"Jason Dagit"
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, E&OE &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 Jon.Fairbairn@cl.cam.ac.uk