[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
groups
(http://groups.google.com/group/erlang-programming/browse_thread/thread/8d339e24ed090311).
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
accumulator.
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
topics).
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
carlsson.richard@REDACTED
01/04/2011 10:46
A
erlang-questions@REDACTED, olivier.boudeville@REDACTED
cc
Objet
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.
/Richard
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
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?
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:
-module(lc).
-export([f/1]).
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) ->
letrec
'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
(_cor2)
in ( [_cor4|_cor5]
-| ['compiler_generated'] )
<[]> when 'true' ->
[]
( <_cor3> when 'true' ->
primop 'match_fail'
({( 'function_clause'
-| [{'name',{'lc$^0',1}}] ),_cor3})
-| ['compiler_generated'] )
end
in apply 'lc$^0'/1
(_cor0)
(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.)
/Richard
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