Lua coding guide

2. The language

2.5.7 - Table Constructors

Table constructors are expressions that create tables. Every time a constructor is evaluated, a new table is created. A constructor can be used to create an empty table or to create a table and initialize some of its fields. The general syntax for constructors is

tableconstructor ::= '{' [fieldlist] '}'

fieldlist ::= field {fieldsep field} [fieldsep]

field ::= '[' exp ']' '=' exp | Name '=' exp | exp

fieldsep ::= ',' | ';'

Each field of the form [exp1] = exp2 adds to the new table an entry with key exp1 and value exp2. A field of the form name = exp is equivalent to ["name"] = exp. Finally, fields of the form exp are equivalent to [i] = exp, where i are consecutive numerical integers, starting with 1. Fields in the other formats do not affect this counting. For example,

a = { [f(1)] = g; "x", "y"; x = 1, f(x), [30] = 23; 45 }

is equivalent to

do

local t = {}

t[f(1)] = g

t[1] = "x" -- 1st exp

t[2] = "y" -- 2nd exp

t.x = 1 -- t["x"] = 1

t[3] = f(x) -- 3rd exp

t[30] = 23

t[4] = 45 -- 4th exp

a = t

end

If the last field in the list has the form exp and the expression is a function call or a vararg expression, then all values returned by this expression enter the list consecutively (see §2.5.8). To avoid this, enclose the function call or the vararg expression in parentheses (see §2.5).

The field list can have an optional trailing separator, as a convenience for machine-generated code.

2.5.8 - Function Calls

A function call in Lua has the following syntax:

functioncall ::= prefixexp args

In a function call, first prefixexp and args are evaluated. If the value of prefixexp has type function, then this function is called with the given arguments. Otherwise, the prefixexp "call" metamethod is called, having as first parameter the value of prefixexp, followed by the original call arguments (see §2.8).

The form

functioncall ::= prefixexp ':' Name args

can be used to call "methods". A call v:name(args) is syntactic sugar for v.name(v,args), except that v is evaluated only once.

Arguments have the following syntax:

args ::= '(' [explist] ')'

args ::= tableconstructor

args ::= String

All argument expressions are evaluated before the call. A call of the form f{fields} is syntactic sugar for f({fields}); that is, the argument list is a single new table. A call of the form f'string' (or f"string" or f[[string]]) is syntactic sugar for f('string'); that is, the argument list is a single literal string.

As an exception to the free-format syntax of Lua, you cannot put a line break before the '(' in a function call. This restriction avoids some ambiguities in the language. If you write