[erlang-questions] foldl vs foldr and ++

Robert Virding <>
Thu May 10 21:30:14 CEST 2012


These say more about it:

http://www.erlang.org/doc/efficiency_guide/myths.html#tail_recursive
http://www.erlang.org/doc/efficiency_guide/listHandling.html#id63380

>From the horse's mouth so to speak,

Robert

----- Original Message -----
> You are right. I should realize it myself instead guessing. Compiler
> has to do something really smart there. Strange thing happen when I
> copied lists:foldl and lists:foldr implementation inside benchmark
> module and compile native. It is far (more than twice) slower than
> BEAM.
> 
> On Thu, May 10, 2012 at 11:08 AM, Ulf Wiger <>
> wrote:
> >
> > Try this benchmark program with, e.g. L = lists:seq(1,100000).
> >
> > -module(lr).
> > -export([l/1, r/1, x/1, foldr/3]).
> >
> >
> > l(L) ->
> >    timer:tc(lists, foldl, [fun erlang:'+'/2, 0, L]).
> >
> > r(L) ->
> >    timer:tc(lists, foldr, [fun erlang:'+'/2, 0, L]).
> >
> > x(L) ->
> >    timer:tc(?MODULE, foldr, [fun erlang:'+'/2, 0, L]).
> >
> > foldr(F, Acc, L) -> lists:foldl(F, Acc, lists:reverse(L, [])).
> >
> >
> > lists:foldl/3 and lists:foldr/3 run a fairly even race, whereas
> > lr:foldr/3 is consistenty slower.
> >
> > BR,
> > Ulf W
> >
> > On 10 May 2012, at 10:53, Hynek Vychodil wrote:
> >
> >> When I see foldr implementation I wonder why it is not simple
> >>
> >> foldr(F, Acc, L) -> foldl(F, Acc, lists:reverse(L, [])).
> >>
> >> There should be less memory consumption. lists:reverse is BIF
> >> which I
> >> believe consume less memory than stack frame per each list element
> >> what current foldr implementation do. It should be faster and both
> >> accepts only proper lists. Only difference which I can found
> >> current
> >> implementation consumes memory on stack but above implementation
> >> would
> >> consume heap and affect GC behavior. Current foldr implementation
> >> would make sense in Haskell but I don't understand its reason in
> >> Erlang.
> >>
> >> On Thu, May 10, 2012 at 10:26 AM, Robert Virding
> >> <> wrote:
> >>> The documentation is referring to the *implementation* of the
> >>> foldl/foldr functions, foldl uses a tail-recursive function
> >>> while foldr cannot do that. The difference in speed comes from
> >>> what you do inside the fold function. As others have pointed out
> >>> the efficiency of ++ depends solely on its left argument.
> >>>
> >>> Robert
> >>>
> >>> ----- Original Message -----
> >>>> Aha I just looked at my own explanation and realized it made no
> >>>> sense. Of course the slow one is slow for exactly the reason I
> >>>> expected. Sorry for the waste, but thank you for the clear
> >>>> complexity analysis.
> >>>>
> >>>> Andrew Ledvina
> >>>>
> >>>> On May 9, 2012, at 2:57 PM, "Richard O'Keefe"
> >>>> <>
> >>>> wrote:
> >>>>
> >>>>>
> >>>>> On 10/05/2012, at 7:00 AM, Andrew Ledvina wrote:
> >>>>>
> >>>>>> Okay, so I was profiling something and it led me to write
> >>>>>> this:
> >>>>>>
> >>>>>> from_left(X) ->
> >>>>>>   lists:foldl(fun(Y,R) -> R ++ [{Y+1,Y*2}] end, [], X).
> >>>>>>
> >>>>>> from_right(X) ->
> >>>>>>   lists:foldr(fun(Y,R) -> [{Y+1,Y*2}] ++ R end, [], X).
> >>>>>
> >>>>> Immediate reaction:  from_left/1 is going to be atrociously
> >>>>> slow.
> >>>>>
> >>>>>> My understanding is that foldl is preferred over foldr because
> >>>>>> it
> >>>>>> is tail recursive.
> >>>>>
> >>>>> ***other things being equal***
> >>>>>
> >>>>> In this case, they are not.  It should be clear that
> >>>>> from_right/1
> >>>>> is
> >>>>> O(|X|) because it does O(1) work per element of X, while
> >>>>> from_left/1 is O(|X|**2) because it does O(|R|) work per
> >>>>> element of
> >>>>> X, and R grows from empty to 2|X|-2 elements long.
> >>>>>
> >>>>> Write [{Y+1,Y*2} || Y <- X] and let the compiler worry about
> >>>>> the
> >>>>> best way to do it.
> >>>>>
> >>>>>> I was also to understand that putting the accumulator on the
> >>>>>> left-hand-side of the ++ operation is super bad. So, the
> >>>>>> following results were a bit of a surprise at first:
> >>>>>
> >>>>> If you understood that long++short is more expensive than
> >>>>> short++long,
> >>>>> you should NOT have been surprised by your result.
> >>>>>
> >>>>>
> >>>> _______________________________________________
> >>>> erlang-questions mailing list
> >>>> 
> >>>> http://erlang.org/mailman/listinfo/erlang-questions
> >>>>
> >>> _______________________________________________
> >>> erlang-questions mailing list
> >>> 
> >>> http://erlang.org/mailman/listinfo/erlang-questions
> >>
> >>
> >>
> >> --
> >> Hynek Vychodil
> >> Chief Scientists
> >>
> >> GoodData
> >> náměstí 28. října 1104/17, 602 00, Brno - Černá Pole
> >> Office:   +420 530 50 7704
> >> E-mail:  
> >> Web:     www.gooddata.com
> >> _______________________________________________
> >> erlang-questions mailing list
> >> 
> >> http://erlang.org/mailman/listinfo/erlang-questions
> >
> > Ulf Wiger, Co-founder & Developer Advocate, Feuerlabs Inc.
> > http://feuerlabs.com
> >
> >
> >
> 
> 
> 
> --
> Hynek Vychodil
> Chief Scientists
> 
> GoodData
> náměstí 28. října 1104/17, 602 00, Brno - Černá Pole
> Office:   +420 530 50 7704
> E-mail:  
> Web:     www.gooddata.com
> 



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