[erlang-questions] Re: List comprehension questions

Olivier BOUDEVILLE olivier.boudeville@REDACTED
Fri Apr 1 13:15:14 CEST 2011

Hello Richard,

First of all, sorry, I had indeed missed your answer. Actually I did not 
see it neither at work nor at home, and I wonder whether there is not a 
mailing-list problem as it does not seem to appear either in gmane or in 
the thread as shown (at least at the time of this writing) by google 

Back on topic: as I understand now (I am not too familiar with Core 
Erlang), list comprehensions generate local functions that are 
body-recursive, i.e. involve actually a direct recursion, not using an 

I just happened to find 
http://www.erlang.org/doc/efficiency_guide/listHandling.html#id61285 which 
gives a similar explanation (maybe a link from the reference manual to the 
efficiency guide could be made, to associate these two list comprehension 

I was wondering if this body-recursiveness did not imply that a list 
comprehension operating on a longer list, whose result would be kept, 
could eat a lot of stack space/take longer to execute than a 
tail-recursive version?

Apparently according to 
http://www.erlang.org/doc/efficiency_guide/myths.html#id58884 this will 
probably not be the case, so using list comprehension does not involve 
much trade-off.

Thanks again,
Best regards,

Olivier Boudeville.
Olivier Boudeville

EDF R&D : 1, avenue du Général de Gaulle, 92140 Clamart, France
Département SINETICS, groupe ASICS (I2A), bureau B-226
Office : +33 1 47 65 59 58 / Mobile : +33 6 16 83 37 22 / Fax : +33 1 47 
65 27 13

01/04/2011 10:46

erlang-questions@REDACTED, olivier.boudeville@REDACTED

Re: [erlang-questions] Re: List comprehension questions

On 04/01/2011 10:00 AM, Olivier BOUDEVILLE wrote:
> Hello Robert,
> Thanks for your explanation. I guess that the reversal (to restore the
> original list order after a first "if selected by filter then apply"
> traversal) is done then in the list:map/2 counterpart!
> My concern was that, for long lists which do not happen to have to
> respect any particular order in their elements, two traversals would be
> done with list comprehensions whereas just one could be sufficient.
> Well, a minor concern as for such cases we can write our own functions
> to do so in one pass.

Perhaps you didn't see my reply to your original mail (repeated below). 
The point that both Robert and I are trying to make is that there is no 
reversal involved in the code, and it only does one traversal, so you 
don't need to worry about this.


On 03/30/2011 01:29 PM, Olivier BOUDEVILLE wrote:
 > 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 
 > 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?

No, the translation does a straightforward recursion over the list 
without accumulator, automatically preserving order. To see what 
happens, take for example the following module:

   f(Input) ->
     [N + 1 || N <- Input].

and compile it to Core Erlang with c(lc,[to_core]) or erlc +to_core 
lc.erl. The resulting file lc.core contains the following code:

   'f'/1 =
     fun (_cor0) ->
             'lc$^0'/1 =
                 fun (_cor3) ->
                     case _cor3 of
                       <[N|_cor2]> when 'true' ->
                           let <_cor4> =
                               call 'erlang':'+'
                                   (N, 1)
                           in  let <_cor5> =
                                   apply 'lc$^0'/1
                               in  ( [_cor4|_cor5]
                                     -| ['compiler_generated'] )
                       <[]> when 'true' ->
                       ( <_cor3> when 'true' ->
                             primop 'match_fail'
                                 ({( 'function_clause'
                                     -| [{'name',{'lc$^0',1}}] ),_cor3})
                         -| ['compiler_generated'] )
         in  apply 'lc$^0'/1

(The annotations ( Expr -| 'compiler_generated') on some subexpressions 
are mostly needed to suppress certain compilation warnings that might 
otherwise be triggered, such as for unused results of expressions.)


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