traveler has asked for the wisdom of the Perl Monks concerning the following question:

I have a program where I let the user specify some expressions using variables in the program. For now I want to use strings like "foo has the value $foo" and see the value of $foo printed out. My code looks like this: 
sub fun{
 return sub {return 'foo has value $foo');
}
...
$coderef = fun;
$foo = 7;

$str = &$coderef;

[download]
Of course, $str is "foo has the value $foo". I'd like to do "variable interpolation" on $str. I thought eval might do that, but is unhappy trying to do just that. I believe that there is a simple answer, but it is one of those days when the mind gets totally fogged...

When I tried double quotes around the string, it tried to evaluate $foo (which was undefined) and created a closure with the string having an undefined value.

As always, any help is appreciated,
--traveler

Replies are listed 'Best First'.
Re: Delaying interpolation
by meonkeys (Chaplain) on Aug 07, 2001 at 04:22 UTC
    How's this?
    use strict;

my $foo = 'nothing yet';

sub fun {
  return sub { return "foo has value $foo" }
}

my $coderef = fun();
$foo = 7; 

my $str = &$coderef;

print "$str\n";

    [download]
[reply]
[d/l]
Re: Delaying interpolation
by abstracts (Hermit) on Aug 07, 2001 at 04:26 UTC
    Hello

       eval EXPR
   eval BLOCK
      In the first form, the return value of EXPR is
      parsed and executed as if it were a little Perl
      program.
    So, to return "foo has value $foo" after interpolation, you can do: 
    my $foo = 7;
my $str =  eval '"foo has $foo"';
print $str;

    [download]
    Notice the additional double quotes inside the single quotes. This makes eval evals 
    "foo has $foo"
    and returns the resulting string. If you don't include the additional quotes, eval evals 
    foo has $foo
    which is not what you want.

    Hope this helps,,,

    Aziz,,,

[reply]
[d/l]
Re: Delaying interpolation
by chipmunk (Parson) on Aug 07, 2001 at 06:39 UTC
[reply]
Re: Delaying interpolation
by traveler (Parson) on Aug 07, 2001 at 05:01 UTC
    I tried each and they worked. Thanks!
[reply]

Back to Seekers of Perl Wisdom