Improved handling of syntax errors in `reduction-relation'

svn: r18431

collects/redex/private/reduction-semantics.ss

@@ -485,6 +485,20 @@
           [withs (make-module-identifier-mapping)])
       (for-each (λ (shortcut)
                   (syntax-case shortcut ()
+                    [((rhs-arrow rhs-from rhs-to)
+                      (lhs-arrow a b))
+                     (not (identifier? #'a))
+                     (raise-syntax-error
+                      orig-name
+                      "malformed shortcut, expected identifier"
+                      shortcut #'a)]
+                    [((rhs-arrow rhs-from rhs-to)
+                      (lhs-arrow a b))
+                     (not (identifier? #'b))
+                     (raise-syntax-error
+                      orig-name
+                      "malformed shortcut, expected identifier"
+                      shortcut #'b)]
                     [((rhs-arrow rhs-from rhs-to)
                       (lhs-arrow lhs-from lhs-to))
                      (begin

collects/redex/tests/tl-test.ss

@@ -1090,6 +1090,22 @@
                   [(~> a b) (==> a b)])
                  #rx"~> relation is not defined")
   
+  (test-syn-err  (reduction-relation
+                  grammar
+                  (==> 1 2)
+                  with
+                  [(--> a b)
+                   (==> a (+ 3 b))])
+                 #rx"expected identifier")
+  
+  (test-syn-err  (reduction-relation
+                  grammar
+                  (==> 1 2)
+                  with
+                  [(--> a b)
+                   (==> (+ 3 a) b)])
+                 #rx"expected identifier")
+  
   (test-syn-err (define-language bad-lang1 (e name)) #rx"name")
   (test-syn-err (define-language bad-lang2 (name x)) #rx"name")
   (test-syn-err (define-language bad-lang3 (x_y x)) #rx"cannot have _")