Basic Functions

Lua includes a variety of basic functions that are useful when working with Lua.

These functions are always available, and can be accessed by name as shown in the following example:

print(type(123)) -- number

The most-commonly used functions are:

assert(cond): Raise an error if cond is not truthy with message "assertion failed!"
assert(cond, msg): Raise an error if cond is not truthy with message msg

dofile(): Reads input from stdin and executes it, returning all values.
dofile(path): Executes the lua file located at path, returning all values..

getmetatable (o): Return the metatable from object o, of nil if o does not have a metatable.

ipairs (list): Returns (index, value) pairs from list. See iterating lists for more information.

pairs (map): Returns (key, value) pairs from map. See iterating maps for more information.

pcall (f, ···): calls function f with optional arguments ... in "protected mode". returns true, followed by an return values when no error occurs, otherwise returns false followed by an error message.

print (···): Prints arguments ... to stdout, passing non-string values to tostring.

require (name): Loads module name and returns it.

setmetatable (table): Removes table's metatable (i.e., sets its metatable to nil) and returns table.
setmetatable (table, t): Sets table's metatable to t and returns table.

tonumber (arg): Converts arg to a number and returns it. arg is expected to be a number or a string that represents a number. Otherwise, returns nil.
tonumber (arg, base): Converts arg to a number with base base and returns it. arg is expected to be a number or a string that represents a number. Otherwise, returns nil.

tostring (arg): Converts arg to a string and returns it.

type (arg): Returns a string specifying the type of arg.

unpack (list): Returns the elements from list.
unpack (list, i): Returns elements from index i through the end of list.
unpack (list, i, j): Returns elements from indices i through j of list

xpcall (f, callback): calls function f in "protected mode". returns true, followed by an return values when no error occurs, otherwise returns false followed by the return value(s) of callback(error), where error is the original error.