On 11 Dec 2006 16:55:17 +0000, Jón Fairbairn <[EMAIL PROTECTED]> wrote:
"Nicola Paolucci" <[EMAIL PROTECTED]> 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.
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