# Documentation¶

Julia enables package developers and users to document functions, types and other objects easily, either via the built-in documentation system in Julia 0.4 or the Docile.jl package in Julia 0.3.

In 0.4:

"Tells you if 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.

"""
The @bar macro will probably make your code 2x faster or something. Use
it like this:

"""
macro bar(ex) ...


Documentation is very free-form; there are no set formatting restrictions or strict conventions. It’s hoped that best practices will emerge fairly naturally as package developers learn to use the new system.

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.

## 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. 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.

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.