[Haskell] really undecidable instances?

Simon Peyton-Jones simonpj at microsoft.com
Mon Oct 17 09:57:44 EDT 2005

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


| -----Original Message-----
| From: haskell-bounces at haskell.org [mailto:haskell-bounces at 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
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