Parsing command line arguments, predicate error

[Zelphir Kaltstahl]

Hi!

I decided to take a look at how one can parse command line arguments in
Guile and was looking for something like argparse in Python. It seems
that (use-modules (ice-9 getopt-long)) does the job, except that I hit
one problem and don't know what the mistake I am making is. It seems to
be connected to the usage of `predicate` in my code.

The following is an example program, which I wrote to try the
getopt-long facilities, adapted from the docs example at
https://www.gnu.org/software/guile/manual/html_node/getopt_002dlong-Example.html#getopt_002dlong-Example:

;; ===== EXAMPLE START =====
(add-to-load-path (dirname (current-filename)))

(use-modules (ice-9 getopt-long))

(define (string-exact-integer? str)
  (exact-integer? (string->number str)))

(define option-spec
  '((version (single-char #\v) (value #f))
    (help    (single-char #\h) (value #f))
    (user-name (value #t) (required? #f))
    (times-hello (value #t)
                 (single-char #\n)
                 (required? #f)
                 (predicate string-exact-integer?))))

(define options (getopt-long (command-line) option-spec))

(option-ref options 'help #f)

(define (main)
  (let* ([help-wanted (option-ref options 'help #f)]
         [version-wanted (option-ref options 'version #f)]
         [user-name (option-ref options 'user-name #f)]
         [times-say-hello (option-ref options 'times-hello 1)])
    (cond [(or version-wanted help-wanted)
           (when version-wanted
             (display "command-line-arguments.scm 1.0.0\n"))
           (when help-wanted
             (display
              (string-join '(""
                             "getopt-long-example [options]"
                             "-v,  --version  Display version"
                             "-h,  --help     Display this help"
                             "")
                           "\n")))]
          [else
           (do ([i 0 (1+ i)])
               ([>= i times-say-hello])
             (display (simple-format #f "Hello, ~a!\n" (if user-name
                                                           user-name
                                                           "World"))))])))

(main)
;; ===== EXAMPLE END =====

This program I call as follows in Emacs' Eshell:

;; ===== COMMAND START =====
guile command-line-arguments.scm -n 3 --user-name test
;; ===== COMMAND END =====

Then I get the following error:

;; ===== ERROR START =====
Backtrace:
           9 (apply-smob/1 #<catch-closure a98b80>)
In ice-9/boot-9.scm:
    705:2  8 (call-with-prompt ("prompt") #<procedure a9a0a0 at
ice-9/eval.scm:330:13 ()> #<procedure…>)
In ice-9/eval.scm:
    619:8  7 (_ #(#(#<directory (guile-user) b2e140>)))
In ice-9/boot-9.scm:
   2312:4  6 (save-module-excursion #<procedure ad6330 at
ice-9/boot-9.scm:3827:3 ()>)
  3822:12  5 (_)
In
/home/xiaolong/development/Guile/examples-and-convenience-procedures/command-line-arguments/command-line-arguments.scm:
    30:16  4 (_)
In ice-9/getopt-long.scm:
    350:6  3 (getopt-long _ _ #:stop-at-first-non-option _)
In ice-9/boot-9.scm:
   260:13  2 (for-each #<procedure acd1e0 at
ice-9/getopt-long.scm:350:16 (spec)> _)
In ice-9/getopt-long.scm:
   209:28  1 (_ "times-hello" _)
In unknown file:
           0 (_ "3")

ERROR: Wrong type to apply: string-exact-integer?
;; ===== ERROR END =====

Maybe I am doing something stupid, but as far as I can see, I am just
doing what the docs say, I am providing a function that takes the
option's value as string and returns #t or #f. Everything worked up to
when I tried the predicate thing.

What am I doing wrong?