Perl Hacks

Hack #27. Pull Multiple Values from an Iterator

Make your iterators and generators highly context-sensitive.

Iterators and generators are fantastically useful for data that takes too long to generate, may never run out, or costs too much memory to keep around. Not every problem works when reading one item at a time though, and finding a nice syntax for pulling only as many items as you need can be tricky.

Perl's notion of context sets it apart from many other programming languages by doing what you mean. When you want a single item, it will give you a single item. When you want nothing, it can give you nothing. When you want a list, it will oblige. That power is yours through the wantarray( ) operator, too.

Wouldn't it be nice if Perl could tell how many items you want from an iterator or generator without you having to be explicit? Good news—it can.

Better Context than wantarray( )

Robin Houston's Want module extends and enhances wantarray( ) to give more details about the calling context of a function. Besides distinguishing between void and scalar context, Want's howmany( ) function can tell how many list items the calling context wants, from one to infinity.

The Code

Consider a simple generator that implements a counter. It takes the initial value, the destination value, and an optional step size (which defaults to 1). When it reaches the destination, it returns the undefined value.

sub counter { my ($from, $to, $step) = @_; $step ||= 1; return sub { return if $from > $to; ...