Re: [Haskell-cafe] Heavy lift-ing

On Fri, Jul 23, 2010 at 11:43:08AM -0700, michael rice wrote:

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]?

It depends upon the semantics of the particular monad. List monads represent nondeterminism. So, for example, [0,1] represents a 0 or 1, and [0,2] represents a 0 or 2. When you add 0 or 1 to 0 or 2, your possible answers are [0,2,1,3]. Alex