The possible forms of an @extref link are as follows:

  1. [title](@extref) where title is a section title
  2. [`name`](@extref) where name is a fully specified code object 🏅
  3. [text](@extref name) where text is an arbitrary link text and name (optionally enclosed in backticks) is a fully specified code object or sluggified section title. 🏅
  4. [title](@extref project) where project is a known project in the underlying InterLinks object
  5. [`name`](@extref project) 🥈
  6. [text](@extref :role:`name`)
  7. [text](@extref :domain:role:`name`)
  8. [text](@extref project name) 🏅
  9. [text](@extref project :role:`name`) 🥈
  10. [text](@extref project :domain:role:`name`)

The most commonly used forms of syntax should be (2), (3), and (8) 🏅, with (5), and (9) being useful in some situations 🥈, see the Recommended Syntax.

Assuming an InterLinks instance links, all of the above will reference the DocInventories.InventoryItem links[project][":domain:role:name"]. If project is not specified, the first project in links that contains a matching item will be used (up to a performance shortcut). If domain or role are not given, any domain or role will match.

Forms (1-3) most directly extend Documenter's built-in @ref syntax. Forms (4) and (5) take precedence over form (3) if project is a known element of links. The use of backticks around name in form (3) would avoid this ambiguity.

Forms (1) and (4) apply a sluggification to transform title into a name. This matches Documenter's @ref behavior for linking to section titles. The specifics of the sluggification algorithm are not guaranteed to be stable between different versions of Documenter, and they do not match the sluggification used by other documentation generators like Sphinx. For this reason, forms (1) and (4) only have limited usefulness.

Performance Tips

Although resolving external references is unlikely to have a significant impact on the build time of a project's documentation, there are some internals that affect the relative performance of the above @extref syntax forms.

When no project is given in the @extref specification, all projects declared in the InterLinks object may have to be searched for a matching item. The projects are searched in order, so the ordering in the definition of InterLinks matters.

However, DocumenterInterLinks implements a short-circuit mechanism to avoid having to specify the project when linking to code objects in most cases: If name starts with the name of a project followed by a period, then that project is searched first.

For example, in order to link to Documenter.makedocs, we can use


to immediately search the inventory links["Documenter"], making the reference lookup as efficient as for the more verbose

[`Documenter.makedocs`](@extref Documenter)

Further @extref calls that will use the short-circuit mechanism for efficient lookup are

[`makedocs`](@extref Documenter.makedocs)
[`makedocs`](@extref `Documenter.makedocs`)
[`makedocs`](@extref :function:`Documenter.makedocs`)
[`makedocs`](@extref :jl:function:`Documenter.makedocs`)

where the latter two are unnecessarily verbose, as Documenter.makedocs is already uniquely specified without the role or domain.

The short-circuit mechanism only works if the project name used in the instantiation of InterLinks matches the package name as it occurs in the fully specified name of any code object. That is, name the project "Documenter", not, e.g., "Documenter121" for version 1.2.1 of Documenter.

When this is not possible, e.g., for the Julia project which contains many different modules without a common prefix (Base, Core, LinearAlgebra, …), it is best to declare that project as the first element in InterLinks. That way,


looks in the Julia project first, avoiding the need for

[`Base.sort!`](@extref Julia)

(although you may still prefer the latter as a matter of clarity).


If possible, use the name of a package as it occurs in the fully specified name of any code objects when declaring the project in InterLinks.

With the Performance Tips in mind, not all of the 10 possible Syntax forms are recommended in practice. For maximum clarity and performance, use the following guidelines:

  1. When referencing section headers in another project, e.g., the Basic Markdown section in Documenter's documentation, look up the appropriate sluggified name:

    links["Documenter"]("Basic Markdown")
    1-element Vector{InventoryItem}:
     InventoryItem(":std:label:`Basic-Markdown`" => "showcase/#\$", dispname="Basic Markdown")

    and use form (8):

    [Basic Markdown](@extref Documenter Basic-Markdown)
  2. When directly referencing a code object, e.g., Documenter.makedocs, use form (2):


    Make sure that Documenter is a project name in links (see Performance Tips).

    This gets slightly more complicated when the code object is a "method" (where the docstring is for specific types of arguments), e.g., You will generally have to look up the full name

    2-element Vector{InventoryItem}:
     InventoryItem(":jl:method:`{AbstractString}`" => "base/file/")
     InventoryItem(":jl:method:`{Function}`" => "base/file/")

    and then use form (3),

    [``](@extref `{AbstractString}`)

    to link to a specific method. The use of backticks around the full method name is optional, but recommended especially when there are spaces in the type description. Note that if there is only a single method for a function, InterLinks by default (due to alias_methods_as_function) will add an alias that links the function name to that method docstring, allowing to use the shorter function name as a convenient target for the reference.

    If the module name of the object cannot match the project name (e.g., for the Julia documentation, which contains docstrings for Base, Core, LinearAlgebra, etc.), use form (5),

    [`Base.sort!`](@extref Julia)
  3. When referencing a page, e.g., the Home page of the Documenter documentation, use form (9):

    [Home page of the Documenter documentation](@extref Documenter :doc:`index`)

    The doc role is not strictly necessary, but it clearly distinguishes references to documents from references to headings (especially when both may exist with the same name).

Thus, the most commonly used forms of syntax for @extref links should be (2), (3), and (8), highlighted with 🏅 in Syntax, with (5), and (9) being useful in some situations (🥈).