[Haskell] really undecidable instances?

Wolfgang Jeltsch wolfgang at jeltsch.net
Mon Oct 17 09:16:59 EDT 2005


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,

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