Documentation

Julia enables package developers and users to document functions, types and other objects easily via a built-in documentation system since Julia 0.4.

Tip

This documentation system can also be used in Julia 0.3 via the Docile.jl package; see the documentation for that package for more details.

The basic syntax is very simple: any string appearing at the top-level right before an object (function, macro, type or instance) will be interpreted as documenting it (these are called docstrings). Here is a very simple example:

"Tell whether there are too foo items in the array."
foo(xs::Array) = ...

Documentation is interpreted as Markdown, so you can use indentation and code fences to delimit code examples from text. Technically, any object can be associated with any other as metadata; Markdown happens to be the default, but one can construct other string macros and pass them to the @doc macro just as well.

Here is a more complex example, still using Markdown:

“”“
bar(x[, y])

Compute the Bar index between x and y. If y is missing, compute the Bar index between all pairs of columns of x.

# Examples `julia julia> bar([1, 2], [1, 2]) 1 ` “”” function bar(x, y) ...

As in the example above, we recommend following some simple conventions when writing documentation:

1. Always show the signature of a function at the top of the documentation, with a four-space indent so that it is printed as Julia code.

This can be identical to the signature present in the Julia code (like mean(x::AbstractArray)), or a simplified form. Optional arguments should be represented with their default values (i.e. f(x, y=1)) when possible, following the actual Julia syntax. Optional arguments which do not have a default value should be put in brackets (i.e. f(x[, y]) and f(x[, y[, z]])). An alternative solution is to use several lines: one without optional arguments, the other(s) with them. This solution can also be used to document several related methods of a given function. When a function accepts many keyword arguments, only include a <keyword arguments> placeholder in the signature (i.e. f(x; <keyword arguments>)), and give the complete list under an # Arguments section (see point 4 below).

2. Include a single one-line sentence describing what the function does or what the object represents after the simplified signature block. If needed, provide more details in a second paragraph, after a blank line.

The one-line sentence should use the imperative form (“Do this”, “Return that”) instead of the third person (do not write “Returns the length...”) when documenting functions. It should end with a period. If the meaning of a function cannot be summarized easily, splitting it into separate composable parts could be beneficial (this should not be taken as an absolute requirement for every single case though).
  1. Do not repeat yourself.
Since the function name is given by the signature, there is no need to start the documentation with “The function bar...”: go straight to the point. Similarly, if the signature specifies the types of the arguments, mentioning them in the description is redundant.
  1. Only provide an argument list when really necessary.

For simple functions, it is often clearer to mention the role of the arguments directly in the description of the function’s purpose. An argument list would only repeat information already provided elsewhere. However, providing an argument list can be a good idea for complex functions with many arguments (in particular keyword arguments). In that case, insert it after the general description of the function, under an # Arguments header, with one * bullet for each argument. The list should mention the types and default values (if any) of the arguments:

"""
...
# Arguments
* `n::Integer`: the number of elements to compute.
* `dim::Integer=1`: the dimensions along which to perform the computation.
...
"""

5. Group examples under an # Examples section and use ``julia blocks instead of standard text.

Examples should consist of verbatim copies of the Julia REPL, including the julia> prompt (see example above). This will be used in the future to allow running examples automatically and checking that their actual output is consistent with that presented in the documentation (a feature called doctests). This way, the code will be tested and examples won’t get out of date without notice.
  1. Use backticks to identify code and equations.
Julia identifiers and code excerpts should always appear between backticks ` to enable highlighting. Equations in the LaTeX syntax can be inserted between double backticks ``. Use Unicode characters rather than their LaTeX escape sequence, i.e. ``α = 1`` rather than ``\\alpha = 1``.
  1. Place the starting and ending """ characters on lines by themselves.

That is, write:

"""
...

...
"""
f(x, y) = ...

rather than:

"""...

..."""
f(x, y) = ...

This makes it more clear where docstrings start and end.

  1. Respect the line length limit used in the surrounding code.
Docstrings are edited using the same tools as code. Therefore, the same conventions should apply. It it advised to add line breaks after 92 characters.

Accessing Documentation

Documentation can be accessed at the REPL or in IJulia by typing ? followed by the name of a function or macro, and pressing Enter. For example,

?fft
?@time
?r""

will bring up docs for the relevant function, macro or string macro respectively. In Juno using Ctrl-D will bring up documentation for the object under the cursor.

Functions & Methods

Functions in Julia may have multiple implementations, known as methods. While it’s good practice for generic functions to have a single purpose, Julia allows methods to be documented individually if necessary. In general, only the most generic method should be documented, or even the function itself (i.e. the object created without any methods by function bar end). Specific methods should only be documented if their behaviour differs from the more generic ones. In any case, they should not repeat the information provided elsewhere. For example:

"""
Multiplication operator. `x*y*z*...` calls this function with multiple
arguments, i.e. `*(x,y,z...)`.
"""
function *(x, y)
  # ... [implementation sold separately] ...
end

"When applied to strings, concatenates them."
function *(x::AbstractString, y::AbstractString)
  # ... [insert secret sauce here] ...
end

help?>*
Multiplication operator. `x*y*z*...` calls this function with multiple
arguments, i.e. `*(x,y,z...)`.

When applied to strings, concatenates them.

When retrieving documentation for a generic function, the metadata for each method is concatenated with the catdoc function, which can of course be overridden for custom types.

