implemented (scheme lazy)

piclib/built-in.scm

@@ -892,3 +892,45 @@
         (write obj port)))))
 
   (export display))
+
+;;; Appendix A. Standard Libraries Lazy
+(define-library (scheme lazy)
+  (import (scheme base)
+	  (picrin macro))
+
+  (define-record-type promise
+    (make-promise% done obj)
+    promise?
+    (done promise-done? promise-done!)
+    (obj promise-value promise-value!))
+
+  (define-syntax delay-force
+    (ir-macro-transformer
+     (lambda (form rename compare?)
+       (let ((expr (cadr form)))
+	 `(make-promise% #f (lambda () ,expr))))))
+  
+  (define-syntax delay
+    (ir-macro-transformer
+     (lambda (form rename compare?)
+       (let ((expr (cadr form)))
+	 `(delay-force (make-promise% #t ,expr))))))
+
+  (define (promise-update! new old)
+    (promise-done! old (promise-done? new))
+    (promise-value! old (promise-value new)))
+  
+  (define (force promise)
+    (if (promise-done? promise)
+	(promise-value promise)
+	(let ((promise* ((promise-value promise))))
+	  (unless (promise-done? promise)
+		  (promise-update! promise* promise))
+	  (force promise))))
+
+  (define (make-promise obj)
+    (if (promise? obj)
+	obj
+	(make-promise% #f obj)))
+
+  (export delay-force delay force make-promise promise?))