optimization of list comprehensions

Robert Virding <>
Mon Mar 6 12:03:22 CET 2006

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 syntax):
> [Acc+1, I || Acc, I <- List](0) -> Acc1
>                              ^
>                              |
>                        Initial Acc's value
> Serge
> 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.
>> 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

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