# Extending roxygen2

## Basics

Roxygen is extensible with user-defined roclets. It means that you can take advantage of Roxygen’s parser and extend it with your own `@tags`.

There are two primary ways to extend roxygen2:

- Add a new tag that generates a new top-level section in `.Rd` files.

- Add a new roclet that does anything you like.

This vignette will introduce you to the key data structures in roxygen2, and then show you how to use these two extension points. This vignette is very rough, so you are expected to have also read some roxygen2 source code to understand all the extension points. Hopefully, it’s useful enough to help you get started, and if you have problems, please file an issue!

    library(roxygen2)

## Key data structures

Before we talk about extending roxygen2, we need to first discuss two important data structures that power roxygen: tags and blocks.

### Tags

A tag (a list with S3 class `roxy_tag`) represents a single tag. It has the following fields:

- `tag`: the name of the tag.

- `raw`: the raw contents of the tag (i.e. everything from the end of this tag to the beginning of the next).

- `val`: the parsed value, which we’ll come back to shortly.

- `file` and `line`: the location of the tag in the package. Used with `roxy_tag_warning()` to produce informative error messages.

You can construct tag objects by hand with `roxy_tag()`:

    roxy_tag("name", "Hadley")
    #> [????:???] @name 'Hadley' {unparsed}
    str(roxy_tag("name", "Hadley"))
    #> List of 5
    #>  $ file: chr NA
    #>  $ line: chr NA
    #>  $ raw : chr "Hadley"
    #>  $ tag : chr "name"
    #>  $ val : NULL
    #>  - attr(*, "class")= chr [1:2] "roxy_tag_name" "roxy_tag"

However, you should rarely need to do so, because you’ll typically have them given to you in a block object, as you’ll see shortly.

### Blocks

A block (a list with S3 class `roxy_block`) represents a single roxygen block. It has the following fields:

- `tags`: a list of `roxy_tags`.
- `call`: the R code associated with the block (usually a function call).
- `file` and `line`: the location of the R code.
- `object`: the evaluated R object associated with the code.

The easiest way to see the basic structure of a `roxy_block()` is to generate one by parsing a roxygen block with `parse_text()`:

    text <- "
      #' This is a title
      #'
      #' This is the description.
      #'
      #' @param x,y A number
      #' @export
      f <- function(x, y) x + y
    "

    # parse_text() returns a list of blocks, so I extract the first
    block <- parse_text(text)[[1]]
    block
    #> <roxy_block> [<text>:8]
    #>   $tag
    #>     [line:  2] @title 'This is a title' {parsed}
    #>     [line:  4] @description 'This is the description.' {parsed}
    #>     [line:  6] @param 'x,y A number' {parsed}
    #>     [line:  7] @export '' {parsed}
    #>     [line:  8] @usage '<generated>' {parsed}
    #>     [line:  8] @.formals '<generated>' {parsed}
    #>     [line:  8] @backref '<generated>' {parsed}
    #>   $call   f <- function(x, y) x + y
    #>   $object <function>
    #>     $topic f
    #>     $alias f

## Adding a new `.Rd` tag

The easiest way to extend roxygen2 is to create a new tag that adds output to `.Rd` files. This requires two steps:

1. Define a `roxy_tag_parse()` method that describes how to parse our new tag.

2. Define a `roxy_tag_rd()` method that describes how to convert the tag into `.Rd` commands.

To illustrate the basic idea, we’ll create a new `@tip` tag that will create a bulleted list of tips about how to use a function. The idea is to take something like this:

    #' @tip The mean of a logical vector is the proportion of `TRUE` values.
    #' @tip You can compute means of dates and date-times!

