Writing robust, performant linters

Accumulated knowledge on best practice writing linters

The {lintr} codebase has a lot of accumulated knowledge about how to write robust and fast linters. This Wiki exists as a repository for tidbits on these topics.

It exists as a Wiki to make editing it open to all and with low overhead.

Tips for robustness

• When writing a test for logical constants like TRUE or FALSE, if you want the condition to match the shorthands T and F, note that the former is a NUM_CONST while the latter is a SYMBOL (c.f. getParseData(parse(text = "TRUE; T"))).
• Keep pipes (%>%, |>) in mind when writing lints based on positional logic (e.g. if it's a lint for the 2nd argument to meet some condition, that will usually become the 1st argument inside a pipe chain).
• The magrittr pipe %>% and the "native pipe" |> show up differently on the parse tree: SPECIAL and PIPE, respectively. Note that all infix operators (e.g. %%, %in%, %*%) show up as SPECIAL, so you'll need to test the text() as well for magrittr pipes.
• Often, it's better to anchor on EQ_SUB instead of SYMBOL_SUB when writing conditions around named arguments. The latter need not be present in all cases, e.g. in foo("a" = 1), which is valid R code, the parse tree will have a STR_CONST for "a", not a SYMBOL_SUB.
• Be wary of * searches like preceding-sibling::*[1]. Are you sure everything counts? One common mistake is to include <COMMENT> nodes here, so the XPath lands on a comment instead of the intended expression. Exclude such comments like preceding-sibling::*[not(self::COMMENT)][1].

Tips for performance

• Avoid //* XPaths like the plague! At least in the current {xml2}, it is almost always slower than alternatives. A good example is https://github.com/r-lib/lintr/pull/2025, which shows a 3x speed-up from avoiding //* even though the replacement is a long, inefficient-seeming chain of //A[expr] | //B[expr]-style repetitive expressions.
• Similarly, avoid //expr XPaths. See https://github.com/r-lib/lintr/issues/1358 -- more than 1/3 of all nodes are <expr>, so //expr only eliminates a relatively small portion of the parse tree. The more specific a node you can anchor on, the better, but the difference among nodes besides <expr> is not as important, so err on the side of readability/comprehensibility.
• If you use //SYMBOL_FUNCTION_CALL as an entry point, use the xml_find_function_calls() helper instead, because it returns cached results much faster, especially when testing for multiple options of text() = 'foo'.