Re: [Haskell-cafe] Heavy lift-ing

On 17/08/10 17:13, Tilo Wiklund wrote:

On 24/07/2010, aditya siram wrote:

...

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

I'm sorry if I'm bumping an old thread, but why should "liftM2 f" be commutative when "f" isn't?

(I hope I'm not responding incorrectly)

I think the point that was being made is that: liftM2 (flip f) /= flip (liftM2 f) This is because the former (well: liftM2 (flip f) a b) effectively does: do {x <- a; y <- b; return (f y x)} Whereas the latter (flip (liftM2 f) a b) effectively does: do {y <- b; x <- a; return (f y x)} That is, the order of the arguments to liftM2 matters because they are executed in that order. So lifting the flipped function has a different effect to flipping the lifted function. Thanks, Neil.