And generate Rd like this:

    \section{Tips and tricks}{
    \itemize{
      \item The mean of a logical vector is the proportion of \code{TRUE} values.
      \item You can compute means of dates and date-times!
    }
    }

The first step is to define a method for `roxy_tag_parse()` that describes how to parse the tag text. The name of the class will be `roxy_tag_{tag}`, which in this case is `roxy_tag_tip`. This function takes a `roxy_tag` as input, and it’s job is to set `x$val` to a convenient parsed value that will be used later by the roclet. Here we want to process the text using Markdown so we can just use `tag_markdown()`:

    roxy_tag_parse.roxy_tag_tip <- function(x) {
      tag_markdown(x)
    }

We check this works by using `parse_text()`:

    text <- "
      #' Title
      #'
      #' @tip The mean of a logical vector is the proportion of `TRUE` values.
      #' @tip You can compute means of dates and date-times!
      #' @md
      f <- function(x, y) {
        # ...
      }
    "
    block <- parse_text(text)[[1]]
    block
    #> <roxy_block> [<text>:7]
    #>   $tag
    #>     [line:  2] @title 'Title' {parsed}
    #>     [line:  4] @tip 'The mean of a logical vector is the proportion ...' {parsed}
    #>     [line:  5] @tip 'You can compute means of dates and date-times!' {parsed}
    #>     [line:  6] @md '' {parsed}
    #>     [line:  7] @usage '<generated>' {parsed}
    #>     [line:  7] @.formals '<generated>' {parsed}
    #>     [line:  7] @backref '<generated>' {parsed}
    #>   $call   f <- function(x, y) { ...
    #>   $object <function>
    #>     $topic f
    #>     $alias f

    str(block$tags[[2]])
    #> List of 5
    #>  $ file: chr "<text>"
    #>  $ line: int 4
    #>  $ tag : chr "tip"
    #>  $ raw : chr "The mean of a logical vector is the proportion of `TRUE` values."
    #>  $ val : chr "The mean of a logical vector is the proportion of \\code{TRUE} values."
    #>  - attr(*, "class")= chr [1:2] "roxy_tag_tip" "roxy_tag"

(Here I explicitly turn Markdown parsing on using `@md`; it’s usually turned on for a package using roxygen options).

Next, we define a method for `roxy_tag_rd()`, which must create an `rd_section()`. We’re going to create a new section called `tip`. It will contain a character vector of tips:

    roxy_tag_rd.roxy_tag_tip <- function(x, base_path, env) {
      rd_section("tip", x$val)
    }

This additional layer is needed because there can be multiple tags of the same type in a single block, and multiple blocks can contribute to the same `.Rd` file. The job of the `rd_section` is to combine all the tags into a single top-level Rd section. Each tag generates an `rd_section` which is then combined with any previous section using `merge()`. The default `merge.rd_section()` just concatenates the values together (`rd_section(x$type, c(x$value, y$value))`); you can override this method if you need more sophisticated behaviour.

We then need to define a `format()` method to convert this object into text for the `.Rd` file:

    format.rd_section_tip <- function(x, ...) {
      paste0(
        "\\section{Tips and tricks}{\n",
        "\\itemize{\n",
        paste0("  \\item ", x$value, "\n", collapse = ""),
        "}\n",
        "}\n"
      )
    }

We can now try this out with `roclet_text()`:

    topic <- roc_proc_text(rd_roclet(), text)[[1]]
    topic$get_section("tip")
    #> \section{Tips and tricks}{
    #> \itemize{
    #>   \item The mean of a logical vector is the proportion of \code{TRUE} values.
    #>   \item You can compute means of dates and date-times!
    #> }
    #> }
    #>

Note that there is no namespacing so if you’re defining multiple new tags I recommend using your package name as the common prefix.

## Creating a new roclet

Creating a new roclet is usually a two part process. First, you define new tags that your roclet will work with. Second, you define a roclet that tells roxygen how to process an entire package.

### Custom tags

In this example we will make a new `@memo` tag to enable printing the memos at the console when the roclet runs. We choose that the `@memo` has this syntax:

    @memo [Headline] Description

As an example:

    @memo [EFFICIENCY] Currently brute-force; find better algorithm.

