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'.
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