Expose the mk-rhs function normalize-definition uses.

I found I wanted this to make a define/stub macro that errors giving the defined identifier:

(define-syntax (define/stub stx)
  (syntax-case stx ()
    [(_ header)
     (let-values ([(id mk-rhs body) (normalize-definition/mk-rhs stx #'lambda #t #t #f)])
       #`(define #,id #,(mk-rhs #`(error '#,id "TODO: stub"))))]))

Closes #508.

pkgs/racket-doc/syntax/scribblings/define.scrbl

@@ -30,3 +30,21 @@ an expression context. The default value of @racket[check-context?] is
 If @racket[opt-kws?] is @racket[#t], then arguments of the form
 @racket[[id expr]], @racket[keyword id], and @racket[keyword [id
 expr]] are allowed, and they are preserved in the expansion.}
+
+@defproc[(normalize-definition/mk-rhs [defn-stx syntax?]
+                                      [lambda-id-stx identifier?]
+                                      [check-context? boolean?]
+                                      [opt+kws? boolean?]
+                                      [err-no-body? boolean?])
+         (values identifier? (-> syntax? syntax?) syntax?)]{
+
+  The helper for @racket[normalize-definition] that produces three values:
+  the defined identifier, a function that takes the syntax of the body
+  and produces syntax that has the expected binding structure, and
+  finally the right-hand side expression that @racket[normalize-definition]
+  gives to the previous function.
+
+  If @racket[err-no-body?] is true, then there must be a right-hand side
+  expression or else it is a syntax error. The @racket[err-no-body?] argument
+  is true for uses of @racket[normalize-definition].
+}

racket/collects/racket/private/norm-define.rkt

@@ -3,11 +3,11 @@
   (#%require "small-scheme.rkt" "stxcase-scheme.rkt"
              "member.rkt" "stx.rkt" "qqstx.rkt")
 
-  (#%provide normalize-definition)
+  (#%provide normalize-definition normalize-definition/mk-rhs)
 
-  (define-values (normalize-definition)
-    (case-lambda 
-     [(stx lambda-stx check-context? allow-key+opt?)
+  (define-values (normalize-definition/mk-rhs)
+    (lambda 
+        (stx lambda-stx check-context? allow-key+opt? err-no-body?)
       (when (and check-context?
 		 (memq (syntax-local-context) '(expression)))
 	(raise-syntax-error 
@@ -17,7 +17,7 @@
       (syntax-case stx ()
 	[(_ id expr)
 	 (identifier? #'id)
-	 (values #'id #'expr)]
+	 (values #'id values #'expr)]
 	[(_ id . rest)
 	 (identifier? #'id)
 	 (raise-syntax-error
@@ -155,11 +155,18 @@
 	      #f
 	      "bad syntax (illegal use of `.' for procedure body)"
 	      stx))
-	   (when (stx-null? #'body)
+	   (when (and err-no-body? (stx-null? #'body))
 	     (raise-syntax-error
 	      #f
 	      "bad syntax (no expressions for procedure body)"
 	      stx))
-	   (values id (mk-rhs #'body)))])]
+	   (values id mk-rhs #'body))])))
+
+  (define-values (normalize-definition)
+    (case-lambda 
+     [(stx lambda-stx check-context? allow-key+opt?)
+      (let-values ([(id mk-rhs body)
+                    (normalize-definition/mk-rhs stx lambda-stx check-context? allow-key+opt? #t)])
+        (values id (mk-rhs body)))]
      [(stx lambda-stx check-context?) (normalize-definition stx lambda-stx check-context? #f)]
      [(stx lambda-stx) (normalize-definition stx lambda-stx #t #f)])))

racket/collects/syntax/define.rkt

@@ -1,3 +1,3 @@
 (module define racket/base
   (require racket/private/norm-define)
-  (provide normalize-definition))
+  (provide normalize-definition normalize-definition/mk-rhs))

racket/collects/syntax/doc.txt

@@ -132,6 +132,18 @@ _define.ss_: handling all the same function forms as `define'
   `keyword id', and `keyword [id expr]' are allowed, and they are
   preserved in the expansion.
 
+> (normalize-definition/mk-rhs defn-stx lambda-id-stx [check-context? opt+kws? err-no-body?]) -
+
+  the helper for `normalize-definition' that produces three values:
+  the defined identifier, a function that takes the syntax of the body
+  and produces syntax that has the expected binding structure, and
+  finally the right-hand side expression that `normalize-definition'
+  gives to the previous function.
+
+  If `err-no-body?' is true, then there must be a right-hand side
+  expression or else it is a syntax error. This is true for uses of
+  `normalize-definition'.
+
 ======================================================================
 _struct.ss_: generating the same names as `define-struct'
 ======================================================================