[Haskell-cafe] what does @ mean?.....
Chaddaï Fouché
chaddai.fouche at gmail.com
Fri Dec 28 06:28:45 EST 2007
2007/12/28, Nicholls, Mark <Nicholls.Mark at mtvne.com>:
> So in the example given...
>
> mulNat a b
> | a <= b = mulNat' a b b
> | otherwise = mulNat' b a a
> where
> mulNat' x@(S a) y orig
> | x == one = y
> | otherwise = mulNat' a (addNat orig y) orig
>
> Is equivalent to
>
> mulNat a b
> | a <= b = mulNat' a b b
> | otherwise = mulNat' b a a
> where
> mulNat' (S a) y orig
> | (S a) == one = y
> | otherwise = mulNat' a (addNat orig y) orig
>
> ?
Yes, but in the second version, it has to reconstruct (S a) before
comparing it to "one" where in the first it could do the comparison
directly. In this cas there may be some optimisation involved that
negate this difference but in many case it can do a real performance
difference.
The "as-pattern" (@ means as) is both practical and performant in most cases.
--
Jedaï
More information about the Haskell-Cafe
mailing list