a faster, accumulating mapM (was Re: [Haskell-cafe] mapM vs mapM_ performance)

[Niklas Broberg]

Hi Ben,

>  mapM2 :: Monad m => (a -> m b) -> [a] -> m [b]
>  {-# INLINE mapM2 #-}
>  mapM2 fn lst = mapM2accum fn lst []
>     where mapM2accum _ [] accum = return accum
>           mapM2accum fn (x:xs) accum = do
>             r <- fn x
>             mapM2accum fn xs (r:accum)

Not that it should matter for performance any, but you really ought to
reverse the result list too, or compute the accumulator in the right
order. :-)

mapM2 :: Monad m => (a -> m b) -> [a] -> m [b]
{-# INLINE mapM2 #-}
mapM2 fn lst = mapM2accum fn lst id
   where mapM2accum _ [] accum = return $ accum []
         mapM2accum fn (x:xs) accum = do
           r <- fn x
           mapM2accum fn xs (accum . (r:))

Cheers,

/Niklas