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Robert Brown <>


New BSD license. See the copyright messages in individual files.

Lisp gflags

Lisp gflags is a Common Lisp implementation of gflags, Google's command line flag parsing library. The library implements functionality similar to that of Google's C++ gflags library and the absl.flags module in Google's Python Abseil libraries.

The code allows you to define command line flags using the DEFINE-FLAG macro. For instance, the following form defines a boolean flag called *DEBUG-FLAG* that is set based on the presence of "--debug" in the application's command line:

(define-flag *debug-flag*
  :default-value nil
  :selector "debug"
  :type boolean
  :help "Turn on debugging mode?"
  :documentation "Is debugging mode turned on?")

If the command line contains "--debug" or "--debug=true", then *DEBUG-FLAG* is set to T. Otherwise, it defaults to NIL.

The function PARSE-COMMAND-LINE is used to parse the command line and initialize flags. It returns the original command line with all recognized flags removed.

The file flag-test.lisp contains many examples.

The Lisp gflags API

define-flag flag-variable &key default-value selector type help parser documentation

Defines a global FLAG-VARIABLE of type TYPE, holding value DEFAULT-VALUE,
that can be set via the Unix command line to "value" with argument
"--SELECTOR=value" or arguments "--SELECTOR value".  As a special case,
flags of type "boolean" can additionally be set to true with "--SELECTOR"
and to false with "--noSELECTOR".

Optionally, associates a HELP string with the flag and a DOCUMENTATION string

Values for flags defined to be of boolean, string, keyword, integer, or
floating point type are parsed by built-in parsers.  For flags of other types
supply PARSER, a function designator for a function that converts a string into
a value of type TYPE.  The parser must return two values, the parsed flag value
and a boolean indicating whether the parse was successful.


    (define-flag *debug-mode*
      :default-value nil
      :selector "debug"
      :type boolean
      :help "Turn on debugging mode?"
      :documentation "Is debugging mode turned on?")

    (define-flag *ip-address*
      :default-value (make-instance 'ip-address ...)
      :selector "ip_address"
      :type (satisfies ip-address-p)
      :parser ip-address-parser
      :help "An internet protocol address.")

parse-command-line arguments

Parses ARGUMENTS, a list of command line argument strings.  If a registered
flag is found in ARGUMENTS, sets the flag's value.  Returns a copy of
ARGUMENTS, but with all recognized flag arguments removed.


Returns the Unix command line as a list of strings.

Dependencies (2)

  • hu.dwim.stefil

Dependents (0)

    • GitHub
    • Quicklisp