Add applicable records

Fixes #267 (closed)

doc/hoot.texi

@@ -1819,8 +1819,9 @@ and @code{hashv} is used by @code{make-eqv-hashtable}.
 @node Records
 @section Records
 
-Hoot extends the R7RS @code{define-record-type} form with additional
-features such as inheritance and opaque types.
+The @code{(hoot records)} module extends the R7RS
+@code{define-record-type} form with additional features such as
+inheritance and opaque types.
 
 @deffn {Syntax} define-record-type name @
        [#:printer] [#:parent] [#:uid] [#:extensible? #t] [#:opaque? #f] @
@@ -1859,6 +1860,59 @@ appear more than once in the fields section.
 
 @end deffn
 
+@subsection Applicable structs
+
+Hoot's record type system can be used to create new types that can be
+applied as if they were regular procedures.  These are known as
+@emph{applicable structs}.  The @code{(hoot applicable-structs)}
+module provides the special @code{<applicable-struct>} record type; it
+defines a single field that should contain a procedure.  Any record
+type that descends from the @code{<applicable-struct>} lineage (via
+@code{#:parent}) becomes procedure-like.
+
+As a contrived example, an incrementing counter type could be
+implemented as an applicable struct:
+
+@lisp
+(define-record-type <counter>
+  #:parent <applicable-struct>
+  (%make-counter procedure count)
+  counter?
+  (count counter-count set-counter-count!))
+
+(define (make-counter)
+  (define (next!)
+    (let ((x (1+ (counter-count counter))))
+      (set-counter-count! counter x)
+      x))
+  (define counter (%make-counter next! 0))
+  counter)
+
+(define c (make-counter))
+
+(counter? c) ;; => #t
+
+(procedure? c) ;; => #t
+
+(c) ;; => 1
+(c) ;; => 2
+@end lisp
+
+Note that @code{<counter>} objects are recognized as both counters
+@emph{and} procedures!
+
+@defvr {Variable} <applicable-struct>
+The record type descriptor for applicable structs.
+@end defvr
+
+@deffn {Procedure} applicable-struct? obj
+Return @code{#t} if @var{obj} is an applicable struct.
+@end deffn
+
+@deffn {Procedure} applicable-struct-procedure obj
+Return the procedure stored within @var{obj}.
+@end deffn
+
 @node Pattern matching
 @section Pattern matching