Lua coding guide

2. The language

The generic for statement works over functions, called iterators. On each iteration, the iterator function is called to produce a new value, stopping when this new value is nil. The generic for loop has the following syntax:

stat ::= for namelist in explist do block end

namelist ::= Name {',' Name}

A for statement like

for var_1, ···, var_n in explist do block end

is equivalent to the code:

do

local f, s, var = explist

while true do

local var_1, ···, var_n = f(s, var)

var = var_1

if var == nil then break end

block

end

end

Note the following:

explist is evaluated only once. Its results are an iterator function, a state, and an initial value for the first iterator variable.

f, s, and var are invisible variables. The names are here for explanatory purposes only.

You can use break to exit a for loop.

The loop variables var_i are local to the loop; you cannot use their values after the for ends. If you need these values, then assign them to other variables before breaking or exiting the loop.

2.4.6 - Function Calls as Statements

To allow possible side-effects, function calls can be executed as statements:

stat ::= functioncall

In this case, all returned values are thrown away. Function calls are explained in §2.5.8.

2.4.7 - Local Declarations

Local variables can be declared anywhere inside a block. The declaration can include an initial assignment:

stat ::= local namelist ['=' explist]

If present, an initial assignment has the same semantics of a multiple assignment (see §2.4.3). Otherwise, all variables are initialized with nil.

A chunk is also a block (see §2.4.1), and so local variables can be declared in a chunk outside any explicit block. The scope of such local variables extends until the end of the chunk.

The visibility rules for local variables are explained in §2.6.

2.5 - 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 §2.1; variables are explained in §2.3; function definitions are explained in §2.5.9; function calls are explained in §2.5.8; table constructors are explained in §2.5.7. Vararg expressions, denoted by three dots ('...'), can only be used when directly inside a vararg function; they are explained in §2.5.9.