Jason Dagit [EMAIL PROTECTED] writes:
Well, this is basically just a zip with a special base case. But you
can't just write it with zipWith because zipWith stops when it exausts
either list.
How about we define zipWith'' like this:
zipWith'' _ [] _ l _ = [l]
zipWith'' _ _ [] _ r = [r]
zipWith'' f (x:xs) (y:ys) l r = f x y : zipWith'' f xs ys l r
Then we can write:
isPrefixOf xs ys = and (zipWith'' (==) xs ys True False)
I wonder if there is mileage to be had from persuing
something like this, rather different (off top of head, very
provisional, EOE c) approach:
extend_infinitely l = map Just l ++ repeat Nothing
promote1 rel (Just a) b = rel a b
promote1 rel Nothing b = False
is_pfx_of l1 l2 = and (zipWith (promote1 (==)) (extend_infinitely l2) l1)
? For at least the few minutes I've thought about it, it
seems like promote and extend_infinitely might be more
generally userful. I've probably missed something better,
too.
A point free reduction might look like the following and probably
isn't worth it:
isPrefixOf = (and .) . flip flip False . flip flip True . zipWith'' (==)
probably?
--
Jón Fairbairn [EMAIL PROTECTED]
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