Editing WoW:Common Lua shortcuts

{{wow/uihowto}}
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 a trigraph operator.

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

A notable limitation of using short-cut evaluation as a trigraph operator is that if the ''true'' expression may return a false value, the false expression will be returned:
 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
This occurs because nothing prevents the evaluation of the or operator as its left-hand operand is false/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 <code>_G</code> 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 [[API getglobal|getglobal]]("name") function enables similar behavior, but at a marginally higher cost.
[[Category:Interface customization]]