Add error checking for contracted form, also added documentation in reference

svn: r13135

collects/mzlib/private/unit-keywords.ss

@@ -3,7 +3,7 @@
 
 (provide only except prefix rename tag
          import export init-depend link
-         extends)
+         extends contracted)
 
 (define-syntax-rule (define-syntax-for-error name message)
   (define-syntax name
@@ -34,3 +34,5 @@
   "misuse of compound-unit keyword")
 (define-syntax-for-error extends
   "misuse of define-signature keyword")
+(define-syntax-for-error contracted
+  "misuse of define-signature keyword")

collects/mzlib/unit.ss

@@ -17,7 +17,7 @@
   
   (provide define-signature-form struct open
            define-signature provide-signature-elements
-           only except rename import export prefix link tag init-depend extends
+           only except rename import export prefix link tag init-depend extends contracted
            unit?
            (rename :unit unit) define-unit 
            compound-unit define-compound-unit compound-unit/infer define-compound-unit/infer
@@ -252,6 +252,13 @@
                     val-defs
                     stx-defs
                     (append (syntax->list #'(z ...)) ctcs)))
+             ((x . z)
+              (and (identifier? #'x)
+                   (module-identifier=? #'x #'contracted))
+              (raise-syntax-error 
+               'define-signature
+               "expected a list of [id contract] pairs after the contracted keyword"
+               (car sig-exprs)))
              ((x . y)
               (and (identifier? #'x)
                    (or (module-identifier=? #'x #'define-values)

collects/scribblings/reference/units.scrbl

@@ -160,6 +160,7 @@ the corresponding import. Each @scheme[tagged-sig-id] in an
 
  [sig-elem
   id
+  (contracted [id contract] ...)
   (define-syntaxes (id ...) expr)
   (define-values (value-id ...) expr) 
   (open sig-spec) 
@@ -176,6 +177,15 @@ of bindings for import or export:
  importing the signature, and @scheme[id] must be defined by units
  exporting the signature.}
       
+ @item{Each @scheme[contracted] form in a signature declaration means
+ that a unit exporting the signature must supply a variable definition
+ for each @scheme[id] in that form.  If the signature is imported, then
+ uses of @scheme[id] inside the unit are protected by the appropriate
+ contracts using the unit as the negative blame.  If the signature is
+ exported, then the exported values are protected by the appropriate
+ contracts which use the unit as the positive blame, but internal uses
+ of the exported identifiers are not protected.}
+
  @item{Each @scheme[define-syntaxes] form in a signature declaration
  introduces a macro to that is available for use in any unit that
  imports the signature.  Free variables in the definition's