Hello i am trying to do the following:
module MyMonad where
f::(Monad m),=>m (a->b->c)->m a -> m b -> m c
f mf ma mb=
ma >>= \a ->
mb >>= \b ->
mf >>= \c ->
return (c a b)
and use it like this :
f (Just 3) (Just 4)
And i get the following error:
* Non type-variable argument in the constraint: Num (a -> b -> c)
(Use FlexibleContexts to permit this)
* When checking the inferred type
it :: forall a b c.
(Num a, Num (a -> b -> c)) =>
Maybe b -> Maybe c
I didn't know how to put multiple type constraints so i tried like this:
f (Just [3]) (Just [4]) (++) -- (knowing (++) can be applied to any type - being a monoid).
In this case i get the following exception:
* Couldn't match expected type `Maybe b0'
with actual type `[a1] -> [a1] -> [a1]'
* Probable cause: `(++)' is applied to too few arguments
In the third argument of `f', namely `(++)'
In the expression: f (Just [3]) (Left [3]) (++)
In an equation for `it': it = f (Just [3]) (Left [3]) (++)
f requires a monad-wrapped function as the first argument. In your first try, you didn't pass the function at all; in the second, you pass (++) as the last argument.
The following works fine:
> f (Just (++)) (Just [3]) (Just [4])
Just [3,4]
liftM2 (and more generally liftA2) already does something very similar to what you want.
> import Control.Monad (liftM2)
> liftM2 (++) (Just [3]) (Just [4])
Just [3,4]
> import Control.Applicative (liftA2)
> liftA2 (++) (Just [3]) (Just [4])
Just [3,4]