define-macro: check that ... only follow wildcards

beautiful-racket-lib/br/define.rkt

@@ -141,6 +141,8 @@
     [(_ id:id leading-pat:expr ... else-pat:else-clause trailing-pat0:expr trailing-pat:expr ...)
      (raise-syntax-error 'define-macro-cases "`else` clause must be last" (syntax->datum #'id))]
     [(_ id:id (pat:expr . result-exprs:expr) ... else-clause:else-clause)
+     (unless (all-...-follow-wildcards #'(pat ...))
+       (raise-syntax-error 'define-macro-cases "found ellipses after non-wildcard variable" (syntax->datum stx)))
      (with-syntax ([(LITERAL ...) (generate-literals #'(pat ...))])
        #'(define-macro id
            (λ (stx)

beautiful-racket-lib/br/private/generate-literals.rkt

@@ -1,6 +1,6 @@
 #lang racket/base
 (require "syntax-flatten.rkt")
-(provide generate-literals)
+(provide all-...-follow-wildcards generate-literals)
 
 (define (literal-identifier? pat-datum)
   (and (symbol? pat-datum)
@@ -9,6 +9,11 @@
          (or (not (regexp-match #rx"[A-Z]" pat-str)) ; either doesn't contain at least one uppercase letter ...
              (not (equal? (string-upcase pat-str) pat-str)))))) ;...  or doesn't contain all uppercase letters
 
+(define (wildcard-identifier? pat-datum)
+  (and (symbol? pat-datum)
+       (not (literal-identifier? pat-datum))
+       (not (memq pat-datum '(... _)))))
+
 ;; generate literals for any symbols that are not ... or _ and not IN_CAPS
 (define (generate-literals pats)
   (for*/list ([pat-arg (in-list (syntax-flatten pats))]
@@ -16,6 +21,34 @@
               #:when (literal-identifier? pat-datum))
     pat-arg))
 
+(define (all-...-follow-wildcards pats)
+  (define prev-datum (box #f))
+  (and
+    (for*/and ([pat-arg (in-list (syntax-flatten pats))]
+               [pat-datum (in-value (syntax->datum pat-arg))])
+      ;; OK if there's no previous datum,
+      (and
+        (when (eq? pat-datum '...)
+          (wildcard-identifier? (unbox prev-datum)))
+        (set-box! prev-datum pat-datum)))
+    #true))
+
 (module+ test
   (require rackunit)
-  (check-equal? (map syntax->datum (generate-literals #'(foo 42 BAR _ (... ...) bar <=> 3Bar 3bar))) '(foo bar <=> 3Bar 3bar)))
+  (check-equal? (map syntax->datum (generate-literals #'(foo 42 BAR _ (... ...) bar <=> 3Bar 3bar))) '(foo bar <=> 3Bar 3bar))
+
+  (test-case "wildcard-identifier?"
+    (check-true (wildcard-identifier? 'FOO))
+    (check-true (wildcard-identifier? 'TOPPING))
+
+    (check-false (wildcard-identifier? 'piZZa))
+    (check-false (wildcard-identifier? 'please)))
+
+  (test-case "all-...-follow-wildcards"
+    (check-true (all-...-follow-wildcards #'()))
+    (check-true (all-...-follow-wildcards (datum->syntax #f '(a b))))
+    (check-true (all-...-follow-wildcards (datum->syntax #f '(a b C ...))))
+
+    (check-false (all-...-follow-wildcards (datum->syntax #f '(...))))
+    (check-false (all-...-follow-wildcards (datum->syntax #f '(a ...))))
+    (check-false (all-...-follow-wildcards (datum->syntax #f '(A ... b ...))))))