# Creating new linters

#### lintr maintainers

#### 2023-11-07

This vignette describes the steps necessary to create a new linter.

See the last section for some details specific to writing new linters for `{lintr}`.

A good example of a simple linter is the `pipe_call_linter`.

    #' Pipe call linter
    #'
    #' Force explicit calls in magrittr pipes, e.g.,
    #' `1:3 %>% sum()` instead of `1:3 %>% sum`.
    #'
    #' @evalRd rd_tags("pipe_call_linter")
    #' @seealso [linters] for a complete list of linters available in lintr.
    #' @export
    pipe_call_linter <- function() {
      xpath <- "//expr[preceding-sibling::SPECIAL[text() = '%>%'] and *[1][self::SYMBOL]]"

      Linter(function(source_expression) {
        if (!is_lint_level(source_expression, "expression")) {
          return(list())
        }

        xml <- source_expression$xml_parsed_content

        bad_expr <- xml2::xml_find_all(xml, xpath)

        xml_nodes_to_lints(
          bad_expr,
          source_expression = source_expression,
          lint_message = "Use explicit calls in magrittr pipes, i.e., `a %>% foo` should be `a %>% foo()`.",
          type = "warning"
        )
      })
    }

Let’s walk through the parts of the linter individually.

## Writing the linter

    #' Pipe call linter
    #'
    #' Force explicit calls in magrittr pipes, e.g.,
    #' `1:3 %>% sum()` instead of `1:3 %>% sum`.

Describe the linter, giving it a title and briefly covering the usages that are discouraged when the linter is active.

    #' @evalRd rd_tags("pipe_call_linter")
    #' @seealso [linters] for a complete list of linters available in lintr.
    #' @export

These lines (1) generate a Tags section in the documentation for the linter1; (2) link to the full table of available linters; and (3) mark the function for export. The most unfamiliar here is probably (1), which can be skipped outside of `lintr` itself.

    pipe_call_linter <- function() {

Next, we define the name of the new linter. The convention is to suffix all linter names with `_linter`. All `_linter` functions are function factories that return a closure that will do the actual linting function. We could define additional parameters that are useful for the linter in this function declaration (see, e.g. `assignment_linter`), but `pipe_call_linter` requires no additional arguments.

    xpath <- "//expr[preceding-sibling::SPECIAL[text() = '%>%'] and *[1][self::SYMBOL]]"

Here is the core linter logic. `xpath` is an XPath expression for expressions matching the discouraged usage. `xpath` is saved inside the factory code (as opposed to inside the linter itself) for efficiency. Often, the `xpath` will be somewhat complicated / involve some assembly code using `paste()` or `glue::glue()`[^See `infix_spaces_linter()` for an example of this], in which case it is preferable to execute this code only once when creating the linter; the cached XPath is then re-used on each expression in each file where the linter is run.

Let’s examine the XPath a bit more closely:

    //expr                  # global search (//) for 'expr' nodes (R expressions), at any nesting level
    [                       # node[...] looks for any 'node' satisfying conditions in ...
      preceding-sibling::   # "siblings" are at the same nesting level in XML
        SPECIAL[            # 'SPECIAL' is the parse token for infix operators like %% or %+%
          text() = '%>%'    # text() returns the string associated with this node
        ]                   #
      and                   # combine conditions with 'and'
      *                     # match any node
      [1]                   # match the first such node
      [self::SYMBOL]        # match if the current node is a 'SYMBOL' (i.e., a 'name' in R)
    ]                       #

Taken together, that means we want to match `expr` nodes preceded by the `%>%` infix operator whose first child node is a `name`. That maps pretty closely to the description of what the `pipe_call_linter` is looking for, but there is subtlety in mapping between the R code you’re used to and how they show up in the XML representation. `expr` nodes in particular take some practice to get accustomed to – use the plentiful XPath-based linters in `lintr` as a guide to get extra practice2. Note: `xml2` implements XPath 1.0, which lacks some helpful features available in XPath 2.0.

    Linter(function(source_expression) {

This is the closure. It will be called on the `source_expression` variable that contains the top level expressions in the file to be linted. The call to `Linter()` gives this closure the class ‘linter’ (it also guesses the name of the linter; see `?Linter` for more details).

The raw text of the expression is available from `source_file$content`. However, it is not generally possible to implement linters from the raw text – consider `equals_na_linter`. If we just look for `== NA` in the text of the file, we’ll generate many false positives, e.g. in comments (such as `# note: is.na() is the proper way to check == NA`) or inside character literals (such as `warning("don't use == NA to check missingness")`). We’re also likely to generate false negatives, for example when `==` and `NA` appear on different lines! Working around these issues using only the un-parsed text in every situation amounts to re-implementing the parser.

Therefore it is recommended to work with the tokens from `source_file$parsed_content` or `source_file$xml_parsed_content`, as they are tokenized from the `R` parser. These tokens are obtained from `parse()` and `utils::getParseData()` calls done prior to calling the new linter. `getParseData()` returns a `data.frame` with information from the source parse tree of the file being linted. A list of tokens is available from r-source/src/main/gram.y.

`source_file$xml_parsed_content` uses `xmlparsedata::xml_parse_data()` to convert the `getParseData()` output into an XML tree, which enables writing linter logic in XPath, a powerful language for expressing paths within the nested XML data structure. Most linters in `lintr` are built using XPath because it is a powerful language for computation on the abstract syntax tree / parse tree.

    if (!is_lint_level(source_expression, "expression")) {
      return(list())
    }

Here, we return early if `source_expression` is not the expression-level object. `get_source_expression()` returns an object that parses the input file in two ways – once is done expression-by-expression, the other contains all of the expressions in the file. This is done to facilitate caching. Suppose your package has a long source file (e.g., 100s of expressions) – rather than run linters on every expression every time the file is updated, when caching is activated `lintr` will only run the linter again on expressions that have changed.

