r{5,6}rs: change letrec to signal use-before-definition errors

The old `letrec` implementation used the R5RS macro with `undefined`,
which differents from Racket's `letrec` in that no variable is
initialized until all right-hand sides are evaluated.  It turns out
to be simpler and produce the same effect to use `letrec-values`
with a single clause: `(letrec ([x e1] [y e2]) ....)` ->
`(letrec-values ([(x y) (values e1 e2)]) ....)`.

pkgs/r5rs-pkgs/r5rs-doc/r5rs/r5rs.scrbl

@@ -134,6 +134,13 @@ not allow redefinition of top-level bindings, and expressions
 evaluated through @racket[load] and @racket[eval] cannot automatically
 access bindings defined within the module.
 
+
+@history[#:changed "6.0.1.4" @elem{When an identifier bound by
+                                   @racket[letrec] is referenced
+                                   before it is initialized, an
+                                   exception is raised, instead of
+                                   producing @|undefined-const|.}]
+
 @; --------------------
 
 @subsection{Non-@|r5rs| Bindings from @racketmodname[r5rs]}

pkgs/r5rs-pkgs/r5rs-lib/r5rs/main.rkt

@@ -242,55 +242,18 @@
             ;; None, so just use R5RS quote:
             #'(r5rs:quote form))]))
 
-  ;; Copied from R5rS, but with an added `let' around body,
+  ;; The difference between R5RS `letrec` and Racket `letrec` (or
-  ;; and with optimization for precedure letrecs
+  ;; R6RS `letrec*`) is that all right-hand sides are evaluated
-  (define-for-syntax (immediate-value? stx)
+  ;; before any binding is initialized. We can get this effect
-    (let ([v (syntax-e stx)])
+  ;; by using `letrec-values` and a single clause:
-      (or (number? v)
-          (boolean? v)
-          (string? v)
-          (syntax-case stx (r5rs:lambda quote r5rs:quote #%datum)
-            [(r5rs:lambda . _rest) #t]
-            [(quote . _) #t]
-            [(r5rs:quote . _) #t]
-            [(#%datum . _) #t]
-            [_ #f]))))
   (define-syntax (r5rs:letrec stx)
     (syntax-case stx (r5rs:lambda)
-      ((r5rs:letrec ((var1 rhs) ...) body ...)
-       (andmap immediate-value? (syntax->list #'(rhs ...)))
-       (syntax/loc stx (letrec ((var1 rhs) ...) (r5rs:body body ...))))
       ((r5rs:letrec ((var1 init1) ...) body ...)
        (syntax/loc stx
-         (r5rs:letrec "generate_temp_names"
+         (letrec-values ([(var1 ...)
-                      (var1 ...)
+                          (values init1 ...)])
-                      ()
+           (r5rs:body
-                      ((var1 init1) ...)
+            body ...))))))
-                      body ...)))
-      ((r5rs:letrec "generate_temp_names"
-                    ()
-                    (temp1 ...)
-                    ((var1 init1) ...)
-                    body ...)
-       (syntax/loc stx
-         (let ((var1 undefined) ...)
-           (let ((temp1 init1) ...)
-             (set! var1 temp1)
-             ...
-             (let ()
-               (r5rs:body
-                body ...))))))
-      ((r5rs:letrec "generate_temp_names"
-                    (x y ...)
-                    (temp ...)
-                    ((var1 init1) ...)
-                    body ...)
-       (syntax/loc stx
-         (r5rs:letrec "generate_temp_names"
-                      (y ...)
-                      (newtemp temp ...)
-                      ((var1 init1) ...)
-                      body ...)))))
 
   (define-syntax (r5rs:lambda stx)
     ;; Convert rest-arg list to mlist, and use r5rs:body:

pkgs/r6rs-pkgs/r6rs-doc/r6rs/scribblings/r6rs.scrbl

@@ -382,10 +382,6 @@ several known ways:
        @racket[string-upcase], and @racket[string-titlecase] are not
        determined as specified by Unicode Standard Annex #29.}
 
- @item{When an identifier bound by @racket[letrec] or @racket[letrec*]
-       is referenced before it is bound, an exception is not raised;
-       instead, the reference produces @|undefined-const|.}
-
  @item{A custom textual port must represent positions using integers,
        and the positions must correspond to bytes in a UTF-8 encoding
        of the port's data. For custom ports (byte or character) that
@@ -407,9 +403,13 @@ several known ways:
        @racketidfont{#%top}, and @racketidfont{#%top-interaction} are
        imported into every library and program, and at every phase
        level for which the library or program has imports.}
-
 ]
 
+@history[#:changed "6.0.1.4" @elem{When an identifier bound by
+                                   @racket[letrec] or @racket[letrec*]
+                                   is referenced before it is initialized,
+                                   an exception is raised, instead of
+                                   producing @|undefined-const|.}]
 
 @; ----------------------------------------------------------------------