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 export MyType, foo type MyType x end bar(x) = 2x foo(a::MyType) = bar(a.x) + 1 import Base.show 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
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 in
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
Once a variable is imported this way (or, equivalently, with the
keyword), 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.
Method definitions are a bit special: they do not search modules named in
using statements. The definition
function foo() creates a new
foo in the current module, unless
foo has already been imported from
elsewhere. For example, in
MyModule above we wanted to add a method
to the standard
show function, so we had to write
Modules and files¶
Files and file names are 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
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
using Core, since you can’t do anything without those
Base is the standard library (the contents of base/). All modules implicitly
using Base, since this is needed in the vast majority of cases.
Default top-level definitions and bare modules¶
In addition to
using Base, a module automatically contains a definition
eval function, which evaluates expressions within the context of
If these definitions are not wanted, modules can be defined using the
baremodule instead. In terms of
baremodule, a standard
module looks like this:
baremodule Mod using Base eval(x) = Core.eval(Mod, x) eval(m,x) = Core.eval(m, x) ... end
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.
Macros must be exported if they are intended to be used outside their
defining module. Macro names are written with
@ in import and
export statements, e.g.
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.