Therefore, it is preferable to write expression-level linters whenever possible. Two types of exceptions observed in `lintr` are (1) when several or all expressions are required to ensure the linter logic applies (e.g., `conjunct_test_linter` looks for consecutive calls to `stopifnot()`, which will typically appear on consecutive expressions) or (2) when the linter logic is very simple & fast to compute, so that the overhead of re-running the linter is low (e.g., `single_quotes_linter`). In those cases, use `is_lint_level(source_expression, "file")`.

    xml <- source_expression$xml_parsed_content

    bad_expr <- xml2::xml_find_all(xml, xpath)

`source_expression$xml_parsed_content` is copied to a local variable (this is not strictly necessary but facilitates debugging). Then `xml2::xml_find_all()` is used to execute the XPath on this particular expression. Keep in mind that it is typically possible for a single expression to generate more than one lint – for example, `x %>% na.omit %>% sum` will trigger the `pipe_call_linter()` twice3.

    xml_nodes_to_lints(
      bad_expr,
      source_expression = source_expression,
      lint_message = "Use explicit calls in magrittr pipes, i.e., `a %>% foo` should be `a %>% foo()`.",
      type = "warning"
    )

Finally, we pass the matching XML node(s) to `xml_nodes_to_lints()`, which returns `Lint` objects corresponding to any “bad” usages found in `source_expression`. See `?Lint` and `?xml_nodes_to_lints` for details about the arguments. Note that here, the `message` for the lint is always the same, but for many linters, the message is customized to more closely match the observed usage. In such cases, `xml_nodes_to_lint()` can conveniently accept a function in `lint_message` which takes a node as input and converts it to a customized message. See, for example, `seq_linter`.

## Writing linter tests

(NB: this section uses the `assignment_linter()` which has simpler examples than `pipe_continuation_linter()`.)

`{lintr}` works best inside the `{testthat}` unit testing framework, in particular, `{lintr}` exports `lintr::expect_lint()` which is designed as a companion to other testthat expectations.

You can define tests inside separate `test_that` calls. Most of the linters use the same default form.

    test_that("returns the correct linting", {

You then test a series of expectations for the linter using `expect_lint`. Please see `?expect_lint` for a full description of the parameters.

The main three aspects to test are:

1. Linter returns no lints when there is nothing to lint, e.g.

    expect_lint("blah", NULL, assignment_linter())

2. Linter returns a lint when there is something to lint, e.g.

    expect_lint("blah=1",
      rex("Use <-, not =, for assignment."),
      assignment_linter()
    )

3. As many edge cases as you can think of that might break it, e.g.

    expect_lint("fun((blah = fun(1)))",
      rex("Use <-, not =, for assignment."),
      assignment_linter()
    )

You may want to test that additional `lint` attributes are correct, such as the type, line number, column number, e.g.

    expect_lint("blah=1",
      list(message = "assignment", line_number = 1, column_number = 5, type = "style"),
      assignment_linter()
    )

Finally, it is a good idea to test that your linter reports multiple lints if needed, e.g.

    expect_lint("blah=1; blah=2",
      list(
        list(line_number = 1, column_number = 5),
        list(line_number = 1, column_number = 13),
      )
      assignment_linter()
    )

It is always better to write too many tests rather than too few.

## Other utilities for writing custom linters

Besides `is_lint_level()`, `{lintr}` also exports some other helpers generally useful for writing custom linters; these are used a lot in the internals of our own helpers, and so they’ve been tested and demonstrated their utility already.

- `get_r_string()`: Whenever your linter needs to examine the value of a character literal (e.g., whether an argument value is set to some string), use this to extract the string exactly as R will see it. This is especially important to make your logic robust to R-4-style raw strings like `R"-(hello)-"`, which is otherwise difficult to express, for example as an XPath.

## Contributing to `{lintr}`

### More details about writing tests for new `{lintr}` linters

The `{lintr}` package uses testthat for testing. You can run all of the currently available tests using `devtools::test()`. If you want to run only the tests in a given file use the `filter` argument to `devtools::test()`.

Linter tests should be put in the tests/testthat/ folder. The test filename should be the linter name prefixed by `test-`, e.g. `test-pipe_continuation_linter.R`.

### Adding your linter to the default_linters

If your linter implements part of the tidyverse style guide you can add it to `default_linters`. This object is created in the file `zzz.R` (this name ensures that it will always run after all the linters are defined). Add your linter name to the `default_linters` list before the `NULL` at the end, and add a corresponding test case to the test script `./tests/testthat/default_linter_testcode.R`.

### Submit pull request

Push your changes to a branch of your fork of the lintr repository, and submit a pull request to get your linter merged into lintr!

1. NB: this is a helper function for generating custom Rd styling. See R/linter_tags.R.↩︎

2. The W3schools tutorials are also quite helpful; see https://www.w3schools.com/xml/xpath_intro.asp↩︎

3. This is particularly important if you want the `message` field in the resulting `Lint()` to vary depending on the exact violation that’s found. For `pipe_call_linter()`, the message is always the same. See `assignment_linter()` for an example where the `message` can vary.↩︎