---
title: "Auto-linking"
output: rmarkdown::html_vignette
description: >
  Learn how pkgdown's automatic linking works, and how you can 
  customise it.
vignette: >
  %\VignetteIndexEntry{Auto-linking}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r, include = FALSE}
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)
```

## Within a package

pkgdown will automatically link to documentation and articles wherever it's possible to do unambiguously. This includes:

* Bare function calls, like `build_site()`.
* Calls to `?`, like `?build_site` or `package?pkgdown`.
* Calls to `help()`, like `help("pkgdown")`.
* Calls to `vignette()`, like `vignette("pkgdown")`.

## Across packages

Linking to documentation in another package is straightforward. Just adapt the call in the usual way:

* `purrr::map()`, `MASS::addterm()`.
* `?purrr::map`, `?MASS::addterm`.
* `vignette("other-langs", package = "purrr")`, `vignette("longintro", package = "rpart")`
* `{purrr}`

If pkgdown can find a pkgdown site for the remote package, it will link to it; otherwise, it will link to <https://rdrr.io/> for documentation and CRAN for vignettes. In order for a pkgdown site to be findable, it needs to be listed in two places:

*   In the `URL` field in the `DESCRIPTION`, as in
    [dplyr](https://github.com/tidyverse/dplyr/blob/85faf79c1fd74f4b4f95319e5be6a124a8075502/DESCRIPTION#L15):
  
    ```
    URL: https://dplyr.tidyverse.org, https://github.com/tidyverse/dplyr
    ```

*   In the `url` field in `_pkgdown.yml`, as in 
    [dplyr](https://github.com/tidyverse/dplyr/blob/master/_pkgdown.yml#L1)
    
    ```yaml
    url: https://dplyr.tidyverse.org
    ```
    
    When this field is defined, pkgdown generate a public facing
    [`pkgdown.yml` file](https://dplyr.tidyverse.org/pkgdown.yml) that provides
    metadata about the site:
    
    ```yaml
    pandoc: '2.2'
    pkgdown: 1.3.0
    pkgdown_sha: ~
    articles:
      compatibility: compatibility.html
      dplyr: dplyr.html
      dplyr_0.8.0: future/dplyr_0.8.0.html
      dplyr_0.8.0_new_hybrid: future/dplyr_0.8.0_new_hybrid.html
      programming: programming.html
      two-table: two-table.html
      window-functions: window-functions.html
    urls:
      reference: https://dplyr.tidyverse.org/reference
      article: https://dplyr.tidyverse.org/articles
    ```

Now, when you build a pkgdown site for a package that links to the dplyr documentation (e.g., `dplyr::mutate()`), pkgdown looks first in dplyr's `DESCRIPTION` to find its website, then it looks for `pkgdown.yml`, and uses the metadata to generate the correct links.

To allow your package to be linked by other locally installed packages, even if your website is not reachable at build time, the following option needs to be set in `_pkgdown.yml`:

```yaml
deploy:
  install_metadata: true
```

This allows locally installed packages to access package index metadata from the locally installed copy, which may be useful
if your website require auth, or you build behind a firewall.