---
title: "Auto-linking"
output: rmarkdown::html_vignette
description: >
  Learn how pkgdown's automatic linking works, and how you can 
  customise it.
vignette: >
  %\VignetteIndexEntry{Linking between pkgdown sites}
  %\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")`

If pkgdown can find a pkgdown site for the remote package, it will link to it; otherwise it will link to <http://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: http://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.