On Sun, Feb 04, 2007 at 08:20:23AM +0000, Dominic Steinitz wrote:
Someone suggested
pad :: Num a => [a] -> [a] pad = pad' 0 where pad' !l [] = [0x80] ++ ps ++ lb where pl = (64-(l+9)) `mod` 64 ps = replicate pl 0x00 lb = i2osp 8 (8*l) pad' !l (x:xs) = x : pad' (l+1) xs
but that didn't compile
*Main> :r [2 of 2] Compiling Main ( allInOne.hs, interpreted )
allInOne.hs:83:14: Parse error in pattern Failed, modules loaded: Codec.Utils.
Dominic.
The '!' is a GHC extension, enabled using the flag '-fbang-patterns'. Equivalently, you can use Haskell 98's "seq" : pad :: Num a => [a] -> [a] pad = pad' 0 where pad' l [] | l `seq` False = undefined pad' l [] = [0x80] ++ ps ++ lb where pl = (64-(l+9)) `mod` 64 ps = replicate pl 0x00 lb = i2osp 8 (8*l) pad' l (x:xs) = x : pad' (l+1) xs The first alternative never succeeds, but to see that the compiler must force the evaluation of 'l'.