Re: [Haskell-cafe] Problem with forall type in type declaration

Anyone could help on this? On Fri, May 4, 2012 at 11:12 PM, Magicloud Magiclouds wrote:

Sorry to use Monad as the example, I meant this one:  run :: MonadTrans m => m IO a -> IO a

And Daniel, I do not think adding another type "b" a good idea. Since "run" could actually return any inside type (depending on another function that passed to it). Even simple as different tuples would destroy this solution.

On Fri, May 4, 2012 at 10:05 PM, Daniel Díaz Casanueva wrote:

...

If one parameter is not enough, you always can add more:

Test m a b = Test { f :: m IO a -> IO b }

This way, if

run :: m IO a -> IO a

then

Test run :: Test m a a

But for other type for your run function

run' :: m IO a -> IO b

you get

Test run' :: Test m a b

So you can have different types in input and output. Anyway, your type 'm' is applied to other two types (m IO a), so it cannot be a monad, because monads have arity 1 as type constructors, i.e. monads have kind (* -> *). Is perhaps 'm' some kind of monad transformer?

Well, that's all I can say from your explanation of the problem! Hope it helps!

Daniel Díaz.

-- 竹密岂妨流水过 山高哪阻野云飞

And for G+, please use magiclouds#gmail.com.

-- 竹密岂妨流水过 山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.