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A boolean data type is one that by definition only has two states, "true" and "false".
Booleans in Lua
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!"; else print "They're different!"; end
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 ...