optimization of list comprehensions
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:
fun(I) -> I end,
N > 10
[I || I <- lists:seq(1, 20), I > 20].
Why not having a similar concise representation for the following case?
fun(I, Acc) -> Acc+I end,
N > 10
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.
> 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
>> [Acc+1, I || Acc, I <- List](0) -> Acc1
>> Initial Acc's value
>> Robert Virding wrote:
>>> I have been thinking about this a bit and I wonder if the
>>> 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.
>>> 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
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