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copied!<p>Perl offers a system called subroutine prototypes that allow you to write user subs that get parsed in a way similar to the builtin functions. The builtins that you want to emulate are <code>map</code>, <code>grep</code>, or <code>sort</code> which can each take a block as their first argument.</p> <p>To do that with prototypes, you use <code>sub name (&) {...}</code> where the <code>&</code> tells perl that the first argument to the function is either a block (with or without <code>sub</code>), or a literal subroutine <code>\&mysub</code>. The <code>(&)</code> prototype specifies one and only one argument, if you need to pass multiple arguments after the code block, you could write it as <code>(&@)</code> which means, a code block followed by a list.</p> <pre><code>sub higher_order_fn (&@) { my $code = \&{shift @_}; # ensure we have something like CODE for (@_) { $code->($_); } } </code></pre> <p>That subroutine will run the passed in block on every element of the passed in list. The <code>\&{shift @_}</code> looks a bit cryptic, but what it does is shifts off the first element of the list, which should be a code block. The <code>&{...}</code> dereferences the value as a subroutine (invoking any overloading), and then the <code>\</code> immediately takes the reference to it. If the value was a CODE ref, then it is returned unchanged. If it was an overloaded object, it is turned into code. If it could not be coerced into CODE, an error is thrown.</p> <p>To call this subroutine, you would write:</p> <pre><code>higher_order_fn {$_ * 2} 1, 2, 3; # or higher_order_fn(sub {$_ * 2}, 1, 2, 3); </code></pre> <p>The <code>(&@)</code> prototype that lets you write the argument as a <code>map</code>/<code>grep</code> like block only works when using the higher order function as a function. If you are using it as a method, you should omit the prototype and write it this way:</p> <pre><code>sub higher_order_method { my $self = shift; my $code = \&{shift @_}; ... $code->() for @_; } ... $obj->higher_order_method(sub {...}, 'some', 'more', 'args', 'here'); </code></pre>

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