<html><head><style type='text/css'>p { margin: 0; }</style></head><body><div style='font-family: Times New Roman; font-size: 12pt; color: #000000'>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.<br><br><div style="text-align: left;">The expanded code is in fact quite simple and is close to what you probably write yourself.<br><br>So order is preserved and their is no call to lists:reverse when generating the output list.<br><br>Robert<br><br></div>----- "Olivier BOUDEVILLE" <olivier.boudeville@edf.fr> wrote:
<br>>
<br>> <font face="sans-serif" size="2">Hi,</font>
<br>>
<br>> <font face="sans-serif" size="2">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:</font>
<br>>
<br>> <font face="sans-serif" size="2">1> [ X || X <- [1,2,3] ].</font>
<br>> <font face="sans-serif" size="2">[1,2,3]<br>>
</font>
<br>> <font face="sans-serif" size="2">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?</font>
<br>>
<br>> <font face="sans-serif" size="2">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?</font>
<br>>
<br>> <font face="sans-serif" size="2">Thanks in advance for any answer,</font>
<br>> <font face="sans-serif" size="2">Best regards,</font>
<br>> <font face="sans-serif" size="2"><br>>
Olivier Boudeville.<br>>
---------------------------<br>>
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