4 Command Line Flags🔗ℹ

When parsing command line flags, Denxi does not store user-defined values as a side effect. Instead, it defers binding values from a command line to settings. Once a process is ready to determine a runtime configuration, it binds values from a command line using call-with-bound-cli-flags.

denxi/cli-flag provides identifiers that are not documented in this section. Each of these provided identifiers match the --<id> form of available setting names, so the cli-flag instance for DENXI_VERBOSE is provided as --DENXI_VERBOSE. You can view all acceptable flags for an instance using cli-flag-strings.

An instance of this structure corresponds to a suitable flag table entry in parse-command-line. The value of kind has the same meaning as it does in parse-command-line.

An instance is used to generate deferred bindings for setting, such that the strings passed in the command line are turned into a Racket values for the setting using convert.

Each instance of cli-flag implicitly carries a longform flag equal to (format "--~a" (setting-id setting)). additional-flag-strings holds acceptable flags recognized by an instance.

convert and help-strings are related in the same way as they are in a program using parse-command-line (see example below). However, arity describes the number of formals accepted by convert minus the flag argument. Therefore it must be the case that (equal? (procedure-arity convert) (add1 arity)).

The below example shows a setting that holds a list of lists, where each sublist holds three numbers. The following cli-flag instance describes the corresponding command line flag and the conversion from user-provided strings to a valid value for the setting.

(define-setting TRIPLET_LIST

(listof (list/c real? real? real?))

null)

(define --triplet

(cli-flag TRIPLET_LIST

'multi

'("-t" "--triplet")

3

(λ (flag a b c)

(cons (list (string->number a)

(string->number b)

(string->number c))

(TRIPLET_LIST)))

'("number" "number" "number")))

flag-string is bound to the exact flag used by the user. bind applies a given thunk such that the setting in flag-definition is bound to a user-defined value in the extent of said thunk.

A particular use of --triplet from the earlier example could generate this state.

; Generated from -t 9 7 22 -t 10 0 0 -t 2 4 5

(cli-flag-state "-t" --triplet

(lambda (proc)

(TRIPLET_LIST '((2 4 5) (10 0 0) (9 7 22))

proc)))