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A boolean data type is one that by definition only has two states, "true" and "false".

Booleans in Lua

In Lua, the programming language used by the World of Warcraft User Interface, real booleans were not introduced until version 5.0.

Early versions of World of Warcraft used Lua v4, which shows in that the majority of APIs still return 1 for "true", and nil for "false".

In Lua, there are only two values that evaluate to a boolean false: false and nil.

ALL OTHER VALUES EVALUATE TO TRUE. Including 0, which is an important distinction for e.g. C/C++ programmers.

Gotchas with Booleans in Lua

When used in if ... then clauses a 1 result is the same as a true result, so you do not have to worry there. But beware of comparing the returns of supposedly-boolean functions with eachother without forcing them to actually be boolean.


function Func1() return 1; end     -- old-style function returning 1 for true
function Func2() return true; end  -- new-style function actually using true

if(Func1() == Func2()) then
  print "They're both the same!";
  print "They're different!";

This would output "They're different!", because 1 and true are not the same thing in Lua.

You can force a value to be boolean by NOTing it twice, like so:

definitelyBoolean = not not kindaBoolean;

In the earlier disfunctional example, we'd get away with NOTing it just once since we're only testing if they're the same:

if( (not Func1()) == (not Func2()) ) then