[Haskell] really undecidable instances?
wolfgang at jeltsch.net
Mon Oct 17 11:12:25 EDT 2005
Am Montag, 17. Oktober 2005 15:57 schrieben Sie:
> This one
[That is the instance declaration instance C Int a => D Char [a].]
> 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
[Could you please put citations at the top of the respective mail?]
Section 188.8.131.52 of the GHC User's guide says about the restrictions for
These restrictions ensure that context reduction terminates: each reduction
step removes one type constructor.
But as far as I can see, in my example there is also one type constructor
removed. Why isn't this allowed then?
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