darcs patch: Improve Control.Monad.filterM:

[Bulat Ziganshin]

Hello Simon,

Tuesday, August 8, 2006, 4:18:01 PM, you wrote:

>> -filterM _ []     =  return []
>> -filterM p (x:xs) =  do
>> -   flg <- p x
>> -   ys  <- filterM p xs
>> -   return (if flg then x:ys else ys)

> The new definition looks less lazy than the original, so it's not a drop-in
> replacement.  Also, we would need some measurements to test whether this version
> doesn't lose efficiency - it probably fuses better, but might be slower when it
> doesn't fuse.  Rules to turn the foldr version back into the recursive version
> might be needed (or aggressive inlining).

jfyi - original filterM isn't tail recursive, so it overflows stack on
really large data.


-- 
Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com