Modules

Modules in Julia are separate global variable workspaces. They are delimited syntactically, inside module Name ... end. Modules allow you to create top-level definitions without worrying about name conflicts when your code is used together with somebody else’s. Within a module, you can control which names from other modules are visible (via importing), and specify which of your names are intended to be public (via exporting).

The following example demonstrates the major features of modules. It is not meant to be run, but is shown for illustrative purposes:

module MyModule
using Lib

using BigLib: thing1, thing2

import Base.show

importall OtherLib

export MyType, foo

type MyType
    x
end

bar(x) = 2x
foo(a::MyType) = bar(a.x) + 1

show(io, a::MyType) = print(io, "MyType $(a.x)")
end

Note that the style is not to indent the body of the module, since that would typically lead to whole files being indented.

This module defines a type MyType, and two functions. Function foo and type MyType are exported, and so will be available for importing into other modules. Function bar is private to MyModule.

The statement using Lib means that a module called Lib will be available for resolving names as needed. When a global variable is encountered that has no definition in the current module, the system will search for it among variables exported by Lib and import it if it is found there. This means that all uses of that global within the current module will resolve to the definition of that variable in Lib.

The statement using BigLib: thing1, thing2 is a syntactic shortcut for using BigLib.thing1, BigLib.thing2.

The import keyword supports all the same syntax as using, but only operates on a single name at a time. It does not add modules to be searched the way using does. import also differs from using in that functions must be imported using import to be extended with new methods.

In MyModule above we wanted to add a method to the standard show function, so we had to write import Base.show. Functions whose names are only visible via using cannot be extended.

The keyword importall explicitly imports all names exported by the specified module, as if import were individually used on all of them.

Once a variable is made visible via using or import, a module may not create its own variable with the same name. Imported variables are read-only; assigning to a global variable always affects a variable owned by the current module, or else raises an error.

Summary of module usage

To load a module, two main keywords can be used: using and import. To understand their differences, consider the following example:

module MyModule

export x, y

x() = "x"
y() = "y"
p() = "p"

end

In this module we export the x and y functions (with the keyword export), and also have the non-exported function p. There are several different ways to load the Module and its inner functions into the current workspace:

Import Command What is brought into scope Available for method extension
using MyModule All export ed names (x and y), MyModule.x, MyModule.y and MyModule.p MyModule.x, MyModule.y and MyModule.p
using MyModule.x, MyModule.p x and p  
using MyModule: x, p x and p  
import MyModule MyModule.x, MyModule.y and MyModule.p MyModule.x, MyModule.y and MyModule.p
import MyModule.x, MyModule.p x and p x and p
import MyModule: x, p x and p x and p
importall MyModule All export ed names (x and y) x and y

Modules and files

Files and file names are mostly unrelated to modules; modules are associated only with module expressions. One can have multiple files per module, and multiple modules per file:

module Foo

include("file1.jl")
include("file2.jl")

end

Including the same code in different modules provides mixin-like behavior. One could use this to run the same code with different base definitions, for example testing code by running it with “safe” versions of some operators:

module Normal
include("mycode.jl")
end

module Testing
include("safe_operators.jl")
include("mycode.jl")
end

Standard modules

There are three important standard modules: Main, Core, and Base.

Main is the top-level module, and Julia starts with Main set as the current module. Variables defined at the prompt go in Main, and whos() lists variables in Main.

Core contains all identifiers considered “built in” to the language, i.e. part of the core language and not libraries. Every module implicitly specifies using Core, since you can’t do anything without those definitions.

Base is the standard library (the contents of base/). All modules implicitly contain using Base, since this is needed in the vast majority of cases.

Default top-level definitions and bare modules

In addition to using Base, all operators are explicitly imported, since one typically wants to extend operators rather than creating entirely new definitions of them. A module also automatically contains a definition of the eval function, which evaluates expressions within the context of that module.

If these definitions are not wanted, modules can be defined using the keyword baremodule instead. In terms of baremodule, a standard module looks like this:

baremodule Mod

using Base

importall Base.Operators

eval(x) = Core.eval(Mod, x)
eval(m,x) = Core.eval(m, x)

...

end

Relative and absolute module paths

Given the statement using Foo, the system looks for Foo within Main. If the module does not exist, the system attempts to require("Foo"), which typically results in loading code from an installed package.

However, some modules contain submodules, which means you sometimes need to access a module that is not directly available in Main. There are two ways to do this. The first is to use an absolute path, for example using Base.Sort. The second is to use a relative path, which makes it easier to import submodules of the current module or any of its enclosing modules:

module Parent

module Utils
...
end

using .Utils

...
end

Here module Parent contains a submodule Utils, and code in Parent wants the contents of Utils to be visible. This is done by starting the using path with a period. Adding more leading periods moves up additional levels in the module hierarchy. For example using ..Utils would look for Utils in Parent‘s enclosing module rather than in Parent itself.

Note that relative-import qualifiers are only valid in using and import statements.

Module file paths

The global variable LOAD_PATH contains the directories Julia searches for modules when calling require. It can be extended using push!:

push!(LOAD_PATH, "/Path/To/My/Module/")

Putting this statement in the file ~/.juliarc.jl will extend LOAD_PATH on every Julia startup. Alternatively, the module load path can be extended by defining the environment variable JULIA_LOAD_PATH.

Miscellaneous details

If a name is qualified (e.g. Base.sin), then it can be accessed even if it is not exported. This is often useful when debugging.

Macro names are written with @ in import and export statements, e.g. import Mod.@mac. Macros in other modules can be invoked as Mod.@mac or @Mod.mac.

The syntax M.x = y does not work to assign a global in another module; global assignment is always module-local.

A variable can be “reserved” for the current module without assigning to it by declaring it as global x at the top level. This can be used to prevent name conflicts for globals initialized after load time.