14. Bigloo The Programming Language (4.6a) User manual for version 4.6a January 2025 -- Command Line Parsing

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Command Line Parsing

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Bigloo supports command line argument parsing. That is, when an application is spawn from an Unix shell, the `main` function is called and its argument is bound to the list of the command line arguments, See Module Declaration. The `args-parse` form may be used to parse these.

args-parse list rules [null-rule] [else-rule] ... | bigloo syntax

The argument `list` is a list of strings. `Rules` is defined by the following grammar:

    <rule>      ==> (section <string>)
                  | ((<option> <help>) <s-expression>)
                  | ((<option>) <s-expression>)
                  | ((<flag> <var> <var> ...) <s-expression>)
                  | ((<flag> <var> <var> ... <help>) <s-expression>)
    <null-rule> ==> (() <s-expression>)
    <else-rule> ==> (else <s-expression>)
    <option>    ==> <flag>
                  | <string><var>
    <flag>      ==> <string>
                  | (<string>+)
    <var>       ==> an identifier leaded by the ? character
    <help>      ==> (help <s-expression>)
                  | (help <string> <s-expression>)

Each elements of `list` are match against the `rules`. If one of these matches, `args-parse` proceeds as follows:

1. The matched argument elements of `list` are removed from the list.
2. The `<s-expression>` associated to the matching rule is evaluated in an environment where the rule variables are bound.
3. The argument parsing is resumed with the rest of `list`.

In addition to parsing the command line arguments, `args-parse` enables help message printing.

args-parse-usage fmt | bigloo procedure

This is a procedure of one argument, an boolean. `Args-parse-usage` constructs an help message from all the option described in a `args-parse` form. `Args-parse-usage` is only defined in the `<s-expression>` of an `args-parse` form.

At last, if no rule matches an argument and if the `args-parse` form contains an `else` rule, this is evaluated. In the `<s-expression>` part of that rule, the pseudo-variable `else` is bound to the first unmatched argument and the pseudo-variable `rest` is bound to all the unmatched arguments.
Here is an example of argument parsing deploying all the possible rules:

    (module args-example
       (main main))

    (define (main argv)
       (args-parse (cdr argv)
          (section "Help")
          (("?")
           (args-parse-usage #f))
          ((("-h" "--help") (help "?,-h,--help" "This help message"))
           (args-parse-usage #f))
          (section "Misc")
          ((("-v" "--version") (help "Version number"))
           (print *version*))
          (("-o" ?file (help "The output file"))
           (set! *dest* file))
          (("--input=?file" (help "The input file"))
           (set! *input* file))
          (else
           (print "Illegal argument `" else "'. Usage:")
           (args-parse-usage #f))))

Invoking the compiled `args-example` module could produce:

    > bigloo.new args.scm
    args.scm:
    > a.out toto
    Illegal argument `toto'. Usage:

    Help:
       ?,-h,--help    --  This help message

    Misc:
       -v,--version   --  Version number
       -o <file>      --  The output file
       --input=<file> --  The input file

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Last update Mon Jan 27 07:55:49 2025.