I've not had any problems with them, though of course your mileage may
vary. Have a look at section 7.4.4.3 in
http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#multi-param-type-classes
for an explanation. Basically, if you have a cyclic class dependency
graph, the type checker won't terminate. Well, it will, but only because
there's a limit on the recursion depth. The trivial example is
instance C a => C a where ...
In practice, I'm not sure that this is really a big issue, and it does
come in handy. Perhaps someone who's actually been bitten by a problem
with undecidable instances can comment?
Bryn
Joel Reymont wrote:
This requires
{-# OPTIONS_GHC -fallow-undecidable-instances #-}
but since I'm using -fglasgow-exts in a lot of places I'm wondering if
adding undecidable instances would be a bad habit. I guess not... not
until I shoot myself in the foot :-).
Any explanation of undecidable instances, the good and the bad?
Joel
On Oct 27, 2005, at 6:49 PM, Bryn Keller wrote:
How about this?
class ArbitraryDefault a where {}
instance (Integral a, Bounded a, ArbitraryDefault a) => Arbitrary a
where
arbitrary = arbitraryBound
coarbitrary a = error "Not implemented"
instance ArbitraryDefault Word16 instance ArbitraryDefault Word32
instance ArbitraryDefault Word64
--
http://wagerlabs.com/
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