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Condition formatting is a set of operations applied to raw inputs for error messages that includes:

  • Transforming a character vector of lines to a width-wrapped list of error bullets. This makes it easy to write messages in a list format where each bullet conveys a single important point.

    abort(c(
      "The error header",
      "*" = "An error bullet",
      "i" = "An info bullet",
      "x" = "A cross bullet"
    ))
    #> Error:
    #> ! The error header
    #> * An error bullet
    #> i An info bullet
    #> x A cross bullet

    See the tidyverse error style guide for more about this style of error messaging.

  • Applying style (emphasis, boldness, ...) and colours to message elements.

While the rlang package embeds rudimentary formatting routines, the main formatting engine is implemented in the cli package.

Formatting messages with cli

By default, rlang uses an internal mechanism to format bullets. It is preferable to delegate formatting to the cli package by using cli::cli_abort(), cli::cli_warn(), and cli::cli_inform() instead of the rlang versions. These wrappers enable cli formatting with sophisticated paragraph wrapping and bullet indenting that make long lines easier to read. In the following example, a long ! bullet is broken with an indented newline:

rlang::global_entrace(class = "errorr")
#> Error in `rlang::global_entrace()`:
#> ! `class` must be one of "error", "warning", or "message",
#>   not "errorr".
#> i Did you mean "error"?

The cli wrappers also add many features such as interpolation, semantic formatting of text elements, and pluralisation:

inform_marbles <- function(n_marbles) {
  cli::cli_inform(c(
    "i" = "I have {n_marbles} shiny marble{?s} in my bag.",
    "v" = "Way to go {.code cli::cli_inform()}!"
  ))
}

inform_marbles(1)
#> i I have 1 shiny marble in my bag.
#> v Way to go `cli::cli_inform()`!

inform_marbles(2)
#> i I have 2 shiny marbles in my bag.
#> v Way to go `cli::cli_inform()`!

Transitioning from abort() to cli_abort()

If you plan to mass-rename calls from abort() to cli::cli_abort(), be careful if you assemble error messages from user inputs. If these individual pieces contain cli or glue syntax, this will result in hard-to-debug errors and possibly unexpected behaviour.

user_input <- "{base::stop('Wrong message.', call. = FALSE)}"
cli::cli_abort(sprintf("Can't handle input `%s`.", user_input))
#> Error: Wrong message.

To avoid this, protect your error messages by using cli to assemble the pieces:

user_input <- "{base::stop('Wrong message.', call. = FALSE)}"
cli::cli_abort("Can't handle input {.code {user_input}}.")
#> Error:
#> ! Can't handle input `{base::stop('Wrong message.', call. = FALSE)}`.

Enabling cli formatting globally

To enable cli formatting for all abort() calls in your namespace, call local_use_cli() in the onLoad hook of your package. Using on_load() (make sure to call run_on_load() in your hook):

Enabling cli formatting in abort() is useful for:

  • Transitioning from abort() to cli::cli_abort() progressively.

  • Using abort() when you'd like to disable interpolation syntax.

  • Creating error conditions with error_cnd(). These condition messages will be automatically formatted with cli as well.