Add applicable records

Fixes #267 (closed)

doc/hoot.texi

@@ -1826,8 +1826,9 @@ 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] @
@@ -1866,6 +1867,65 @@ appear more than once in the fields section.
 
 @end deffn
 
+@c @subsection Applicable records
+
+@c Hoot's record type system can be used to create new types that can be
+@c applied as if they were regular procedures.  These are known as
+@c @emph{applicable records}.  The @code{<applicable-reord>} type defines
+@c a single field that should contain a procedure.  Any record type that
+@c descends from the @code{<applicable-record>} lineage (via
+@c @code{#:parent}) becomes procedure-like.
+
+@c As a contrived example, an incrementing counter could be implemented
+@c like this:
+
+@c @lisp
+@c (use-modules (hoot records))
+
+@c (define-record-type <counter>
+@c   #:parent <applicable-record>
+@c   (%make-counter procedure count)
+@c   counter?
+@c   (count counter-count set-counter-count!))
+
+@c (define (make-counter)
+@c   (define (next!)
+@c     (let ((x (1+ (counter-count counter))))
+@c       (set-counter-count! counter x)
+@c       x))
+@c   (define counter (%make-counter next! 0))
+@c   counter)
+
+@c (define c (make-counter))
+
+@c (counter? c) ;; => #t
+
+@c (procedure? c) ;; => #t
+
+@c (c) ;; => 1
+@c (c) ;; => 2
+@c @end lisp
+
+@c Note that @code{<counter>} objects are recognized as both counters
+@c @emph{and} procedures!
+
+@c @defvr {Variable} <applicable-record>
+@c The record type descriptor for applicable records.
+@c @end defvr
+
+@c @deffn {Procedure} applicable-record? obj
+@c Return @code{#t} if @var{obj} is an applicable record.
+@c @end deffn
+
+@c @deffn {Procedure} applicable-record-procedure obj
+@c Return the procedure stored within the applicable record @var{obj}.
+@c @end deffn
+
+@c @deffn {Procedure} set-applicable-record-procedure! obj proc
+@c Set the procedure stored within the applicable record @var{obj} to
+@c @var{proc}.
+@c @end deffn
+
 @node Pattern matching
 @section Pattern matching