Bug in GHC 5.02.1

[Simon Peyton-Jones]

Thank you.  I fixed this in the HEAD, but your message
reminded us to transfer that fix to the 5.02 branch for the
upcoming 5.02 release.

Workaround: write a selector for the newtype manually
instead of using the record syntax.

Simon

| -----Original Message-----
| From: Arthur Baars [mailto:arthurb at cs.uu.nl] 
| Sent: 21 December 2001 12:41
| To: glasgow-haskell-bugs at haskell.org
| Subject: Bug in GHC 5.02.1
| 
| 
| I got the following message from GHC version 5.02.1 when 
| compiling one of my programs. I compiled using the 
| -fglasgow-exts flag on a Windows 2000 machine; on Solaris 
| (with GHC 5.02) the same error occurs.
| 
| ghc: panic! (the `impossible' happened, GHC version 5.02.1):
|         applyTypeToArgs eqa{-r5S-} eqf{-srd-} f1{-sre-}
| 
| Please report it as a compiler bug to 
| glasgow-haskell-bugs at haskell.org, or 
| http://sourceforge.net/projects/ghc/.
| 
| 
| The module Test is a stripped version of my program that 
| still produces this
| error:
| 
| module Test where
| 
| class Apply f where
|   (.$.) :: f (a->b) -> f a -> f b
|   lift  :: a ->  f a
| 
| data MyDynamic f               =  forall x . f x ::: Type x
| 
| newtype Equal a b              =  Equal{ getEqual :: forall f 
| . f a -> f b }
| 
| data Type a                    =  forall x y . Func (Equal a (x -> y))
|                                                     (Type x)
|                                                     (Type y)
|                                |  Int (Equal a Int)
| 
| match                          :: Type a -> Type b -> Maybe 
| (Equal a b)
| match  _ _                     =  Nothing
| 
| rewriteArg                     :: Equal b d
|                                -> Equal a (b->c)
|                                -> Equal a (d->c)
| rewriteArg a b                 =  let Proj_Arg r = getEqual a 
| (Proj_Arg b)
|                                   in  r
| 
| newtype Proj_Arg a y x         =  Proj_Arg (Equal a (x->y))
| 
| apply                          :: Apply f
|                                => MyDynamic f
|                                -> MyDynamic f
|                                -> Maybe (MyDynamic f)
| apply (f ::: ft) (a ::: at)    =
|  do Func eqf tx  ty            <- return ft
|     eqa                        <- tx `match` at
|     let  f'                    =  getEqual (rewriteArg eqa eqf) f
|     return (f' .$. a ::: ty)
| 
| 
| I have no clue what the error is about.
| I suspect that the type variable f in the type of apply and 
| the application of the rewriteArg function somehow cause 
| things to go wrong in GHC.
|  *) If I instantiate the apply function for the identity type 
| constructor
|     then the error does not occur anymore:
| 
| apply                          :: MyDynamic Id
|                                -> MyDynamic Id
|                                -> Maybe (MyDynamic Id)
| newtype Id x = Id x
| instance Apply Id
| 
|  *) If I set the right-hand side of the definition of f' to 
| undefined then
|     the error does not occur either.
| 
| 
| 
| Cheers Arthur
| 
| 
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