Pass conditions to a subroutine

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

Hi all, I'm attempting to pass a conditional statement to a subroutine for evaluation in a while loop, but I'm having trouble getting Perl to DWIM. Here's a code snippet:

my $condition = '!-d $someDir';
my $result = checkForDirectory($condition);

sub checkForDirectory
{
    my $condition = shift;
    
    # This is where the problem is. I want to interpolate
    # the text in this variable and use it as my condition.
    # How can I get Perl to DWIM here?
    
    if ($condition)
    {
        doSomethingElse();
    }
}
[download]

Comment on Pass conditions to a subroutine Download Code

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Re: Pass conditions to a subroutine by ikegami (Patriarch) on May 13, 2008 at 17:37 UTC
You could use `eval EXPR`, but passing a sub reference is safer, clearer and easier. `my $condition = sub { !-d $_[0] }; my $result = checkForDirectory($condition); sub checkForDirectory { my $condition = shift; ... if ($condition->($someDir)) { doSomethingElse(); } }` [download]	[reply] [d/l] [select]
Re^2: Pass conditions to a subroutine by Fletch (Bishop) on May 13, 2008 at 19:15 UTC
Of course you could take advantage of one of the few times you would actually want to use a prototype and allow a more streamlined syntax: `sub checkForDirectory (&) { my $cond = shift; ## ... } ## ... my $result = checkForDirectory { ! -d $_[0] }; my $also_result = checkForDirectory sub { -M $_[0] > 2 }` [download] Although doing it this way means that if your coderefs aren't inline (say they're coming from an array or hash based on some other external condition) you'll need to use a leading & to override the prototype check . . . `my $predicate = sub { $_[0] =~ /^\d{4}-\d{2}-\d{2}/ }; my $other_result = &checkForDirectory( $predicate );` [download] The cake is a lie. The cake is a lie. The cake is a lie.	[reply] [d/l] [select]
Re^2: Pass conditions to a subroutine by PerlRob (Sexton) on May 13, 2008 at 18:08 UTC
Thanks to all--exactly what I was looking for!	[reply]
Re: Pass conditions to a subroutine by pc88mxer (Vicar) on May 13, 2008 at 17:37 UTC
An anonymous subroutine would work here: `my $cond = sub { !-d $_[0] }; perform_check($cond); sub perform_check { my $cond = shift; ... if ($cond->($possible_directory)) { doSomethingElse(); } }` [download]	[reply] [d/l]
Re: Pass conditions to a subroutine by dragonchild (Archbishop) on May 13, 2008 at 17:38 UTC
`my $condition = sub { !-d $_[0] }; my $result = check( $condition ); sub check { my ($condition) = @_; # Obviously, you'll need to find $param from somewhere. if ( $condition->( $param ) ) { blah(); } }` [download] My criteria for good software: Does it work? Can someone else come in, make a change, and be reasonably certain no bugs were introduced?	[reply] [d/l]