Found "ghc-6.4: panic!"

[pupeno@…]

I have just gotten:

ghc-6.4: panic! (the `impossible' happened, GHC version 6.4):
        Unify.unifyTauTyLists: mismatched type lists!

Please report it as a compiler bug to glasgow-haskell-bugs@haskell.org,
or http://sourceforge.net/projects/ghc/.

from ghci. The code I loaded is:

module ProgramOptions
    where

-- newtype Command n = Command [Option n]
--                   deriving (Show)
-- 
-- data Option n = Option { keys :: [String],
--                          value :: Value n,
--                          description :: String }
--               deriving (Show)
data Option = Option [String] Value String

data Num n => Value n = Boolean Bool
                      | Numeric n
                      | String String
instance Num n => Show (Value n) where
    show (Boolean b) = show b
    show (Numeric n) = show n
    show (String s) = show s

and the whole session was:

$ ghci
   ___         ___ _
  / _ \ /\  /\/ __(_)
 / /_\// /_/ / /  | |      GHC Interactive, version 6.4, for Haskell 98.
/ /_\\/ __  / /___| |      http://www.haskell.org/ghc/
\____/\/ /_/\____/|_|      Type :? for help.

Loading package base-1.0 ... linking ... done.
Prelude> :l ProgramOptions
Compiling ProgramOptions   ( ProgramOptions.hs, interpreted )
Ok, modules loaded: ProgramOptions.
*ProgramOptions> Option ["n", "name"] (String "Pupeno") "Name"
ghc-6.4: panic! (the `impossible' happened, GHC version 6.4):
        Unify.unifyTauTyLists: mismatched type lists!

Please report it as a compiler bug to glasgow-haskell-bugs@haskell.org,
or http://sourceforge.net/projects/ghc/.


*ProgramOptions>

I am reporting mainly because the interpreter asked for it, I am very new to Haskell.

[anonymous]

Fixed in 6.4.1; see 456