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15.5.1 Creating Loggers
15.5.2 Logging Events
15.5.3 Receiving Logged Events

15.5 Logging

A logger accepts events that contain information to be logged for interested parties. A log receiver represents an interested party that receives logged events asynchronously. Each event has a topic and level of detail, and a log receiver subscribes to logging events at a certain level of detail (and higher) for a specific topic or for all topics. The levels, in increasing order of detail, are 'fatal, 'error, 'warning, 'info, and 'debug.

To help organize logged events, a logger can have a default topic and/or a parent logger. Every event reported to a logger is also propagated to its parent (if any), while the event message is prefixed with the logger’s topic (if any) if the message doesn’t already have a topic. Furthermore, events that are propagated from a logger to its parent can be filtered by level and topic.

On start-up, Racket creates an initial logger that is used to record events from the core run-time system. For example, an 'debug event is reported for each garbage collection (see Garbage Collection). For this initial logger, two log receivers are also created: one that writes events to the process’s original error output port, and one that writes events to the system log. The level of written events in each case is system-specific, and the default can be changed through command-line flags (see Command Line) or through environment variables:

The current-logger parameter determines the current logger that is used by forms such as log-warning. On start-up, the initial value of this parameter is the initial logger. The run-time system sometimes uses the current logger to report events. For example, the bytecode compiler sometimes reports 'warning events when it detects an expression that would produce a run-time error if evaluated.

15.5.1 Creating Loggers


(logger? v)  boolean?

  v : any/c
Returns #t if v is a logger, #f otherwise.


(make-logger [topic    
  propagate-topic ...]    
  ...)  logger?
  topic : (or/c symbol? #f) = #f
  parent : (or/c logger? #f) = #f
  propagate-level : (or/c 'none 'fatal 'error 'warning 'info 'debug)
   = 'debug
  propagate-topic : (or/c #f symbol?) = #f
Creates a new logger with an optional topic and parent.

The optional propagate-level and propagate-topic arguments constrain the events that are propagated from the new logger to parent (when parent is not #f) in the same way that events are described for a log receiver in make-log-receiver. By default, all events are propagated to parent.

Changed in version of package base: Removed an optional argument to specify a notification callback, and added propagate-level and propagate-topic constraints for events to propagate.


(logger-name logger)  (or/c symbol? #f)

  logger : logger?
Reports logger’s default topic, if any.


(current-logger)  logger?

(current-logger logger)  void?
  logger : logger?
A parameter that determines the current logger.


(define-logger id)

Defines log-id-fatal, log-id-error, log-id-warning, log-id-info, and log-id-debug as forms like log-fatal, log-error,log-warning, log-info, and log-debug. The define-logger form also defines id-logger, which is a logger with default topic 'id that is a child of (current-logger); the log-id-fatal, etc. forms use this new logger. The new logger is created when define-logger is evaluated.

15.5.2 Logging Events


(log-message logger    
  [prefix-message?])  void?
  logger : logger?
  level : (or/c 'fatal 'error 'warning 'info 'debug)
  topic : (or/c symbol? #f) = (logger-name logger)
  message : string?
  data : any/c
  prefix-message? : any/c = #t
Reports an event to logger, which in turn distributes the information to any log receivers attached to logger or its ancestors that are interested in events at level or higher.

Log receivers can filter events based on topic. In addition, if topic and prefix-message? are not #f, then message is prefixed with the topic followed by ": " before it is sent to receivers.

Changed in version of package base: Added the prefix-message? argument.


(log-level? logger level [topic])  boolean?

  logger : logger?
  level : (or/c 'fatal 'error 'warning 'info 'debug)
  topic : (or/c symbol? #f) = #f
Reports whether any log receiver attached to logger or one of its ancestors is interested in level events (or potentially lower) for topic. If topic is #f, the result indicates whether a log receiver is interested in events at level for any topic.

