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Dolan, Ryanne Thomas (UMR-Student) wrote:
Exactly.  The only problem with nil/false is that people use these values incorrectly.  nil is by no means synonomous with false, so it makes sense that nil ~= false.  Being false and not defined are two very different states.

I agree. To me, this discussion is just like the one with NULL's in RDBMS (1). NULL is not a value, it represents unknow, undetermined, not present etc. It is not 0, it is not "", it is not false (in systems that support boolean natively), it is not 0.0 nor it is ASCII NULL.

And to get worse, a table couldn't be checked for emptiness because it may have a metatable which, incidentally, may have __index and __newindex defined. It may be "physically" empty, but who knows what those methods will return?


userOptions = {}

groupOptions = {
    sendEmail = true;


if userOptions.sendEmail then
  mail.send{to = ..., subject = ..., ...}

Note that userOptions has no value inside it (no key, no value) but it is not empty. It's "parent" metatable has the values. Also note that 'nil' is different from 'false', because 'false' would mean the user don't want to receive e-mails, while 'nil' means he/she just don't care.

This discussion remembers me of some posts from The Daily WTF, where now-and-then some smart guy declares something like:

enum Bool = (TRUE,FALSE,MAYBE)


1. Except, of course, to the fact that in Lua, nil==nil, while in some RDBMS, NULL <> NULL.