This one can't. But it's hard to formulate a general rule. -fallow-undecidable-instances simply says that you, the programmer, take responsibility for termination. Without the flag, GHC uses a simple but sometimes over-conservative rule Simon | -----Original Message----- | From: haskell-bounces@haskell.org [mailto:haskell-bounces@haskell.org] On Behalf Of Wolfgang | Jeltsch | Sent: 17 October 2005 14:17 | To: Haskell ML | Subject: [Haskell] really undecidable instances? | | Hello, | | what ist the problem with instance declarations like the following: | | instance C Int a => D Char [a] | | Why are such declarations only allowed with -fallow-undecidable-instances in | GHC? How can they result in undecidability? | | Best wishes, | Wolfgang | _______________________________________________ | Haskell mailing list | Haskell@haskell.org | http://www.haskell.org/mailman/listinfo/haskell