Add documentation for predicate signatures.

Author: mikesperber

htdp-doc/scribblings/htdp-langs/advanced.scrbl

@@ -338,7 +338,7 @@ level as they did in the @secref["intermediate-lam"] level.
 
 @section[#:tag "advanced-signatures"]{Signatures}
 
-@(signature-forms ("advanced") advanced : signature enum mixed -> ListOf)
+@(signature-forms ("advanced") advanced : signature enum mixed -> ListOf predicate)
 
 @pre-defined-fun
 

htdp-doc/scribblings/htdp-langs/beginner-abbr.scrbl

@@ -100,7 +100,7 @@ Abbreviations} level as they did in the @secref["beginner"] level.
 
 @section[#:tag "beginner-abbr-signatures"]{Signatures}
 
-@(signature-forms ("beginner-abbr") define-struct : signature enum mixed -> ListOf)
+@(signature-forms ("beginner-abbr") define-struct : signature enum mixed -> ListOf predicate)
 
 @; ----------------------------------------
 

htdp-doc/scribblings/htdp-langs/beginner.scrbl

@@ -96,7 +96,7 @@ A quoted @racket[name] is a symbol. A symbol is a value, just like
 
 @section[#:tag "beginner-signatures"]{Signatures}
 
-@(signature-forms ("beginner") define-struct : signature enum mixed -> ListOf)
+@(signature-forms ("beginner") define-struct : signature enum mixed -> ListOf predicate)
 
 @; --------------------------------------------------
 

htdp-doc/scribblings/htdp-langs/intermediate-lambda.scrbl

@@ -141,7 +141,7 @@ level as they did in the @secref["intermediate"] level.
 
 @section[#:tag "intermediate-lambda-signatures"]{Signatures}
 
-@(signature-forms ("intermediate-lambda") define-struct : signature enum mixed -> ListOf)
+@(signature-forms ("intermediate-lambda") define-struct : signature enum mixed -> ListOf predicate)
 
 @pre-defined-fun
 

htdp-doc/scribblings/htdp-langs/intermediate.scrbl

@@ -114,7 +114,7 @@ did in the @secref["beginner-abbr"] level.
 
 @section[#:tag "intermediate-signatures"]{Signatures}
 
-@(signature-forms ("intermediate") define-struct : signature enum mixed -> ListOf)
+@(signature-forms ("intermediate") define-struct : signature enum mixed -> ListOf predicate)
 
 @section[#:tag "intermediate-pre-defined" ]{Pre-defined Functions}
 

htdp-doc/scribblings/htdp-langs/prim-ops.rkt

@@ -735,24 +735,26 @@ see note on @racket[check-expect] for details.  }
 
 (define-syntax-rule (signature-forms 
                      (section-prefix)
-                     define-struct : signature enum mixed -> ListOf)
+                     define-struct : signature enum mixed -> ListOf predicate)
   (gen-signature-forms section-prefix
                        #'define-struct @racket[define-struct]
                        #': @racket[:]
                        #'signature @racket[signature]
                        #'enum @racket[enum]
                        #'mixed @racket[mixed]
                        #'-> @racket[->]
-                       #'ListOf @racket[ListOf]))
+                       #'ListOf @racket[ListOf]
+                       #'predicate @racket[predicate]))
 
 (define (gen-signature-forms section-prefix
                              define-struct-id define-struct-elem
                              :-id :-elem
                              signature-id signature-elem
                              enum-id enum-elem
-                             mixed-id mixed-enum
+                             mixed-id mixed-elem
                              ->-id ->-elem
-                             ListOf-id ListOf-elem)
+                             ListOf-id ListOf-elem
+                             predicate-id predicate-elem)
   (list
    @; ----------------------------------------------------------------------
 
@@ -862,6 +864,12 @@ This signature describes a list where the elements are described by
 @racket[signature-form].
 }
 
+@defform[#:id [predicate predicate-id] (predicate expression)]{
+This signature describes values through a predicate:
+@racket[expression] must evaluate to a function of one argument that
+returns a boolean.  The signature matches all values for which
+the predicate returns @racket[#true].
+}
 
 @subsection[#:tag (string-append section-prefix "-struct-signatures")]{Struct Signatures}