These functions provide a mechanism for handling unusual conditions, including errors and warnings.
tryCatch(expr, …, finally)
withCallingHandlers(expr, …)signalCondition(cond)
simpleCondition(message, call = NULL)
simpleError (message, call = NULL)
simpleWarning (message, call = NULL)
simpleMessage (message, call = NULL)
# S3 method for condition
as.character(x, …)
# S3 method for error
as.character(x, …)
# S3 method for condition
print(x, …)
# S3 method for restart
print(x, …)
conditionCall(c)
# S3 method for condition
conditionCall(c)
conditionMessage(c)
# S3 method for condition
conditionMessage(c)
withRestarts(expr, …)
computeRestarts(cond = NULL)
findRestart(name, cond = NULL)
invokeRestart(r, …)
invokeRestartInteractively(r)
isRestart(x)
restartDescription(r)
restartFormals(r)
suspendInterrupts(expr)
allowInterrupts(expr)
.signalSimpleWarning(msg, call)
.handleSimpleError(h, msg, call)
.tryResumeInterrupt()
a condition object.
call expression.
a condition object.
expression to be evaluated.
expression to be evaluated before returning or exiting.
function.
character string.
character string.
character string naming a restart.
restart object.
object.
additional arguments; see details below.
The condition system provides a mechanism for signaling and handling unusual conditions, including errors and warnings. Conditions are represented as objects that contain information about the condition that occurred, such as a message and the call in which the condition occurred. Currently conditions are S3-style objects, though this may eventually change.
Conditions are objects inheriting from the abstract class
condition. Errors and warnings are objects inheriting
from the abstract subclasses error and warning.
The class simpleError is the class used by stop
and all internal error signals. Similarly, simpleWarning
is used by warning, and simpleMessage is used by
message. The constructors by the same names take a string
describing the condition as argument and an optional call. The
functions conditionMessage and conditionCall are
generic functions that return the message and call of a condition.
Conditions are signaled by signalCondition. In addition,
the stop and warning functions have been modified to
also accept condition arguments.
The function tryCatch evaluates its expression argument
in a context where the handlers provided in the …
argument are available. The finally expression is then
evaluated in the context in which tryCatch was called; that
is, the handlers supplied to the current tryCatch call are
not active when the finally expression is evaluated.
Handlers provided in the … argument to tryCatch
are established for the duration of the evaluation of expr.
If no condition is signaled when evaluating expr then
tryCatch returns the value of the expression.
If a condition is signaled while evaluating expr then
established handlers are checked, starting with the most recently
established ones, for one matching the class of the condition.
When several handlers are supplied in a single tryCatch then
the first one is considered more recent than the second. If a
handler is found then control is transferred to the
tryCatch call that established the handler, the handler
found and all more recent handlers are disestablished, the handler
is called with the condition as its argument, and the result
returned by the handler is returned as the value of the
tryCatch call.
Calling handlers are established by withCallingHandlers. If
a condition is signaled and the applicable handler is a calling
handler, then the handler is called by signalCondition in
the context where the condition was signaled but with the available
handlers restricted to those below the handler called in the
handler stack. If the handler returns, then the next handler is
tried; once the last handler has been tried, signalCondition
returns NULL.
User interrupts signal a condition of class interrupt that
inherits directly from class condition before executing the
default interrupt action.
Restarts are used for establishing recovery protocols. They can be
established using withRestarts. One pre-established restart is
an abort restart that represents a jump to top level.
findRestart and computeRestarts find the available
restarts. findRestart returns the most recently established
restart of the specified name. computeRestarts returns a
list of all restarts. Both can be given a condition argument and
will then ignore restarts that do not apply to the condition.
invokeRestart transfers control to the point where the
specified restart was established and calls the restart's handler with the
arguments, if any, given as additional arguments to
invokeRestart. The restart argument to invokeRestart
can be a character string, in which case findRestart is used
to find the restart.
New restarts for withRestarts can be specified in several ways.
The simplest is in name = function form where the function is
the handler to call when the restart is invoked. Another simple
variant is as name = string where the string is stored in the
description field of the restart object returned by
findRestart; in this case the handler ignores its arguments
and returns NULL. The most flexible form of a restart
specification is as a list that can include several fields, including
handler, description, and test. The
test field should contain a function of one argument, a
condition, that returns TRUE if the restart applies to the
condition and FALSE if it does not; the default function
returns TRUE for all conditions.
One additional field that can be specified for a restart is
interactive. This should be a function of no arguments that
returns a list of arguments to pass to the restart handler. The list
could be obtained by interacting with the user if necessary. The
function invokeRestartInteractively calls this function to
obtain the arguments to use when invoking the restart. The default
interactive method queries the user for values for the
formal arguments of the handler function.
Interrupts can be suspended while evaluating an expression using
suspendInterrupts. Subexpression can be evaluated with
interrupts enabled using allowInterrupts. These functions
can be used to make sure cleanup handlers cannot be interrupted.
.signalSimpleWarning, .handleSimpleError, and
.tryResumeInterrupt are used internally and should not be
called directly.
The tryCatch mechanism is similar to Java
error handling. Calling handlers are based on Common Lisp and
Dylan. Restarts are based on the Common Lisp restart mechanism.
stop and warning signal conditions,
and try is essentially a simplified version of
tryCatch.
assertCondition in package tools tests
that conditions are signalled and works with several of the above
handlers.
# NOT RUN {
tryCatch(1, finally = print("Hello"))
e <- simpleError("test error")
# }
# NOT RUN {
stop(e)
tryCatch(stop(e), finally = print("Hello"))
tryCatch(stop("fred"), finally = print("Hello"))
# }
# NOT RUN {
tryCatch(stop(e), error = function(e) e, finally = print("Hello"))
tryCatch(stop("fred"), error = function(e) e, finally = print("Hello"))
withCallingHandlers({ warning("A"); 1+2 }, warning = function(w) {})
# }
# NOT RUN {
{ withRestarts(stop("A"), abort = function() {}); 1 }
# }
# NOT RUN {
withRestarts(invokeRestart("foo", 1, 2), foo = function(x, y) {x + y})
##--> More examples are part of
##--> demo(error.catching)
# }
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