Re: [Haskell-cafe] Heavy lift-ing

Perhaps I'm being unclear again. All I was trying to say was that: liftM2 (-) [0,1] [2,3] /= liftM2 (-) [2,3] [0,1] -deech On Sat, Jul 24, 2010 at 9:30 AM, Max Rabkin wrote:

On Sat, Jul 24, 2010 at 4:08 PM, aditya siram wrote:

...

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

No, it's not. The type of liftM2 makes this clear:

liftM2 :: (Monad m) => (a -> b -> r) -> m a -> m b -> m r

The arguments to the function *must* come in the right order, because there is no way to match (a) with (m b) or (b) with (m a). Since liftM2 is parametrically polymorphic in (a) and (b), it can't behave differently in the case where (a = b).

--Max