Re: [Haskell-cafe] Heavy lift-ing

I wouldn't-it was a bad example. My only point was that because of the way (>>=) is implemented for lists the order of the arguments 'a' and 'b' in 'liftM2 f a b' matters. -deech On Sat, Jul 24, 2010 at 1:37 AM, Lennart Augustsson wrote:

Why would you expect swapped operands to (-) ?

Sent from my iPad

On Jul 23, 2010, at 20:12, aditya siram wrote:

...

Lists are non-deterministic, but the function taken by liftM2 does not necessarily generate all possible outcomes. In the case of (+) it does, not in the case of (-): liftM2 (-) [0,1] [2,3] => [0-1,0-2,1-2,1-3] => [-2,-3,-1,-2] if all possible cases were generated between the two lists we have to include also: [2-0,2-1,3-0,3-1]

-deech

On Fri, Jul 23, 2010 at 3:44 PM, Alex Stangl wrote:

...

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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe