Here is another example:
f1 n ~(x:xs) = (n, x) f2 n (x:xs) = (n,x)
f1 5 [] = (5, error "irrefutable pattern match failure")
f2 5 [] = error "pattern match failure"
In particular:
fst (f1 5 []) = 5
fst (f2 5 []) = error "pattern match failure"
The "~" delays the pattern match until evaluation of the variables
inside the pattern is demanded. If the variable is never demanded,
the pattern match doesn't happen.
It's especially useful for single-constructor datatypes (like pairs)
if you want the code to be more lazy.
-- ryan
On Wed, Aug 27, 2008 at 11:56 AM, Neil Mitchell
Hi
At the same place, I found that example, but wasn't wise enough to figure out what it does:
(f *** g) ~(x,y) = (f x, g y)
Can you help me understand it?
It means exactly the same as:
(f *** g) xy = (f (fst xy), g (snd xy))
i.e. if you call (f *** g) undefined, you will get (f undefined, g undefined). If the pattern was strict (i.e. no ~) you would get undefined.
Please update the keyword wiki so it makes sense to you, after you have got your head round it.
Thanks
Neil _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe