Mojo reference

This section includes the API reference for the Mojo standard library, as well as a reference for Mojo decorators and the Mojo CLI.

How to read the Mojo API docs

Mojo syntax is covered in detail in the Mojo manual. Here's a quick cheat-sheet on reading struct and function signatures.

Arguments

Function arguments appear in parentheses after the function name:

fn example_fn(pos: Int, /, pos_or_kw: Int, *, kw_only: Bool = False):
    ...
fn example_fn(pos: Int, /, pos_or_kw: Int, *, kw_only: Bool = False):
    ...

Here's a quick overview of some special syntax in the argument list:

Slash (/): arguments declared before a slash are positional-only arguments.
Star (*): a star by itself in place of an argument indicates that the arguments after the star are keyword-only.
An equals sign (=) introduces a default value for an optional argument.

You may also see argument names prefixed with one or two stars (*):

def myfunc2(*names, **attributes):
def myfunc2(*names, **attributes):

An argument name prefixed by a single star character, like *names identifies a variadic argument.
An argument name prefixed with a double star, like **attributes identifies a variadic keyword-only argument.

An argument may also be preceded by an argument convention, which indicates how the value is passed:

fn sort(mut names: List[String]):
fn sort(mut names: List[String]):

The most common conventions are:

read(default): the callee receives an immutable reference to the value.
mut: the callee receives a mutable reference to the value.
owned: the callee receives ownership of a value.

For details and a complete list of argument conventions, see Argument conventions.

Parameters

Mojo structs and functions can take parameters. Parameters are evaluated at compilation time, and act as constants at runtime. Parameter lists are enclosed in square brackets:

struct ExampleStruct[size: Int, //, thing: Thing[size]]:
struct ExampleStruct[size: Int, //, thing: Thing[size]]:

Parameters that occur before a double-slash (//) in the parameter list are infer-only parameters. You usually don't need to specify infer-only parameters; as the name suggests, they're usually inferred.

Like arguments, parameters can be positional-only, keyword-or-positional, or keyword-only, and they can be required or optional. The /, *, and = characters have the same meaning in parameter lists as they do in argument lists.

Mojo standard library

The Mojo standard library provides nearly everything you'll need for writing Mojo programs, including basic data types like Int and SIMD, collection types like List, reusable algorithms and modules to support GPU programming.

Here are all the top-level packages:

algorithm

Implements the algorithm package.

base64

Implements the base64 package.

benchmark

Implements the benchmark package for runtime benchmarking.

bit

Implements the bit package.

buffer

Implements the buffer package.

builtin