Common Lua shortcuts

This article covers several syntactic features of Lua that are commonly used in addon code.

Logical short-cut evaluation
When evaluating the results of boolean operators and and or, Lua only evaluates the right-hand operand if necessary. This makes it possible to provide default values to nil or false expressions, or implement something resembling the C/C++ ternary expression (A?B:C):

For instance: MySVTable = MySVTable or {} print(UnitExists("target") and "You have a target" or "You don't have a target")

A notable limitation of using short-cut evaluation as a ternary expression is that the middle expression must not be false/nil. (After all, there is no actual ternary expression; you are simply chaining boolean expressions and must still follow their rules of precedence.) function isFrameShown(frame, checkParentFrames) -- The expression below returns the wrong value if frame's parent is hidden. return checkParentFrames and frame:IsVisible or frame:IsShown end If you find yourself writing code of the style A and B or C, you may need to rewrite it to ensure that B can never be false or nil.

Colon syntax for defining methods
The colon syntax is used for defining methods -- functions that have an implicit first parameter self. The following function definitions are equivalent: t = {} function t:foo(bar) end function t.foo(self, bar) end

When you call a function using the colon syntax, you implicitly supply the table left of the colon as the first argument to the function. The following two function calls are equivalent: t:foo("baz") t.foo(t, "baz")

This notation is frequently used to emulate something resembling objects.

The global environment table
The global environment table  is defined in most function environments. You can look up global variables by using their names as a key in this table. For example: -- Suppose MyBar1, MyBar2, MyBar3 are global variables (Frames created in XML, for example) for i=1, 3 do _G["MyBar" .. i]:Show end

The getglobal("name") function used to do the same thing, but has now been deprecated and removed.