SUMMARY Arrays should always have known lengths because that's what arrays do. This requirement is enforced culturally, not programmatically.
DETAILS I submit for consideration the idea that if an array doesn't always have a defined length then it ceases to be that incredibly handy construct that we currently call "array". If arrays can answer "I dunno" when asked how long they are, that's taking away a useful feature, not adding one. If we dumb down arrays we're going to have to spend a lot of time smarting up our code. Suddenly every time you want to use an array in scalar or numeric context you first have to check if it has a defined length. That doesn't appeal to my sense of laziness, and it triggers my sense of impatience. OTOH, it doesn't seem Perlish to force the programmer to do something he or she may really not want to do. As always, Larry is wise in saying "an array implementation can return anything it jolly well pleases". So my proposal is that building array classes is like tying arrays and hashes is right now: there are certain expectations and people won't like your code if you don't follow the norms. You should always return a defined length. Documentation for arrays should indicate that they should always return a defined length. If you don't then you've broken the social contract and people will frown at you. If applicable to Perl6: the base class for tied arrays that most people will use to create their own arrays should set the length method as abstract so it needs to be overridden. -Miko