Advanced Usage

The @doc macro associates its first argument with its second in a per-module dictionary called META. By default, documentation is expected to be written in Markdown, and the doc"" string macro simply creates an object representing the Markdown content. In the future it is likely to do more advanced things such as allowing for relative image or link paths.

When used for retrieving documentation, the @doc macro (or equally, the doc function) will search all META dictionaries for metadata relevant to the given object and return it. The returned object (some Markdown content, for example) will by default display itself intelligently. This design also makes it easy to use the doc system in a programmatic way; for example, to re-use documentation between different versions of a function:

@doc "..." foo!
@doc (@doc foo!) foo

Or for use with Julia’s metaprogramming functionality:

for (f, op) in ((:add, :+), (:subtract, :-), (:multiply, :*), (:divide, :/))
    @eval begin
        $f(a,b) = $op(a,b)
    end
end
@doc "`add(a,b)` adds `a` and `b` together" add
@doc "`subtract(a,b)` subtracts `b` from `a`" subtract

Documentation written in non-toplevel blocks, such as if, for, and let, are not automatically added to the documentation system. @doc must be used in these cases. For example:

if VERSION > v"0.4"
    "..."
    f(x) = x
end

will not add any documentation to f even when the condition is true and must instead be written as:

if VERSION > v"0.4"
    @doc "..." ->
    f(x) = x
end

Syntax Guide

A comprehensive overview of all documentable Julia syntax.

In the following examples "..." is used to illustrate an arbitrary docstring which may be one of the follow four variants and contain arbitrary text:

"..."

doc"..."

"""
...
"""

doc"""
...
"""

@doc_str should only be used when the docstring contains $ or \ characters that should not be parsed by Julia such as LaTeX syntax or Julia source code examples containing interpolation.

Functions and Methods

"..."
function f end

"..."
f

Adds docstring "..." to Function f. The first version is the preferred syntax, however both are equivalent.

"..."
f(x) = x

"..."
function f(x)
    x
end

"..."
f(x)

Adds docstring "..." to Method f(::Any).

"..."
f(x, y = 1) = x + y

Adds docstring "..." to two Methods, namely f(::Any) and f(::Any, ::Any).

Types

"..."
abstract T

"..."
type T end

"..."
immutable T end

Adds the docstring "..." to type T.

"..."
type T
    "x"
    x
    "y"
    y
end

Adds docstring "..." to type T, "x" to field T.x and "y" to field T.y. Also applicable to immutable types.

"..."
typealias A T

Adds docstring "..." to the Binding A.

Bindings are used to store a reference to a particular Symbol in a Module without storing the referenced value itself.

Macros

"..."
macro m() end

"..."
:(@m)

Adds docstring "..." to the Binding @m. Adding documentation at the definition is the preferred approach.

Modules

"..."
module M end

module M

"..."
M

end

Adds docstring "..." to the Module M. Adding the docstring above the Module is the preferred syntax, however both are equivalent.

"..."
baremodule M
# ...
end

baremodule M

import Base: call, @doc

"..."
f(x) = x

end

Documenting a baremodule by placing a docstring above the expression automatically imports call and @doc into the module. These imports must be done manually when the module expression is not documented. Empty baremodules cannot be documented.

Global Variables

"..."
const a = 1

"..."
b = 2

"..."
global c = 3

Adds docstring "..." to the Bindings a, b, and c.

"..."
sym

Adds docstring "..." to the value associated with sym. Users should prefer documenting sym at it’s definition.

Multiple Objects

"..."
a, b

Adds docstring "..." to a and b each of which should be a documentable expression. This syntax is equivalent to

"..."
a

"..."
b

Any number of expressions many be documented together in this way. This syntax can be useful when two functions are related, such as non-mutating and mutating versions f and f!.

Macro-generated code

"..."
@m expression

Adds docstring "..." to expression generated by expanding @m expression. This allows for expressions decorated with @inline, @noinline, @generated, or any other macro to be documented in the same way as undecorated expressions.

Macro authors should take note that only macros that generate a single expression will automatically support docstrings. If a macro returns a block containing multiple subexpressions then the subexpression that should be documented must be marked using the @__doc__() macro.

The @enum macro makes use of @__doc__ to allow for documenting Enums. Examining it’s definition should serve as an example of how to use @__doc__ correctly.

@__doc__(ex)

Low-level macro used to mark expressions returned by a macro that should be documented. If more than one expression is marked then the same docstring is applied to each expression.

macro example(f)
    quote
        $(f)() = 0
        @__doc__ $(f)(x) = 1
        $(f)(x, y) = 2
    end |> esc
end

@__doc__ has no effect when a macro that uses it is not documented.

Markdown Syntax Notes

Julia’s Markdown parser supports most of the basic Markdown elements, including paragraphs, code blocks, bulleted lists and basic links. It’s also a work in progress, however, and support for more advanced things like tables is in the works.

Markdown.jl supports interpolation in a very similar way to basic string literals, with the difference that it will store the object itself in the Markdown tree (as opposed to converting it to a string). When the Markdown content is rendered the usual writemime methods will be called, and these can be overridden as usual. This design allows the Markdown to be extended with arbitrarily complex features (such as references) without cluttering the basic syntax.

In principle, the Markdown parser itself can also be arbitrarily extended by packages, or an entirely custom flavour of Markdown can be used, but this should generally be unnecessary.