[Haskell-cafe] Heavy lift-ing

[michael rice]

Hi,

I don't understand what's taking place here.

>From Hoogle:

=================

liftM2 :: Monad  m => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r

Promote a function to a monad, scanning the monadic arguments from left to right. For example,

    liftM2 (+) [0,1] [0,2] = [0,2,1,3]
    liftM2 (+) (Just 1) Nothing = Nothing

=================

What does it mean to "promote a function to a monad?"

It would seem that the monad values must understand the function that's being promoted, like Ints understand (+).

Prelude Control.Monad> liftM2 (+) (Just 1) (Just 1)
Just 2

But how does one add [0,1] and [0,2] to get [0,2,1,3]?

Michael 



      
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