Add log library based on log4p

examples/logger.metta

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+;; Test logger library.
+;;
+;; You may need to install log4p by running the following command:
+;;
+;; ```
+;; swipl pack install log4p
+;; ````
+
+;; Import logger library
+!(import! &self (library lib_logger))
+
+;; Get current filepath
+!(test
+  (logger-filepath)
+  "petta.log")
+
+;; Get current stdout flag
+!(test
+  (logger-stdout)
+  False)
+
+;; Get current timestamp flag
+!(test
+  (logger-timestamp)
+  True)
+
+;; Get current log-level
+!(test
+  (logger-level)
+  info)
+
+;; Get all possible log levels
+!(test
+  (logger-levels)
+  (trace debug info warn error fatal))
+
+;; Set log level to debug
+!(test
+  (logger-set-level debug)
+  True)
+
+;; Get current log-level
+!(test
+  (logger-level)
+  debug)
+
+;; Log message at the trace level (should be ignored)
+!(test
+  (logger-trace "Log message at the trace level")
+  True)
+
+;; Log message at the debug level
+!(test
+  (logger-debug "Log message at the debug level")
+  True)
+
+;; Log message at the info level
+!(test
+  (logger-info "Log message at the info level")
+  True)
+
+;; Log message at the warn level
+!(test
+  (logger-warn "Log message at the warn level")
+  True)
+
+;; Log message at the error level
+!(test
+  (logger-error "Log message at the error level")
+  True)
+
+;; Log message at the fatal level
+!(test
+  (logger-fatal "Log message at the fatal level")
+  True)
+
+;; Log message at the given info level
+!(test
+  (logger-log info "Log message at the given info level")
+  True)
+
+;; Log message containing MeTTa terms at the given info level
+!(test
+  (logger-logf info
+               "Log terms %w, %w and %w at the given info level"
+               ((+ 21 21) "Hello World!" (R A B)))
+  True)
+
+;; Log message containing MeTTa terms at the info level
+!(test
+  (logger-infof "Log terms %w, %w and %w at the info level"
+                ((+ 21 21) "Hello World!" (R A B)))
+  True)
+
+;; Reset log filepath
+!(test
+  (logger-set-filepath "my-petta.log")
+  True)
+!(test
+  (logger-info "Log message to \"my-petta.log\" file")
+  True)
+
+;; Enable logging to stdout
+!(test
+  (logger-set-stdout True)
+  True)
+!(test
+  (logger-info "Message to stdout as well")
+  True)
+
+;; Disable prefixing the message with timestampe
+!(test
+  (logger-set-timestamp False)
+  True)
+!(test
+  (logger-info "Message without timestamp")
+  True)

lib/lib_logger.metta

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+;; Logger library to log timestamped messages into a log file.  New
+;; messages never replace old messages, instead they are appended to
+;; the end of the log file.  The responsability of deleting a log file
+;; is left the user.
+;;
+;; This logger library is based on
+;; [log4p](https://www.swi-prolog.org/pack/list?p=log4p).  To install
+;; log4p you may need to run the following command:
+;;
+;; ```
+;; swipl pack install log4p
+;; ```
+;;
+;; The logger library contains the following methods:
+;;
+;; - (logger-filepath) returns a string of the filepath where log
+;;   messages are written.  Default: "petta.log".
+;;
+;; - (logger-stdout) returns True iff messages are written to stdout.
+;;   Default: False.
+;;
+;; - (logger-timestamp) returns True iff messages are prefixed with
+;;   date and time.  Default: True.
+;;
+;; - (logger-set-filepath FILEPATH) sets the destination of the
+;;   filepath to FILEPATH, where log messages are written.
+;;
+;; - (logger-set-stdout FLAG) sets the flag to write log messages to
+;;   stdout iff FLAG is True.
+;;
+;; - (logger-set-timestamp FLAG) sets the flag to prefix log messages
+;;   with date and time iff FLAG is True.
+;;
+;; - (logger-level) returns the current log level.  Only messages are
+;;   at this level or higher are logged.  For instance, if the current
+;;   log level is info, then messages at levels info, warn, error and
+;;   fatal will be logged.  Default: info.
+;;
+;; - (logger-levels) returns a tuple with all possible log levels,
+;;   which are trace, debug, info, warn, error and fatal.
+;;
+;; - (logger-trace MESSAGE) log MESSAGE at the trace level.
+;;
+;; - (logger-debug MESSAGE) log MESSAGE at the debug level.
+;;
+;; - (logger-info MESSAGE) log MESSAGE at the info level.
+;;
+;; - (logger-warn MESSAGE) log MESSAGE at the warn level.
+;;
+;; - (logger-error MESSAGE) log MESSAGE at the error level.
+;;
+;; - (logger-fatal MESSAGE) log MESSAGE at the fatal level.  Note that
+;;   this will not halt the process. This responsability is left to
+;;   the programmer.
+;;
+;; - (logger-log LEVEL MESSAGE) log MESSAGE at level LEVEL.
+;;
+;; - (logger-logf LEVEL MESSAGE ARGUMENTS) log MESSAGE at level LEVEL,
+;;   formatting the message with ARGUMENTS according
+;;   https://www.swi-prolog.org/pldoc/man?predicate=format/3.
+;;
+;; - (logger-tracef MESSAGE ARGUMENTS) like logger-trace but expects a
+;;   formatted message.
+;;
+;; - (logger-debugf MESSAGE ARGUMENTS) like logger-debug but expects a
+;;   formatted message.
+;;
+;; - (logger-infof MESSAGE ARGUMENTS) like logger-info but expects a
+;;   formatted message.
+;;
+;; - (logger-warnf MESSAGE ARGUMENTS) like logger-warn but expects a
+;;   formatted message.
+;;
+;; - (logger-errorf MESSAGE ARGUMENTS) like logger-error but expects a
+;;   formatted message.
+;;
+;; - (logger-fatalf MESSAGE ARGUMENTS) like logger-fatal but expects a
+;;   formatted message.
+
+;; Import prolog logger library
+!(import! &self (library lib_import))
+!(import_prolog_functions_from_file (library lib_logger.pl)
+                                    (logger-filepath
+                                     logger-stdout
+                                     logger-timestamp
+                                     logger-set-filepath
+                                     logger-set-stdout
+                                     logger-set-timestamp
+                                     logger-level
+                                     logger-levels
+                                     logger-set-global-level
+                                     logger-set-local-level
+                                     logger-set-level
+                                     logger-trace
+                                     logger-debug
+                                     logger-info
+                                     logger-warn
+                                     logger-error
+                                     logger-fatal
+                                     logger-log
+                                     logger-logf
+                                     logger-tracef
+                                     logger-debugf
+                                     logger-infof
+                                     logger-warnf
+                                     logger-errorf
+                                     logger-fatalf))