Create a format method for an object using cli tools

This method can be typically used in format() S3 methods. Then the print() method of the class can be easily defined in terms of such a format() method. See examples below.

cli_format_method(expr, theme = getOption("cli.theme"))

Arguments

expr

Expression that calls cli_* methods, base::cat() or base::print() to format an object's printout.

theme

Theme to use for the formatting.

Value

Character vector, one element for each line of the printout.

Examples

# Let's create format and print methods for a new S3 class that
# represents the an installed R package: `r_package`

# An `r_package` will contain the DESCRIPTION metadata of the package
# and also its installation path.
new_r_package <- function(pkg) {
  tryCatch(
    desc <- packageDescription(pkg),
    warning = function(e) stop("Cannot find R package `", pkg, "`")
  )
  file <- dirname(attr(desc, "file"))
  if (basename(file) != pkg) file <- dirname(file)
  structure(
    list(desc = unclass(desc), lib = dirname(file)),
    class = "r_package"
  )
}

format.r_package <- function(x, ...) {
  cli_format_method({
    cli_h1("{.pkg {x$desc$Package}} {cli::symbol$line} {x$desc$Title}")
    cli_text("{x$desc$Description}")
    cli_ul(c(
      "Version: {x$desc$Version}",
      if (!is.null(x$desc$Maintainer)) "Maintainer: {x$desc$Maintainer}",
      "License: {x$desc$License}"
    ))
    if (!is.na(x$desc$URL)) cli_text("See more at {.url {x$desc$URL}}")
  })
}

# Now the print method is easy:
print.r_package <- function(x, ...) {
  cat(format(x, ...), sep = "\n")
}

# Try it out
new_r_package("cli")
#> $desc
#> $desc$Package
#> [1] "cli"
#> 
#> $desc$Title
#> [1] "Helpers for Developing Command Line Interfaces"
#> 
#> $desc$Version
#> [1] "3.6.3"
#> 
#> $desc$`Authors@R`
#> [1] "c(\n    person(\"Gábor\", \"Csárdi\", , \"csardi.gabor@gmail.com\", role = c(\"aut\", \"cre\")),\n    person(\"Hadley\", \"Wickham\", role = \"ctb\"),\n    person(\"Kirill\", \"Müller\", role = \"ctb\"),\n    person(\"Salim\", \"Brüggemann\", , \"salim-b@pm.me\", role = \"ctb\",\n           comment = c(ORCID = \"0000-0002-5329-5987\")),\n    person(\"Posit Software, PBC\", role = c(\"cph\", \"fnd\"))\n  )"
#> 
#> $desc$Description
#> [1] "A suite of tools to build attractive command line interfaces\n    ('CLIs'), from semantic elements: headings, lists, alerts, paragraphs,\n    etc. Supports custom themes via a 'CSS'-like language. It also\n    contains a number of lower level 'CLI' elements: rules, boxes, trees,\n    and 'Unicode' symbols with 'ASCII' alternatives. It support ANSI\n    colors and text styles as well."
#> 
#> $desc$License
#> [1] "MIT + file LICENSE"
#> 
#> $desc$URL
#> [1] "https://cli.r-lib.org, https://github.com/r-lib/cli"
#> 
#> $desc$BugReports
#> [1] "https://github.com/r-lib/cli/issues"
#> 
#> $desc$Depends
#> [1] "R (>= 3.4)"
#> 
#> $desc$Imports
#> [1] "utils"
#> 
#> $desc$Suggests
#> [1] "callr,\ncovr,\ncrayon,\ndigest,\nglue (>= 1.6.0),\ngrDevices,\nhtmltools,\nhtmlwidgets,\nknitr,\nmethods,\nmockery,\nprocessx,\nps (>= 1.3.4.9000),\nrlang (>= 1.0.2.9003),\nrmarkdown,\nrprojroot,\nrstudioapi,\ntestthat,\ntibble,\nwhoami,\nwithr"
#> 
#> $desc$`Config/Needs/website`
#> [1] "r-lib/asciicast,\nbench,\nbrio,\ncpp11,\ndecor,\ndesc,\nfansi,\nprettyunits,\nsessioninfo,\ntidyverse/tidytemplate,\nusethis,\nvctrs"
#> 
#> $desc$`Config/testthat/edition`
#> [1] "3"
#> 
#> $desc$Encoding
#> [1] "UTF-8"
#> 
#> $desc$RoxygenNote
#> [1] "7.2.3"
#> 
#> $desc$Author
#> [1] "Gábor Csárdi [aut, cre],\n  Hadley Wickham [ctb],\n  Kirill Müller [ctb],\n  Salim Brüggemann [ctb] (<https://orcid.org/0000-0002-5329-5987>),\n  Posit Software, PBC [cph, fnd]"
#> 
#> $desc$Maintainer
#> [1] "Gábor Csárdi <csardi.gabor@gmail.com>"
#> 
#> $desc$Built
#> [1] "R 4.4.1; x86_64-pc-linux-gnu; 2025-05-23 18:07:00 UTC; unix"
#> 
#> attr(,"file")
#> [1] "/tmp/RtmpgHUc9l/temp_libpath39da712951c2e/cli/Meta/package.rds"
#> 
#> $lib
#> [1] "/tmp/RtmpgHUc9l/temp_libpath39da712951c2e"
#> 
#> attr(,"class")
#> [1] "r_package"

# The formatting of the output depends on the current theme:
opt <- options(cli.theme = simple_theme())
print(new_r_package("cli"))
#> 
#> ── cli ─ Helpers for Developing Command Line Interfaces ────────────────────────
#> A suite of tools to build attractive command line interfaces ('CLIs'), from
#> semantic elements: headings, lists, alerts, paragraphs, etc. Supports custom
#> themes via a 'CSS'-like language. It also contains a number of lower level
#> 'CLI' elements: rules, boxes, trees, and 'Unicode' symbols with 'ASCII'
#> alternatives. It support ANSI colors and text styles as well.
#> • Version: 3.6.3
#> • Maintainer: Gábor Csárdi <csardi.gabor@gmail.com>
#> • License: MIT + file LICENSE
#> See more at <https://cli.r-lib.org, https://github.com/r-lib/cli>
options(opt)  # <- restore theme