Claus Reinke wrote:
mplus' :: MonadPlus m => Maybe a -> m a -> m a
mplus' m l = maybeToMonad m `mplus` l
maybeToMonad :: Monad m => Maybe a -> m a
maybeToMonad = maybe (fail "Nothing") return
In general, however, this operation can't be done. For example,
how would you write:
mplus' :: IO a -> [a] -> [a]
Perhaps the question should be: is there an interesting structure
that would allow us to capture when this kind of merging Monads
is possible?
For me, it seems that Foldable is the other side of Alternative. A
functor F supports Alternative if (F a) supports a monoidal structure
for the construction of values, and it supports Foldable if (F a)
supports a monoidal structure for the decomposition of values. That
means that we can give a translation from every Foldable functor to
every Alternative functor as follows:
foldable2alternative = foldr (<|>) empty . fmap pure
Tillmann
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