Use this function to avoid work generating an event for log-message if no receiver is interested in the information; this shortcut is built into log-fatal, log-error, log-warning, log-info, log-debug, and forms bound by define-logger, however, so it should not be used with those forms.

The result of this function can change if a garbage collection determines that a log receiver is no longer accessible (and therefore that any event information it receives will never become accessible).

Changed in version of package base: Added the topic argument.


(log-max-level logger [topic])

  (or/c #f 'fatal 'error 'warning 'info 'debug)
  logger : logger?
  topic : (or/c symbol? #f) = #f
Similar to log-level?, but reports the maximum-detail level of logging for which log-level? on logger and topic returns #t. The result is #f if log-level? with logger and topic currently returns #f for all levels.

Changed in version of package base: Added the topic argument.


(log-all-levels logger)

(list/c (or/c #f 'fatal 'error 'warning 'info 'debug)
        (or/c #f symbol?)
        ... ...)
  logger : logger?
Summarizes the possible results of log-max-level on all possible interned symbols. The result list contains a sequence of symbols and #f, where the first, third, etc., list element corresponds to a level, and the second, fourth, etc., list element indicates a corresponding topic. The level is the result that log-max-level would produce for the topic, where the level for the #f topic (which is always present in the result list) indicates the result for any interned-symbol topic that does not appear in the list.

The result is suitable as a sequence of arguments to make-log-receiver (after a logger argument) to create a new receiver for events that currently have receivers in logger.

Added in version of package base.


(log-level-evt logger)  evt?

  logger : logger?
Creates a synchronizable event that is ready for synchronization when the result of log-level?, log-max-level, or log-all-levels can be different than before log-level-evt was called. The event’s synchronization result is the event itself.

The condition reported by the event is a conservative approximation: the event can become ready for synchronization even if the results of log-level?, log-max-level, and log-all-levels are unchanged. Nevertheless, the expectation is that events produced by log-level-evt become ready infrequently, because they are triggered by the creation of a log receiver.

Added in version of package base.


(log-fatal string-expr)

(log-fatal format-string-expr v ...)


(log-error string-expr)

(log-error format-string-expr v ...)


(log-warning string-expr)

(log-warning format-string-expr v ...)


(log-info string-expr)

(log-info format-string-expr v ...)


(log-debug string-expr)

(log-debug format-string-expr v ...)
Log an event with the current logger, evaluating string-expr or (format format-string-expr v ...) only if the logger has receivers that are interested in the event. In addition, the current continuation’s continuation marks are sent to the logger with the message string.

These form are convenient for using the current logger, but libraries should generally use a logger for a specific topic—typically through similar convenience forms generated by define-logger.

For each log-level,

(log-level string-expr)

is equivalent to

(let ([l (current-logger)])
  (when (log-level? l 'level)
    (log-message l 'level string-expr


(log-level format-string-expr v ...)

is equivalent to

(log-level (format format-string-expr v ...))

15.5.3 Receiving Logged Events


(log-receiver? v)  boolean?

  v : any/c
Returns #t if v is a log receiver, #f otherwise.


(make-log-receiver logger level [topic ...] ...)  log-receiver?

  logger : logger?
  level : (or/c 'none 'fatal 'error 'warning 'info 'debug)
  topic : (or/c #f symbol?) = #f
Creates a log receiver to receive events of detail level and lower as reported to logger and its descendants, as long as either topic is #f or the event’s topic matches topic.

A log receiver is a synchronizable event. It becomes ready for synchronization when a logging event is received, so use sync to receive an logged event. The log receiver’s synchronization result is an immutable vector containing four values: the level of the event as a symbol, an immutable string for the event message, an arbitrary value that was supplied as the last argument to log-message when the event was logged, and a symbol or #f for the event topic.

Multiple pairs of level and topic can be provided to indicate different specific levels for different topics (where topic defaults to #f only for the last given level). A level for a #f topic applies only to events whose topic does not match any other provided topic. If the same topic is provided multiple times, the level provided with the last instance in the argument list takes precedence.