As above, we first define a parse method:

    roxy_tag_parse.roxy_tag_memo <- function(x) {
      if (!grepl("^\\[.*\\].*$", x$raw)) {
        roxy_tag_warning(x, "Invalid memo format")
        return()
      }

      parsed <- stringi::stri_match(str = x$raw, regex = "\\[(.*)\\](.*)")[1, ]

      x$val <- list(
        header = parsed[[2]],
        message = parsed[[3]]
      )
      x
    }

Then check if it works with `parse_text()`:

    text <- "
      #' @memo [TBI] Remember to implement this!
      #' @memo [API] Check best API
      f <- function(x, y) {
        # ...
      }
    "
    block <- parse_text(text)[[1]]
    block
    #> <roxy_block> [<text>:4]
    #>   $tag
    #>     [line:  2] @memo '[TBI] Remember to implement this!' {parsed}
    #>     [line:  3] @memo '[API] Check best API' {parsed}
    #>     [line:  4] @usage '<generated>' {parsed}
    #>     [line:  4] @.formals '<generated>' {parsed}
    #>     [line:  4] @backref '<generated>' {parsed}
    #>   $call   f <- function(x, y) { ...
    #>   $object <function>
    #>     $topic f
    #>     $alias f

    str(block$tags[[1]])
    #> List of 5
    #>  $ file: chr "<text>"
    #>  $ line: int 2
    #>  $ tag : chr "memo"
    #>  $ raw : chr "[TBI] Remember to implement this!"
    #>  $ val :List of 2
    #>   ..$ header : chr "TBI"
    #>   ..$ message: chr " Remember to implement this!"
    #>  - attr(*, "class")= chr [1:2] "roxy_tag_memo" "roxy_tag"

### The roclet

Next, we create a constructor for the roclet, which uses `roclet()`. Our `memo` roclet doesn’t have any options so this is very simple:

    memo_roclet <- function() {
      roclet("memo")
    }

To give the roclet behaviour, you need to define methods. There are two methods that almost every roclet will use:

- `roclet_process()` is called with a list of blocks, and returns an object of your choosing.

- `roclet_output()` produces side-effects (usually writing to disk) using the result from `roclet_process()`.

For this roclet, we’ll have `roclet_process()` collect all the memo tags into a named list:

    roclet_process.roclet_memo <- function(x, blocks, env, base_path) {
      results <- list()

      for (block in blocks) {
        tags <- block_get_tags(block, "memo")

        for (tag in tags) {
          msg <- paste0("[", tag$file, ":", tag$line, "] ", tag$val$message)
          results[[tag$val$header]] <- c(results[[tag$val$header]], msg)
        }
      }

      results
    }

And then have `roclet_output()` just print them to the screen:

    roclet_output.roclet_memo <- function(x, results, base_path, ...) {
      for (header in names(results)) {
        messages <- results[[header]]
        cat(paste0(header, ": ", "\n"))
        cat(paste0(" * ", messages, "\n", collapse = ""))
      }

      invisible(NULL)
    }

Then you can test if it works by using `roc_proc_text()`:

    results <- roc_proc_text(memo_roclet(), "
    #' @memo [TBI] Remember to implement this!
    #' @memo [API] Check best API
    f <- function(x, y) {
      # ...
    }

    #' @memo [API] Consider passing z option
    g <- function(x, y) {
      # ...
    }
    ")
    roclet_output(memo_roclet(), results)
    #> TBI:
    #>  * [<text>:2]  Remember to implement this!
    #> API:
    #>  * [<text>:3]  Check best API
    #>  * [<text>:8]  Consider passing z option

## Adding a roclet to your workflow

To use a roclet when developing a package, call

    roxygen2::roxygenize(roclets = "yourPackage::roclet")

where `yourPackage::roclet` is the function which creates the roclet, e.g. `memo_roclet` above.

You can also add the roclet to the target package’s DESCRIPTION file, like this:

    Roxygen: list(roclets = c("collate", "rd", "namespace", "yourPackage::roclet"))

Optionally, you can add your roclet package to the target package as a `Suggests:` dependency:

    usethis::use_dev_package("yourPackage", type = "Suggests", remote = "yourGithubID/yourPackage")

You don’t have to do this, but it will help other developers working on the target package.