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A Gotcha With perl’s “each()”

It’s my bi-monthly perl blog entry, to earn my place on planet.perl.org! ;)

Here’s an interesting “gotcha”. Take this code:

    perl -e '%t=map{$_=>1}qw/1 2 3/;
    while(($k,$v)=each %t){print "1: $k\n"; last;}
    while(($k,$v)=each %t){print "2: $k\n";}'

In other words, iterate through all the key-value pairs in %t once, then do it again — but exit early in the first loop.

You would expect to get something like this output:

    1: 1
    2: 1
    2: 3
    2: 2

instead, you see:

    1: 1
    2: 3
    2: 2

The “1” entry in the second loop is AWOL. Here’s why — as “perldoc -f each” notes:

There is a single iterator for each hash, shared by all “each”, “keys”, and “values” function calls in the program

That’s all “each” calls, throughout the entire codebase, possibly in a different class entirely. Argh.

The workaround: reset the iterator using “keys” between calls to “each”:

    perl -e '%t=map{$_=>1}qw/1 2 3/;
    while(($k,$v)=each %t){print "1: $k\n"; last;}
    keys %t;
    while(($k,$v)=each %t){print "2: $k\n";}'

This got us in SpamAssassin — bug 4829.

To be honest, having to call “keys” after the loop is kludgy — as you can see if you check the patch in bug 4829 there, we had to change from a “return inside loop” pattern to a “set variable and exit loop, reset state, then return” pattern. It’d be nice to have a scoped version of each(), instead of this global scope, so that this would work:

    perl -e '%t=map{$_=>1}qw/1 2 3/;
    { while(($k,$v)=scoped_each %t){print "1: $k\n"; last;} }
    # that each() iterator is now out of scope, so GC'd;
    # the next call uses a new iterator, starting from scratch
    { while(($k,$v)=scoped_each %t){print "2: $k\n";} }'

Scoping, of course, has the benefit of allowing “return early” patterns to work; in my opinion, those are clearer — at the least because they require less lines of code ;)