add support for guild help FOO

* module/scripts/help.scm (main): Add support for guild help FOO.

module/scripts/help.scm

@@ -27,6 +27,7 @@
 
 (define-module (scripts help)
   #:use-module (ice-9 format)
+  #:use-module (ice-9 documentation)
   #:use-module ((srfi srfi-1) #:select (fold append-map)))
 
 (define %summary "Show a brief help message.")
@@ -80,7 +81,7 @@
                   %load-path)
       string<?))))
 
-(define (main . args)
+(define (list-commands all?)
   (display "\
 Usage: guild COMMAND [ARGS]
 
@@ -91,8 +92,6 @@ Usage: guild COMMAND [ARGS]
 Commands:
 ")
 
-  (let ((all? (or (equal? args '("--all"))
-                  (equal? args '("-a")))))
   (for-each
    (lambda (name)
      (let* ((modname `(scripts ,(string->symbol name)))
@@ -106,4 +105,41 @@ Commands:
            (if summary
                (format #t "  ~A ~23t~a\n" name summary)
                (format #t "  ~A\n" name)))))
-     (find-submodules '(scripts)))))
+   (find-submodules '(scripts)))
+  (display "
+For help on a specific command, try \"guild help COMMAND\".
+"))
+
+(define (module-commentary mod)
+  (file-commentary
+   (%search-load-path (module-filename mod))))
+
+(define (main . args)
+  (cond
+   ((null? args)
+    (list-commands #f))
+   ((or (equal? args '("--all")) (equal? args '("-a")))
+    (list-commands #t))
+   ((not (string-prefix? "-" (car args)))
+    ;; help for particular command
+    (let* ((name (car args))
+           (mod (resolve-module `(scripts ,(string->symbol name))
+                                #:ensure #f)))
+      (if mod
+          (let ((commentary (module-commentary mod)))
+            (if commentary
+                (display commentary)
+                (format #t "No documentation found for command \"~a\".\n"
+                        name)))
+          (begin
+            (format #t "No command named \"~a\".\n" name)
+            (exit 1)))))
+   (else
+    (display "Usage: guild help
+       guild help --all
+       guild help COMMAND
+
+Show a help on guild commands.  With --all, show arcane incantations as
+well.  With COMMAND, show more detailed help for a particular command.
+")
+    (exit 1))))