From 6e8c0b6088a8a98367d2222cad3d296542a01657 Mon Sep 17 00:00:00 2001
From: Justin Ethier <justin.ethier@gmail.com>
Date: Wed, 10 Jul 2019 18:53:23 -0400
Subject: [PATCH] Functional build-out of delay

---
 libs/cyclone/delay-promise.scm | 44 ++++++++++++++++++++++++++++++++--
 1 file changed, 42 insertions(+), 2 deletions(-)

diff --git a/libs/cyclone/delay-promise.scm b/libs/cyclone/delay-promise.scm
index 0dcb8ea3..cd5d956c 100644
--- a/libs/cyclone/delay-promise.scm
+++ b/libs/cyclone/delay-promise.scm
@@ -1,3 +1,43 @@
-TODO: support for concurrent delay and promise as implemented by Clojure
+;; TODO: support for concurrent delay and promise as implemented by Clojure
+;; See: https://purelyfunctional.tv/guide/clojure-concurrency/
 
-See: https://purelyfunctional.tv/guide/clojure-concurrency/
+(import (scheme write) (scheme base) (cyclone concurrent) (srfi 18))
+
+(define-record-type <delay>
+  (%make-delay done result lock)
+  delay?
+  (done delay:done delay:set-done!)
+  (value delay:value delay:set-value!) ;; Either thunk or result
+  (lock delay:lock delay:set-lock!))
+
+(define (make-delay thunk)
+  (%make-delay #f thunk (make-mutex)))
+
+(define (delay-deref d)
+  (when (not (delay? d))
+    (error "Expected future but received" d))
+  (mutex-lock! (delay:lock d))
+  (cond
+    ((delay:done d) 
+     (delay:value d))
+    (else
+     (delay:set-value! d 
+       (make-shared ((delay:value d)))) ;; Exec thunk and store result
+     (delay:set-done! d #t)))
+  (mutex-unlock! (delay:lock d))
+)
+
+(define-syntax shared-delay
+  (er-macro-transformer
+   (lambda (expr rename compare)
+     `(make-delay (lambda () ,(cadr expr))))))
+
+
+(define (test)
+  (write '(testing)) (newline)
+  'done)
+
+(define d (shared-delay (test)))
+(write (delay-deref d))(newline)
+(write (delay-deref d))(newline)
+(write (delay-deref d))(newline)