optimization of list comprehensions
Thu Mar 2 15:03:15 CET 2006
indeed. i tend to use lists:foldl, lists:map and lists:foreach a lot. i would
really like to have comprehension syntax for all three of them.
my suggestion for the foldl-like comprehension is
(x(I,'_') || I <- List, '_' <- Acc0) -> Acc1
where '_' is the accumulator. it would shadow a previously bound '_'. the
(x(I) || I <- List) -> X
(where X is x(L) and L is the last element of List) would be a special case
all real language designers should feel free to flame me. it'll be educational.
Serge Aleynikov wrote:
> 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
> 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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