1 files changed, 85 insertions, 0 deletions
diff --git a/shared/data-directed-programming.rkt b/shared/data-directed-programming.rkt
new file mode 100644
index 0000000..c756537
--- /dev/null
+++ b/shared/data-directed-programming.rkt
@@ -0,0 +1,85 @@
+#lang racket
+(provide
+  make-apply
+  make-dispatch-table
+  getter
+  putter
+  attach-tag
+  type-tag
+  contents
+  find-type)
+(require "./lists.rkt")
+
+
+
+;; Type tagged data
+(define (attach-tag type-tag contents)
+  (cons type-tag contents))
+(define (type-tag datum)
+  (if (pair? datum)
+    (car datum)
+    (error "Bad tagged datum -- TYPE-TAG" datum)))
+(define (contents datum)
+  (if (pair? datum)
+    (cdr datum)
+    (error "Bad tagged datum -- CONTENTS" datum)))
+
+(define (find-type type seq)
+  (define (rec items)
+   (cond ((null? items) false)
+         ((equal? type (type-tag (car items))) (car items))
+         (else (rec (cdr items)))))
+  (rec seq))
+
+
+;; dispatch table abstraction barrier/ encapsulation
+;; ('op (list ('(types) item)))
+(define (make-dispatch-table)
+  (define dispatch-table '())
+  (define (get op types)
+    (let ((op-datum (find-type op dispatch-table)))
+      (if op-datum
+        (let ((proc-datum (find-type types (contents op-datum))))
+          (if (pair? proc-datum)
+            (contents proc-datum)
+            false))
+        false)))
+
+  (define (update-op-datum op-datum types proc)
+    (attach-tag (type-tag op-datum)
+                (if (find-type types (contents op-datum))
+                  (map (lambda (proc-datum)
+                        (if (not (eq? (type-tag proc-datum) types))
+                          proc-datum
+                          (attach-tag (type-tag proc-datum) proc)))
+                       (contents op-datum))
+                  (cons (attach-tag types proc) (contents op-datum)))))
+
+  (define (put op types item)
+    (if (find-type op dispatch-table)
+      (set! dispatch-table
+        (map (lambda (op-datum) ;;just copy the table for now, don't want to mutate yet
+               (if (not (equal? (type-tag op-datum) op))
+                 op-datum
+                 (update-op-datum op-datum types item)))
+             dispatch-table))
+      (set! dispatch-table (cons
+                             (attach-tag op
+                                (list (attach-tag types item)))
+                             dispatch-table))))
+  (list dispatch-table get put))
+
+(define (getter t)
+  (cadr t))
+(define (putter t)
+  (caddr t))
+
+(define (make-apply put get)
+  (lambda (op . args)
+     (let ((type-tags (map type-tag args)))
+       (let ((proc (get op type-tags)))
+         (if proc
+           (apply proc (map contents args))
+           (error
+             "No method for these types -- APPLY-GENERIC"
+             (list op type-tags)))))))