Updated panic for minimized file:

ghc: panic! (the 'impossible' happened)
  (GHC version 6.12.1 for x86_64-unknown-linux):
	collectl/go
    ((=={v axe} [lid]
      u{v amk} [lid]) -> {31:56-59}{31:56-59}ghc-prim:GHC.Bool.True{(w) d 6u})

[uzytkownik]

If I change the ViewPattern? into if the problem does not occure.

[uzytkownik]

Even more simplifized example:

{-# LANGUAGE Arrows #-}
{-# LANGUAGE ViewPatterns #-}

import Control.Arrow

testF :: Eq a => a -> (Maybe (Maybe a)) -> Maybe a
testF v = proc x -> case x of
    Just (Just ((==v) -> True)) -> returnA -< Just v
    _                           -> returnA -< Nothing

ghc: panic! (the 'impossible' happened)
  (GHC version 6.12.1 for x86_64-unknown-linux):
	collectl/go
    ((=={v agq} [lid]
      v{v afv} [lid]) -> {8:25-28}{8:25-28}ghc-prim:GHC.Bool.True{(w) d 6u})

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

[simonpj]

Fixing this. Simon

[igloo]

Thanks for a great testcase!

[simonpj]

Great report thanks. Fixed by

Fri Apr  9 17:55:57 BST 2010  simonpj@microsoft.com
  * Fix Trac #3964: view patterns in DsArrows
  
  Just a missing case; I've eliminated the catch-all so 
  that we get a warning next time we extend HsPat

    M ./compiler/deSugar/DsArrows.lhs -1 +2

Maybe merge to 6.12.3

Simon

[igloo]

Merged.