Lazy evaluation?

[Rob Hoelz]

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!

 

[John W. Krahn]

does Perl use lazy evaluation?

AFAIK no it does not.



John

 

[Mirco Wahab]

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

 

[John W. Krahn]

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

 

[Uri Guttman]

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

 

[xhoster]

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