On Fri, Jul 01, 2005 at 12:33:13PM +0200, Gracjan Polak wrote:
Example: I have some function, that can return multiple results. Currently I need only the first one, but in the spirit of NotJustMaybe, I try to be as general as possible.
If I code it like this:
reduction :: (MonadPlus m) => [Rule] -> Expr -> m Expr reduction expr = do rule <- rules reduction rule expr
Variable m gets unified with []. But I want m to stay as general as possible here.
This version works, but I somehow do not like the "smell" of it
reduction expr = do let listmonad = do rule <- rules reduction rule expr msum (map return listmonad)
Is there a better way how to "embed" MonadPlus in other "MonadPlus"?
It might smell better if you wrote it as reduction expr = do rule <- msum (map return rules) reduction rule expr or made a separate function msumVals :: MonadPlus => [a] -> m a msumVals xs = msum (map return xs) But I think you have a further problem, because the type you gave for reduction does not match your definitions. And if it did, I think you would shortly run into another problem, involving infinite types. Maybe you're thinking along the lines of reduction rules expr = do rule <- msumVals rules rules' <- run rule reduction rules' expr for some suitable function run? Andrew