From 3fc61521fbf5f45097764e8a9f49935e29cfae49 Mon Sep 17 00:00:00 2001
From: Justin Ethier <justin.ethier@gmail.com>
Date: Fri, 5 Feb 2016 21:14:39 -0500
Subject: [PATCH] Initial file

---
 scheme/lazy.sld | 46 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)
 create mode 100644 scheme/lazy.sld

diff --git a/scheme/lazy.sld b/scheme/lazy.sld
new file mode 100644
index 00000000..24286f5a
--- /dev/null
+++ b/scheme/lazy.sld
@@ -0,0 +1,46 @@
+(define-library (scheme lazy)
+  (import (scheme base))
+  (export 
+    delay 
+    force 
+    delay-force 
+    make-promise 
+    promise?)
+  (begin
+    (define (make-promise x)
+      (delay x))
+  ;)
+  ;(begin
+    (define (promise? x)
+      (and (pair? x)
+           (null? (cdr x))
+           (pair? (car x))
+           (or (eq? #t (caar x))
+               (and (eq? #f (caar x))
+                    (procedure? (cdar x))))))
+
+  (define-syntax delay-force
+    (er-macro-transformer
+     (lambda (expr rename compare)
+       `(,(rename 'promise) #f (,(rename 'lambda) () ,(cadr expr))))))
+
+  (define-syntax delay
+    (er-macro-transformer
+     (lambda (expr rename compare)
+       `(,(rename 'delay-force) (,(rename 'promise) #t ,(cadr expr))))))
+
+  (define (promise done? proc)
+    (list (cons done? proc)))
+  (define (promise-done? x) (car (car x)))
+  (define (promise-value x) (cdr (car x)))
+  (define (promise-update! new old)
+    (set-car! (car old) (promise-done? new))
+    (set-cdr! (car old) (promise-value new))
+    (set-car! new (car old)))
+  (define (force promise)
+    (if (promise-done? promise)
+        (promise-value promise)
+        (let ((promise* ((promise-value promise))))
+          (if (not (promise-done? promise))
+            (promise-update! promise* promise))
+          (force promise))))))