racket-expand-file: Improve behavior when no steps

When no steps at all, still show "Final" output.

When no steps left, show simple message there is nothing left to
expand (as opposed to having the back end raise an exception).

Although I noticed this when investigating #697, it is adjacent not
strictly related.

racket-stepper.el

@@ -160,20 +160,22 @@ INTO-BASE is treated as a raw command prefix arg and converted to boolp."
                                        ,(and into-base t))
                        #'racket-stepper--insert)))
 
-(defun racket-stepper--insert (steps)
-  (with-current-buffer (racket--stepper-buffer-name)
-    (let ((inhibit-read-only t))
-      (goto-char (point-max))
-      (dolist (step steps)
-        (pcase step
-          (`(original . ,text)
-           (delete-region (point-min) (point-max))
-           (insert "Original\n" text "\n" "\n"))
-          (`(final    . ,text) (insert "Final\n" text "\n"))
-          (`(,label   . ,diff) (insert label "\n" diff "\n"))))
-      (racket-stepper-previous-item)
-      (when (equal (selected-window) (get-buffer-window (current-buffer)))
-        (recenter)))))
+(defun racket-stepper--insert (nothing-or-steps)
+  (if (eq nothing-or-steps 'nothing)
+      (message "Nothing to expand")
+    (with-current-buffer (racket--stepper-buffer-name)
+      (let ((inhibit-read-only t))
+        (goto-char (point-max))
+        (dolist (step nothing-or-steps)
+          (pcase step
+            (`(original . ,text)
+             (delete-region (point-min) (point-max))
+             (insert "Original\n" text "\n" "\n"))
+            (`(final  . ,text) (insert "Final\n" text "\n"))
+            (`(,label . ,diff) (insert label "\n" diff "\n"))))
+        (racket-stepper-previous-item)
+        (when (equal (selected-window) (get-buffer-window (current-buffer)))
+          (recenter))))))
 
 (defun racket-stepper-step (prefix)
   (interactive "P")

racket/commands/macro.rkt

@@ -22,8 +22,10 @@
          macro-stepper/next)
 
 (define step/c (cons/c (or/c 'original string? 'final) string?))
-(define step-proc/c (-> (or/c 'next 'all) (listof step/c)))
+(define step-proc/c (-> (or/c 'next 'all)
+                        (or/c 'nothing (listof step/c))))
 (define step-proc #f)
+(define (nothing-step-proc _) 'nothing)
 
 (define/contract (make-expr-stepper str)
   (-> string? step-proc/c)
@@ -73,7 +75,7 @@
                      (stepper-text stx
                                    (if into-base? (λ _ #t) (not-in-base)))))
   (define step-num #f)
-  (define step-last-after "")
+  (define step-last-after (pretty-format-syntax stx))
   (log-racket-mode-debug "~v ~v ~v" path into-base? raw-step)
   (define/contract (step what) step-proc/c
     (cond [(not step-num)
@@ -131,11 +133,9 @@
   (macro-stepper/next 'next))
 
 (define/contract (macro-stepper/next what) step-proc/c
-  (unless step-proc
-    (error 'macro-stepper "Nothing to expand"))
   (define v (step-proc what))
   (match v
-    [(list (cons 'final _)) (set! step-proc #f)]
+    [(list (cons 'final _)) (set! step-proc nothing-step-proc)]
     [_ (void)])
   v)