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Signal an error, warning, or message

Source: R/cnd-abort.R, R/cnd-signal.R
abort.Rd

These functions are equivalent to base functions base::stop(), base::warning(), and base::message(). They signal a condition (an error, warning, or message respectively) and make it easy to supply condition metadata:

  • Supply class to create a classed condition that can be caught or handled selectively, allowing for finer-grained error handling.

  • Supply metadata with named ... arguments. This data is stored in the condition object and can be examined by handlers.

  • Supply call to inform users about which function the error occurred in.

  • Supply another condition as parent to create a chained condition.

Certain components of condition messages are formatted with unicode symbols and terminal colours by default. These aspects can be customised, see Customising condition messages.

Usage

abort(
  message = NULL,
  class = NULL,
  ...,
  call,
  body = NULL,
  footer = NULL,
  trace = NULL,
  parent = NULL,
  use_cli_format = NULL,
  .inherit = TRUE,
  .internal = FALSE,
  .file = NULL,
  .frame = caller_env(),
  .trace_bottom = NULL,
  .subclass = deprecated()
)

warn(
  message = NULL,
  class = NULL,
  ...,
  body = NULL,
  footer = NULL,
  parent = NULL,
  use_cli_format = NULL,
  .inherit = NULL,
  .frequency = c("always", "regularly", "once"),
  .frequency_id = NULL,
  .subclass = deprecated()
)

inform(
  message = NULL,
  class = NULL,
  ...,
  body = NULL,
  footer = NULL,
  parent = NULL,
  use_cli_format = NULL,
  .inherit = NULL,
  .file = NULL,
  .frequency = c("always", "regularly", "once"),
  .frequency_id = NULL,
  .subclass = deprecated()
)

signal(message = "", class, ..., .subclass = deprecated())

reset_warning_verbosity(id)

reset_message_verbosity(id)

Arguments

message

The message to display, formatted as a bulleted list. The first element is displayed as an alert bullet prefixed with ! by default. Elements named "*", "i", "v", "x", and "!" are formatted as regular, info, success, failure, and error bullets respectively. See Formatting messages with cli for more about bulleted messaging.

If a message is not supplied, it is expected that the message is generated lazily through cnd_header() and cnd_body() methods. In that case, class must be supplied. Only inform() allows empty messages as it is occasionally useful to build user output incrementally.

If a function, it is stored in the header field of the error condition. This acts as a cnd_header() method that is invoked lazily when the error message is displayed.

class

Subclass of the condition.

...

Additional data to be stored in the condition object. If you supply condition fields, you should usually provide a class argument. You may consider prefixing condition fields with the name of your package or organisation to prevent name collisions.

call

The execution environment of a currently running function, e.g. call = caller_env(). The corresponding function call is retrieved and mentioned in error messages as the source of the error.

You only need to supply call when throwing a condition from a helper function which wouldn't be relevant to mention in the message.

Can also be NULL or a defused function call to respectively not display any call or hard-code a code to display.

For more information about error calls, see Including function calls in error messages.

body, footer

Additional bullets.

trace

A trace object created by trace_back().

parent

Supply parent when you rethrow an error from a condition handler (e.g. with try_fetch()).

  • If parent is a condition object, a chained error is created, which is useful when you want to enhance an error with more details, while still retaining the original information.

  • If parent is NA, it indicates an unchained rethrow, which is useful when you want to take ownership over an error and rethrow it with a custom message that better fits the surrounding context.

    Technically, supplying NA lets abort() know it is called from a condition handler. This helps it create simpler backtraces where the condition handling context is hidden by default.

For more information about error calls, see Including contextual information with error chains.

use_cli_format

Whether to format message lazily using cli if available. This results in prettier and more accurate formatting of messages. See local_use_cli() to set this condition field by default in your package namespace.

If set to TRUE, message should be a character vector of individual and unformatted lines. Any newline character "\\n" already present in message is reformatted by cli's paragraph formatter. See Formatting messages with cli.

.inherit

Whether the condition inherits from parent according to cnd_inherits() and try_fetch(). By default, parent conditions of higher severity are not inherited. For instance an error chained to a warning is not inherited to avoid unexpectedly catching an error downgraded to a warning.

.internal

If TRUE, a footer bullet is added to message to let the user know that the error is internal and that they should report it to the package authors. This argument is incompatible with footer.

.file

A connection or a string specifying where to print the message. The default depends on the context, see the stdout vs stderr section.

.frame

The throwing context. Used as default for .trace_bottom, and to determine the internal package to mention in internal errors when .internal is TRUE.

