I can't get searchAll5[1] 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.
/M
[1]: http://en.wikibooks.org/wiki/Haskell/YAHT/Monads#Monad_Transformers
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Magnus Therning (OpenPGP: 0xAB4DFBA4)
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