How can I "store" an operator inside a variable in Perl?

Question

I'm looking for a way to do this in Perl:

$a = "60"; $b = "< 80";

if ( $a $b ) {  then .... }

Here, $b "holds" an operator... can I do this? Maybe some other way?

Answer 1

It's nice to see how people discover functional programming. :-)

Luckily, Perl has capabilities to create and store functions on-the-fly. For example, the sample in your question will look like this:

$a = "60"; $b = sub { $_[0] < 80 };

if ( $b->($a) ) { .... }

In this example, a reference to the anonymous subroutine is stored in $b, the sub having the same syntax for argument passing as a usual one. -> is then used to call-by-reference (the same syntax you probably use for references to arrays and hashes).

But, of course, if you want just to construct Perl expressions from arbitrary strings, you might want to use eval:

$a = "60"; $b = " < 80";

if ( eval ("$a $b") ) { .... }

However, doing this via eval is not safe, if the string you're eval-ing contains parts that come as user input. Sinan Ünür explained it perfectly in his answer-comment.

Answer 2

How about defining a function that wraps the needed condition:

my $cond = sub { $_[0] < 80 };

if ( $cond->( $a ) ) { 
     ...
}