On Fri, Jul 17, 2009 at 10:21 AM, Wolfgang
Jeltsch
Am Freitag, 10. Juli 2009 23:41 schrieben Sie:
Additionally, the second equality you provide is just wrong.
f *> empty = empty is no more true than f *> g = g,
I don’t understand this. The equation f *> g = g is much more general than f *> empty = empty. (<|>) usually denotes non-determinism and empty should be the neutral element of non-determinism, which is failing. This leads me to f *> empty = empty.
That's too strong, unless you want to restrict Alternative to
applicative functors with reversible side-effects.
It's generally accepted that LogicT IO is an instance of MonadPlus, but
liftIO (putStrLn "effects!") >> mzero /= mzero
I would expect LogicT IO to be an instance of Alternative as well.
--
Dave Menendez