[Haskell-cafe] following up on space leak

[Uwe Hollerbach]

On 7/4/09, Marcin Kosiba <marcin.kosiba at gmail.com> wrote:
> On Saturday 04 July 2009, Uwe Hollerbach wrote:
>> Good evening, all, following up on my question regarding space leaks,
>> I seem to have stumbled across something very promising. I said I was
>> using this tiny function "lastOrNil" to get the last value in a list,
>> or the empty (scheme) list if the haskell list was empty. The uses of
>> it were all of the form
>>
>>     lastOrNil (mapM <something> <some list>)
>>
>> so I wrote a different function mapML to do this directly:
>> > mapML fn lst = mapMLA (List []) fn lst
>> >   where mapMLA r _ [] = return r
>> >         mapMLA ro fn (x:xs) =
>> >            do rn <- fn x
>> >               mapMLA rn fn xs
>>
>> This isn't an accumulator, it's a replacer (or, if you like, the
>> "accumulation" is "drop the old one on the floor"), it starts out with
>> the scheme empty list that I want as the default, and it never even
>> builds the list which it'll just dump an instant later. Shazam! Memory
>> usage dropped by roughly an order of magnitude in my little Collatz
>> benchmark, and incidentally runtime improved by 25% or so as well. The
>> horror! :-)
>
> Hi,
> 	IMHO expressing mapML using StateT would be a bit cleaner ;)
>
> mapML :: (Monad m) => (a -> m List) -> [a] -> m List
> mapML fn lst = execStateT mapMLAs (List [])
>   where
>     mapMLAs  = sequence_ $ map mapMLA lst
>     mapMLA x = (lift $ fn x) >>= put
>
> --
> Marcin Kosiba

Yeah, I'm sure there are more-elegant ways to write this, I'm still
very much a beginner in haskell. I'm just very thrilled by the
reduction in memory usage!

Uwe