Expressions
The basic expressions in Lua are the following:
exp ::= prefixexp exp ::= *nil* | *false* | *true* exp ::= Number exp ::= String exp ::= function exp ::= tableconstructor exp ::= `*...*` exp ::= exp binop exp exp ::= unop exp prefixexp ::= var | functioncall | `*(*` exp `*)*`
Numbers and literal strings are explained in Lexical Conventions; variables are explained in Variables; function definitions are explained in Function Definitions; function calls are explained in Function Calls; table constructors are explained in Table Constructors. Vararg expressions, denoted by three dots ('=...='), can only be used inside vararg functions; they are explained in Function Definitions.
Binary operators comprise arithmetic operators (see Arithmetic Operators), relational operators (see Relational Operators), logical operators (see Logical Operators), and the concatenation operator (see Concatenation). Unary operators comprise the unary minus (see Arithmetic Operators), the unary not (see Logical Operators), and the unary length operator (see The Length Operator).
Both function calls and vararg expressions may result in multiple values. If the expression is used as a statement (see Function Calls as Statements) (only possible for function calls), then its return list is adjusted to zero elements, thus discarding all returned values. If the expression is used as the last (or the only) element of a list of expressions, then no adjustment is made (unless the call is enclosed in parentheses). In all other contexts, Lua adjusts the result list to one element, discarding all values except the first one.
Here are some examples:
f() -- adjusted to 0 results
g(f(), x) -- f() is adjusted to 1 result
g(x, f()) -- g gets x plus all results from f()
a,b,c = f(), x -- f() is adjusted to 1 result (c gets nil)
a,b = ... -- a gets the first vararg parameter, b gets
-- the second (both a and b may get nil if there
-- is no corresponding vararg parameter)
a,b,c = x, f() -- f() is adjusted to 2 results
a,b,c = f() -- f() is adjusted to 3 results
return f() -- returns all results from f()
return ... -- returns all received vararg parameters
return x,y,f() -- returns x, y, and all results from f()
{f()} -- creates a list with all results from f()
{...} -- creates a list with all vararg parameters
{f(), nil} -- f() is adjusted to 1 result
An expression enclosed in parentheses always results in only one value. Thus, (f(x,y,z)) is always a single value, even if f returns several values. (The value of (f(x,y,z)) is the first value returned by f or nil if f does not return any values.)