.trace_bottom

Used in the display of simplified backtraces as the last relevant call frame to show. This way, the irrelevant parts of backtraces corresponding to condition handling (tryCatch(), try_fetch(), abort(), etc.) are hidden by default. Defaults to call if it is an environment, or .frame otherwise. Without effect if trace is supplied.

.subclass

[Deprecated] This argument was renamed to class in rlang 0.4.2 for consistency with our conventions for class constructors documented in https://adv-r.hadley.nz/s3.html#s3-subclassing.

.frequency

How frequently should the warning or message be displayed? By default ("always") it is displayed at each time. If "regularly", it is displayed once every 8 hours. If "once", it is displayed once per session.

.frequency_id

A unique identifier for the warning or message. This is used when .frequency is supplied to recognise recurring conditions. This argument must be supplied if .frequency is not set to "always".

id

The identifying string of the condition that was supplied as .frequency_id to warn() or inform().

Details

  • abort() throws subclassed errors, see "rlang_error".

  • warn() temporarily set the warning.length global option to the maximum value (8170), unless that option has been changed from the default value. The default limit (1000 characters) is especially easy to hit when the message contains a lot of ANSI escapes, as created by the crayon or cli packages

Error prefix

As with base::stop(), errors thrown with abort() are prefixed with "Error: ". Calls and source references are included in the prefix, e.g. "Error in my_function() at myfile.R:1:2:". There are a few cosmetic differences:

  • The call is stripped from its arguments to keep it simple. It is then formatted using the cli package if available.

  • A line break between the prefix and the message when the former is too long. When a source location is included, a line break is always inserted.

If your throwing code is highly structured, you may have to explicitly inform abort() about the relevant user-facing call to include in the prefix. Internal helpers are rarely relevant to end users. See the call argument of abort().

Backtrace

abort() saves a backtrace in the trace component of the error condition. You can print a simplified backtrace of the last error by calling last_error() and a full backtrace with summary(last_error()). Learn how to control what is displayed when an error is thrown with rlang_backtrace_on_error.

Muffling and silencing conditions

Signalling a condition with inform() or warn() displays a message in the console. These messages can be muffled as usual with base::suppressMessages() or base::suppressWarnings().

inform() and warn() messages can also be silenced with the global options rlib_message_verbosity and rlib_warning_verbosity. These options take the values:

  • "default": Verbose unless the .frequency argument is supplied.

  • "verbose": Always verbose.

  • "quiet": Always quiet.

When set to quiet, the message is not displayed and the condition is not signalled.

stdout and stderr

By default, abort() and inform() print to standard output in interactive sessions. This allows rlang to be in control of the appearance of messages in IDEs like RStudio.

There are two situations where messages are streamed to stderr:

  • In non-interactive sessions, messages are streamed to standard error so that R scripts can easily filter them out from normal output by redirecting stderr.

  • If a sink is active (either on output or on messages) messages are always streamed to stderr.

These exceptions ensure consistency of behaviour in interactive and non-interactive sessions, and when sinks are active.

See also

Examples

# These examples are guarded to avoid throwing errors
if (FALSE) {

# Signal an error with a message just like stop():
abort("The error message.")


# Unhandled errors are saved automatically by `abort()` and can be
# retrieved with `last_error()`. The error prints with a simplified
# backtrace:
f <- function() try(g())
g <- function() evalq(h())
h <- function() abort("Tilt.")
last_error()

# Use `summary()` to print the full backtrace and the condition fields:
summary(last_error())


# Give a class to the error:
abort("The error message", "mypkg_bad_error")

# This allows callers to handle the error selectively
tryCatch(
  mypkg_function(),
  mypkg_bad_error = function(err) {
    warn(conditionMessage(err)) # Demote the error to a warning
    NA                          # Return an alternative value
  }
)

# You can also specify metadata that will be stored in the condition:
abort("The error message.", "mypkg_bad_error", data = 1:10)

# This data can then be consulted by user handlers:
tryCatch(
  mypkg_function(),
  mypkg_bad_error = function(err) {
    # Compute an alternative return value with the data:
    recover_error(err$data)
  }
)


# If you call low-level APIs it may be a good idea to create a
# chained error with the low-level error wrapped in a more
# user-friendly error. Use `try_fetch()` to fetch errors of a given
# class and rethrow them with the `parent` argument of `abort()`:
file <- "http://foo.bar/baz"
try(
  try_fetch(
    download(file),
    error = function(err) {
      msg <- sprintf("Can't download `%s`", file)
      abort(msg, parent = err)
  })
)

# You can also hard-code the call when it's not easy to
# forward it from the caller
 f <- function() {
  abort("my message", call = call("my_function"))
}
g <- function() {
  f()
}
# Shows that the error occurred in `my_function()`
try(g())

}