From 84d474a06997f66642f5375b24c77ffb764b4a81 Mon Sep 17 00:00:00 2001
From: Justin Ethier <justin.ethier@gmail.com>
Date: Fri, 21 Jun 2019 13:28:50 -0400
Subject: [PATCH] Add futures

---
 libs/cyclone/concurrent.sld | 63 +++++++++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)

diff --git a/libs/cyclone/concurrent.sld b/libs/cyclone/concurrent.sld
index eb2465e8..9d51eb36 100644
--- a/libs/cyclone/concurrent.sld
+++ b/libs/cyclone/concurrent.sld
@@ -13,6 +13,7 @@
 (define-library (cyclone concurrent)
  (import
    (scheme base)
+   (srfi 18)
  )
  (include-c-header "<ck_pr.h>")
  (export
@@ -23,6 +24,11 @@
    deref
    swap!
    compare-and-set!
+   ;; Futures
+   future?
+   future
+   future-call
+   future-deref
    ;; Immutable objects
    immutable?
    ;; Shared objects
@@ -147,5 +153,62 @@
    }
    return_closcall1(data, k, result); ")
 
+;; Futures
+  (define-record-type <future>
+    (make-future done result lock)
+    future?
+    (done get-done set-done!)
+    (result get-result set-result!)
+    (lock get-lock set-lock!))
+
+  ;; macro: (future expr ...)
+  (define-syntax future
+    (er-macro-transformer
+      (lambda (expr rename compare)
+        `(future-call (lambda () ,@(cdr expr))))))
+
+;; From the clojure docs:
+;;
+;; Takes a function of no args and yields a future object that will
+;; invoke the function in another thread, and will cache the result and
+;; return it on all subsequent calls to deref/@. If the computation has
+;; not yet finished, calls to deref/@ will block, unless the variant
+;; of deref with timeout is used. See also - realized?.
+(define (future-call thunk)
+  (let* (
+         (lock (make-mutex))
+         (ftr (make-future #f #f lock))
+         (tfnc (lambda ()
+                 (mutex-lock! lock) 
+                 (let ((result (thunk))) ;; TODO: Catch exceptions (?)
+                   (set-result! ftr result)
+                   (set-done! ftr #t)
+                   (mutex-unlock! lock) 
+                 )))
+         (t (make-thread tfnc))
+        )
+    (thread-start! t)
+    ftr))
+
+;;(define (future-done? ftr)
+;;  (when (not (future? ftr))
+;;    (error "Expected future but received" ftr))
+;; TODO: may be a good candidate for a timed mutex lock, just return #f if minimum timeout is exceeded
+;;)
+
+;; TODO: (future-cancel ftr)
+;; TODO: (future-cancelled? ftr)
+
+;;TODO: custom deref but eventually need to fold this functionality back into the main one
+(define (future-deref ftr)
+  (when (not (future? ftr))
+    (error "Expected future but received" ftr))
+  (let ((result #f))
+    (mutex-lock! (get-lock ftr))
+    (set! result (get-result ftr))
+    (mutex-unlock! (get-lock ftr))
+    result))
+;; END Futures
+
  )
 )