# Documenting functions

In this module, we will look at what is probably the most important part of writing a function: writing its documentation. By the end of this module, you will be able to write a docstring for your function that is accessible through Julia’s help mode.

Documenting functions is essential. It will help you recall what they do and how to use them, and without documentation, your code might as well not exist. Julia offers ways to see the documentation for functions (type ? at the REPL), and ways to develop manuals for your code (see Documenter and DocumenterTools).

There are different levels of documentation. Good code is its own documentation. Good comments convey the purpose of the code, and are a good way to keep track of insights about how it actually works. Good docstrings remove the ambiguities about how to use a function and what it does. Good vignettes and example show how to actually work with these functions. Each of these things (with the exception of examples if you are not building a large package), when neglected, makes users less likely to use your code well, or at all.

Defining the docstring (i.e. a text that serves as an explanation for the function) is done by adding a special long-form string, which uses """ as the delimiters, on the lines immediately before the function declaration. The first line of this docstring is by convention the signature of the function.

We can write a very simple example, using a function called twice, which returns twice its input. This is not terribly interesting, but we can document this function in details:

"""
twice(x::T) where {T <: Real}

The function twice will return, for an input x which is a Real number, the result of the x+x operation.

**Example**:

julia-repl
julia> twice(2)
4

"""
function twice(x::T) where {T <: Real}
return x + x
end

Main.var"##342".twice


Note that the content of the docstring is in the markdown language - we can have a lot of markup, including tables, bullet points, numbered lists, code blocks, etc… In fact, if we print the documentation entry for this function, it will show up (depending on your environment) as a formatted string:

@doc twice

twice(x::T) where {T <: Real}


The function twice will return, for an input x which is a Real number, the result of the x+x operation.

Example:

julia> twice(2)
4


When we add a new method, we can document it as well. This is because documentation entries for a function are collected together:

"""
twice(::Type{R}, x::T) where {R <: Real, T <: Real}

The function twice will return, for an input x which is a Real number, the result of the x+x operation. The result will be converted to the type R given as the first argument, which must also be a real number.

**Example**:

julia-repl
julia> twice(Float32, 2)
4.0f0

"""
function twice(::Type{R}, x::T) where {R <: Real, T <: Real}
return convert(R, x + x)
end

Main.var"##342".twice


If we now ask for the documentation of twice, we will be able to see two entries:

@doc twice

twice(x::T) where {T <: Real}


The function twice will return, for an input x which is a Real number, the result of the x+x operation.

Example:

julia> twice(2)
4

twice(::Type{R}, x::T) where {R <: Real, T <: Real}


The function twice will return, for an input x which is a Real number, the result of the x+x operation. The result will be converted to the type R given as the first argument, which must also be a real number.

Example:

julia> twice(Float32, 2)
4.0f0


The functions in this material almost never have a docstring, which is justified by the fact that they actually do: it is the text around them that explains how they are built. In short, we care more about building the function than about using it, and this very special use-case means that we can justify not having a documentation. In actual research work, functions must be documented.