RE: [Haskell-cafe] what does @ mean?.....

28 Dec 2007


      Lovely....thank you very much....another small step forward.

-----Original Message-----
From: Chaddaï Fouché [mailto:chaddai.fouche@gmail.com] 
Sent: 28 December 2007 11:29
To: Nicholls, Mark
Cc: Alfonso Acosta; haskell-cafe@haskell.org
Subject: Re: [Haskell-cafe] what does @ mean?.....

2007/12/28, Nicholls, Mark :
...
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