# optimization of list comprehensions

Serge Aleynikov serge@REDACTED
Mon Mar 6 14:43:19 CET 2006

```Thanks Robert, I indeed use lists:foldl/3 quite extensively.  However
the idea is that if the parser supports list comprehensions, why not
allowing for folding as a variation of the comprehension syntax, as this
pattern is used as frequently as the basic list comprehension itself?

If we can represent:

lists:map(
fun(I) -> I end,
lists:filter(
fun(N) ->
N > 10
end,
lists:seq(1, 20)).

like this:

[I || I <- lists:seq(1, 20), I > 20].

Why not having a similar concise representation for the following case?

lists:foldl(
fun(I, Acc) -> Acc+I end,
0,
lists:filter(
fun(N) ->
N > 10
end,
lists:seq(1, 20)).

Regards,

Serge

Robert Virding wrote:
> lists:foldl/3 does what you want, you pass in accumulator which is
> modified, passed along and finally returned. You also have lists:foldr/
> which traverses the list right-to-left, and mapfoldl/r which also
> returns the mapped list.
>
> Robert
>
> Serge Aleynikov skrev:
>
>> I do want to throw a vote for Mats' suggestion on the alternative syntax:
>>
>> (I || I <- List) -> ok
>>
>> What I also find limiting is that it's not possible to have an
>> accumulator when using list comprehension.  Perhaps something like
>> this could also be considered (unless someone can suggest a better
>> syntax):
>>
>> [Acc+1, I || Acc, I <- List](0) -> Acc1
>>                              ^
>>                              |
>>                        Initial Acc's value
>> Serge
>>
>> Robert Virding wrote:
>>
>>> constructing of the return list really causes any problems.
>>> Space-wise it will only be as large as the size of the input list, so
>>> I wouldn't worry.
>>>
>>> Robert
>>>
>>> Ulf Wiger (AL/EAB) skrev:
>>>
>>>> I've seen many examples of how people use
>>>> list comprehensions as a form of beautified
>>>> lists:foreach() - that is, they don't care
>>>> about the return value of the comprehension.
>>>>
>>>> I hesitate to say that it's bad practice,
>>>> even though one will build a potentially
>>>> large list unnecessarily, since it's actually looks a lot nicer than
>>>> using lists:foreach().
>>>>
>>>> Question: would it be in some way hideous
>>>> to introduce an optimization where such
>>>> list comprehensions do everything except
>>>> actually build the list? Then they could
>>>> execute in constant space.
>>>>
>>>> /Ulf W

```