Re: [Haskell-cafe] Re: Cannot understand liftM2

On 11 Dec 2006 16:55:17 +0000, Jón Fairbairn wrote:

"Nicola Paolucci" writes:

...

Hi All,

I'm loving learning Haskell quite a bit. It is stretching my brain but in a delightfull way.

I've googled, I've hoogled but I haven't found a clear explanation for what exactly liftM2 does in the context below.

Using the cool lambdabot "pointless" utility I found out that:

...

\x -> snd(x) - fst(x)

is the same as:

...

liftM2 (-) snd fst

I like the elegance of this but I cannot reconcile it with its type. I can't understand it. I check the signature of liftM2 and I get:

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

Can someone help me understand what's happening here ? What does a Monad have to do with a simple subtraction ? What is actually the "m" of my example ?

I am sure if I get this I'll be another step closer to illumination ...

Does typing

:t liftM2 (-) snd

into ghc enlighten you at all?

Make sure to :m + Control.Monad.Reader first, because this instance unfortunately isn't in the Prelude.