«

In a statement like this:

local $\ = "\n";
foreach $num (1..1000_000_000) {
print $num;
}

or this:

local $\ = "\n";
foreach $line (<$handle>) {
chomp $line;
print $line;
}

does Perl use lazy evaluation? Thanks!

Rob Hoelz wrote:
>
> does Perl use lazy evaluation?

AFAIK no it does not.



John
--
Perl isn't a toolbox, but a small machine shop where you can special-order
certain sorts of tools at low cost and in short order. -- Larry Wall

Michele Dondi wrote:
> On Tue, 13 Feb 2007 20:14:13 -0600, Rob Hoelz <> wrote:
>> local $\ = "\n";
>> foreach $num (1..1000_000_000) {
>> print $num;
>> }
>
> I seem to remember that this particular one is optimized, but I'm not
> really sure if up to the point of lazy evaluation.

The above (foreach list) *is* lazy evaluated (at least
in my in 5.8.8/Linux), whereas the list in

print map { exit } (1..1000_000_000);

is not (panic: realloc at lazy.pl al line ##).

Regards

Mirco

Mirco Wahab wrote:
> Michele Dondi wrote:
>> On Tue, 13 Feb 2007 20:14:13 -0600, Rob Hoelz <> wrote:
>>> local $\ = "\n";
>>> foreach $num (1..1000_000_000) {
>>> print $num;
>>> }
>>
>> I seem to remember that this particular one is optimized, but I'm not
>> really sure if up to the point of lazy evaluation.
>
> The above (foreach list) *is* lazy evaluated (at least
> in my in 5.8.8/Linux), whereas the list in

That is not lazy evaluation. Perl is just internally evaluating:

foreach $num (1..1000_000_000) {
print $num;
}

As:

for ( $num = 1; $num <= 1000_000_000; ++$num ) {
print $num;
}


Lazy evaluation would imply that the assignment to $num is delayed until it is
actually required for the print() function which is not what perl does.



John
--
Perl isn't a toolbox, but a small machine shop where you can special-order
certain sorts of tools at low cost and in short order. -- Larry Wall

>>>>> "b" == bugbear <bugbear@trim_papermule.co.uk_trim> writes:

b> John W. Krahn wrote:
>> That is not lazy evaluation. Perl is just internally evaluating:
>> foreach $num (1..1000_000_000) {
>> print $num;
>> }
>> As:
>> for ( $num = 1; $num <= 1000_000_000; ++$num ) {
>> print $num;
>> }

b> You mean the entire construct is recognised and
b> handled as a complete "idiom", as opposed
b> to a more general evaluation model of ".." ?

it is easy to optimize (or make pseudo lazy) a .. in a for loop. all you
have to do is change how that is code generated for that special (and
common) case. passing a large .. to map is a very different story as map
expects a full list it can use. it would require more complex
recognition of a .. (and nothing else) being passed to map and a
different version of map that will do internal iteration of the .. vs
iterating over its standard input list. so they didn't try to optimize
... in the general case as it would need work on every list consuming
operator or function. and as someone said real lazy eval is the norm (in
all cases!) in p6.

uri

--
Uri Guttman ------ -------- http://www.stemsystems.com
--Perl Consulting, Stem Development, Systems Architecture, Design and Coding-
Search or Offer Perl Jobs ---------------------------- http://jobs.perl.org

Rob Hoelz <> wrote:
> In a statement like this:
>
> local $\ = "\n";
> foreach $num (1..1000_000_000) {
> print $num;
> }
>
> or this:
>
> local $\ = "\n";
> foreach $line (<$handle>) {
> chomp $line;
> print $line;
> }
>
> does Perl use lazy evaluation? Thanks!

First one yes, second one no. (For the second, use while instead)

Xho

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