[Haskell-cafe] YAHT: searchAll5
magnus at therning.org
Mon Oct 30 20:51:33 EST 2006
I can't get searchAll5 in Yet Another Haskell Tutorial to run. Ghci
complains that it can't find a MonadPlus that satisfies the required
type; it needs a MonadPlus.
I suspect this is due to the use of 'mzero' and 'mplus', without making
StateT a MonadPlus. My thought for this was to "push" mzero into the
inner monad and have 'mplus' pass on to the inner monad:
instance Monad m => MonadPlus (StateT state m) where
mzero = StateT (\s -> return (s, mzero))
(StateT m1) `mplus` (StateT m2) = StateT (\s -> do
(s1, a) <- m1 s
(s2, b) <- m2 s1
return (s2, a `mplus` b))
What's above isn't accepted by ghci, and I don't even know for sure that
my thought makes sense.
Magnus Therning (OpenPGP: 0xAB4DFBA4)
magnus at therning.org Jabber: magnus.therning at gmail.com
Software is not manufactured, it is something you write and publish.
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by patent law on written works.
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