"panic! (the `impossible' happened): Prelude.head: empty list

[Simon Peyton-Jones]

Thanks!  I've fixed this, and added a test.

Simon

| -----Original Message-----
| From: glasgow-haskell-bugs-bounces at haskell.org
[mailto:glasgow-haskell-bugs-
| bounces at haskell.org] On Behalf Of Ian Lynagh
| Sent: 14 October 2005 20:51
| To: glasgow-haskell-bugs at haskell.org
| Subject: "panic! (the `impossible' happened): Prelude.head: empty list
| 
| 
| When I try to compile this module:
| 
| -----------------------------------
| module Q where
| 
| data FooBar a b = Foo | Bar
| 
| data P a b = P0 | P1 (FooBar a b)
| 
| data PS p a c where
|     (:<=>) :: p a b -> p b c -> PS p a c
| 
| f :: PS P a c -> Int
| f ((P1 _ :: P a b)      :<=> (P0 :: P b c))          = undefined
| f ((P1 Bar :: P a b)    :<=> (P1 Bar :: P b c))      = undefined
| f (P1 (_ :: FooBar a b) :<=> P1 (Bar :: FooBar b c)) = undefined
| -----------------------------------
| 
| I get this panic from ghc 6.4.1:
| 
| $ ghc -fglasgow-exts -c Q.hs
| ghc-6.4.1: panic! (the `impossible' happened, GHC version 6.4.1):
|         Prelude.head: empty list
| 
| Please report it as a compiler bug to
glasgow-haskell-bugs at haskell.org,
| or http://sourceforge.net/projects/ghc/.
| 
| $
| 
| 
| Thanks
| Ian
| 
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