quote(expr =) should be allowed

[aronatkins]

The "missing argument" form of quote(expr =) causes an infix_spaces_linter error. This syntax is suggested by http://adv-r.had.co.nz/Expressions.html#names

If the expression is rewritten as quote(expr = ), it causes a spaces_inside_linter error.

These errors can be avoided by nolint directive comments, but that feels excessive for recommended syntax.

# Code showing both erring syntaxes, and the directive suppressing the issue.
original <- pairlist(path = quote(expr =))
modified <- pairlist(path = quote(expr = ))
avoided <- pairlist(path = quote(expr =)) # nolint: infix_spaces_linter.

# Sample output.
R -s -e 'lintr::lint_dir(".")'
# => examples.R:1:40: style: [infix_spaces_linter] Put spaces around all infix operators.
# => original <- pairlist(path = quote(expr =))
# =>                                        ^
# => examples.R:2:41: style: [spaces_inside_linter] Do not place spaces before parentheses.
# => modified <- pairlist(path = quote(expr = ))
# =>                                         ^

[IndrajeetPatil]

Note that {styler}, which formats code according to tidyverse style guide, adds spaces around infix operators:

styler::style_text("quote(expr =)")
#> quote(expr = )

So the issue is how to not prevent missing_argument_linter() from getting triggered with this pattern:

library(lintr)

lint(text = "pairlist(path = quote(expr = ))", linters = all_linters())
#> <text>:1:28: style: [missing_argument_linter] Missing argument in function call.
#> pairlist(path = quote(expr = ))
#>                            ^

^{Created on 2023-01-05 with reprex v2.0.2}

Also, note that you have linked to the first edition of Advanced R where you find this advice. But the second edition recommends using {rlang} for missing argument:

identical(
  pairlist(path = quote(expr = )),
  pairlist(path = rlang::missing_arg())
)
#> [1] TRUE

^{Created on 2023-01-05 with reprex v2.0.2}

[aronatkins]

Thanks for the rlang suggestion. I encountered this problem when trying to lint the packrat source and it cannot take external dependencies. The implementation of rlang::missing_arg relies on a C implementation to return R_MissingArg and isn't easily adapted for Packrat:
https://github.com/r-lib/rlang/blob/9e261f86ec8d7aabf8a79705f82881f3447a1ea6/src/internal/exported.c#L696-L698

Funny story: The implementation of rlang::is_missing uses quote(expr = ).
https://github.com/r-lib/rlang/blob/4b1696c61677b19abdb4a135937e60420ad2b646/R/arg.R#L449

The CRAN release of lintr (which does not have all_linters):

lintr::lint(text = "pairlist(path = quote(expr = ))", linters = lintr:::default_linters)
#> <text>:1:29: style: [spaces_inside_linter] Do not place spaces before parentheses.
#> pairlist(path = quote(expr = ))
#>                             ^

Feel free to close this as won't-fix if you feel like the use of quote(expr = ) is rare. It can certainly happen with code that relies only on base R, but most folks are probably fine with an rlang dependency.

[aronatkins]

One other data point: The rlang::missing_arg documentation states that quote(expr = ) is the canonical way to create a missing argument object.

https://rlang.r-lib.org/reference/missing_arg.html#other-ways-to-reify-the-missing-argument

[IndrajeetPatil]

I would say that {lintr} should support this pattern.

I think the easiest way would be to include quote() in the vector of exceptions:

lintr/R/missing_argument_linter.R

Line 34 in 6d892a3

missing_argument_linter <- function(except = c("switch", "alist"), allow_trailing = FALSE) {