My answer applies only to OO stuff ( see above regarding when to use prototypes ).

For objects, the ->isa method is commonly used to check "Is an object of this class".

Of course, if you are taking arguments from outside the code you control, there's no way to know if ->isa will work, since isa'ing something that isn't an object will die.

BUT, calling UNIVERSAL::isa as a function directly CAN handle non-reference arguments, and correctly returns false. ( i.e. UNIVERSAL::isa( undef, 'Something' ) works, and correctly returns false )

I'm also a big fan of doing type checking ( where it doesn't get too large of course ) into the variable decleration. I'm also a fan of using return undef; to symbolise an error. Even in the cases where you have true/false responses, you can use true/false/undef ( 1/''/undef )

SO, for most of my OO stuff, and there's a lot of it, I use something like the following. 

package Foo;

use strict;

sub bar {
    my $File = UNIVERSAL::isa( $_[0], 'IO::File' ) ? shift : return un
+def;

    # Do something to file
}

1;

[download]
Of course, all those UNIVERSAL::isa's get virtually RSI like to type, so for convenience, I use the following. 
package Foo;

use strict;
use UNIVERSAL 'isa';

sub Foo {
    my $File = isa( $_[0], 'IO::File' ) ? shift : return undef;

    # Do something to the file
}

1;

[download]
I find this highly useful, especially for reasonably large quantities of arguments. Because you don't shift first and check later, you never forget to check.

In reply to Re: validating function input by adamk
in thread validating function input by bobdeath

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