[Haskell-cafe] Combining computations

[Felipe Lessa]

I don't know if I understood your intentions, but let's go.  The
problem is that you're trying to combine different monads.  We
have

  mplus :: MonadPlus m => m a -> m a -> m a,

so you never leave 'm', but you want

  mplus' :: ??? => n a -> m a -> m a

where 'n' could be a different monad.  In some specific cases
where you know the internal structure of the monad, you can write
'mplus'', for example:

  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]

?


HTH,

--
Felipe.