Add optional Parameter to Prototype

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In a module (not written by me) I have a prototype sub that takes a code ref and returns result of eval:

sub proto (&) {
    my $code = shift;
    my $r    = eval {&$code};
    return $r;
}

sub foo {
    my $x = 1;
    my $y = proto { $x = $x + 1; };
}

foo();

The prototype sub is called very often in many modules. Now I need to change it and want to add an optional parameter like:

sub proto (&) {
    my $code = shift;
    my $OptionalParameter = shift || 0;
    ...
}

sub foo {
    my $x = 1;
    my $OptionalParameter = 1;
    my $y = proto { $x = $x + 1; }, $OptionalParameter;
}

But that results in warning "Useless use of private variable in void context" for $OptionalParameter. I also changed prototype declaration to (&;$) which results in same warning.

If I remove the prototype declaration (&), I get syntax errors where proto is called: syntax error at Test.pl line 47, near "; }" Global symbol "$OptionalParameter" requires explicit package name (did you forget to declare "my $OptionalParameter"?) at TestQuery.pl line 47.

So, how can I declare a sub that takes a code ref and an optional parameter?

parameter-passing prototype perl
1 Answers

use a ';' in the prototype declaration to separate mandatory and optional parameters. And don't include a comma after the code block:

sub foo(&;$) { my ($fn, $arg) = @_; ... }
foo { ... };
foo { ... } 1;

But in general it's best to avoid using prototypes unless you're trying to emulate the syntax of a built-in perl function:

sub foo { my ($fn, $arg) = @_; ... }
foo(sub { ... });
foo(sub { ... }, 1);
user_97494580
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