in reply to Dumb, non-question: How to return 'nothing'.

See Higher Order Perl, Chapter 4.5 THE SEMIPREDICATE PROBLEM 
# array reference version
sub make_iterator {
    ... return Iterator {
        my $return_value;
        ...
          if (exhausted)
        {
            return;
        }
        else {
            return [$return_value];
        }
    }
}

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Replies are listed 'Best First'.
Re^2: Dumb, non-question: How to return 'nothing'.
by hardburn (Abbot) on Jun 17, 2011 at 20:09 UTC

    That's a nice solution, but is there any particular reason to use an arrayref rather than a scalarref? It'd make the iterating code just a bit cleaner:

    while( my $next = $iter->() ) {
    $next = $$next;
    ...
}

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    As opposed to:

    while( my $next = $iter->() ) {
    ($next) = @$next;
    ...
}

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    "There is no shame in being self-taught, only in not trying to learn in the first place." -- Atrus, Myst: The Book of D'ni.

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      I think I've settled on requiring that the iterator be called in a list context. Eg.:

      sub i{ 
    my $n = shift; 
    return sub{ $n ? $n-- : () } 
};;

$i = i( 5 ); 
print $_ while ( $_ ) = $i->();;
5
4
3
2
1

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      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.
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[d/l]
        Would be worthwhile to add carp "Iterator not called in list context" unless wantarray; so it can't be used improperly?
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[d/l]
      That's a nice solution, but is there any particular reason to use an arrayref rather than a scalarref?

      Its copy/paste from the book, the first solution -- I guess its more generic, re-usability and all that :)

[reply]

        Seeing as a scalar is always being returned, 

        while( my $next = $iter->() ) {
    ($next) = @$next;
    ...
}

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        can be shortened to 

        while( my ($next) = @{ $iter->() } ) {
    ...
}

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In Section Meditations