[erlang-questions] Re: List comprehension questions

[Robert Virding]

List comprehensions work as a combined lists:map and lists:filter. So you map over a list filtering out elements you don't want in the output list. They have the added functionality that they can handle this for nested lists. 


The expanded code is in fact quite simple and is close to what you probably write yourself. 

So order is preserved and their is no call to lists:reverse when generating the output list. 

Robert 

----- "Olivier BOUDEVILLE" <olivier.boudeville@REDACTED> wrote: 
> 
> Hi, 
> 
> Reading http://www.erlang.org/doc/reference_manual/expressions.html#id77052 it is not obvious that the order induced by a single generator is necessarily preserved by the comprehension, as nevertheless suggested by all experiments like: 
> 
> 1> [ X || X <- [1,2,3] ]. 
> [1,2,3] 
> 
> I suppose this order preservation is true. Then, I imagine that, internally, list comprehensions are translated to code very similar to the one that would be produced if the developer had written an appropriate accumulator-based tail-recursive function? 
> 
> If so, does it imply that using a list comprehension on a semantically unordered set of elements will involve a useless reversal (like lists:reverse/1) of the resulting list? 
> 
> Thanks in advance for any answer, 
> Best regards, 
> 
> Olivier Boudeville. 
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