3 files changed, 68 insertions, 0 deletions
diff --git a/stdlib/source/lux/codata/thunk.lux b/stdlib/source/lux/codata/thunk.lux
new file mode 100644
index 000000000..2c0db0db5
--- /dev/null
+++ b/stdlib/source/lux/codata/thunk.lux
@@ -0,0 +1,38 @@
+##  Copyright (c) Eduardo Julian. All rights reserved.
+##  This Source Code Form is subject to the terms of the Mozilla Public License, v. 2.0.
+##  If a copy of the MPL was not distributed with this file,
+##  You can obtain one at http://mozilla.org/MPL/2.0/.
+
+(;module:
+  lux
+  (lux [io]
+       (control monad)
+       (concurrency ["A" atom])
+       [compiler]
+       (macro ["s" syntax #+ syntax:])))
+
+(type: #export (Thunk a)
+  (-> [] a))
+
+(def: #hidden (freeze' generator)
+  (All [a] (-> (-> [] a) (-> [] a)))
+  (let [cache (: (A;Atom (Maybe ($ +0)))
+                 (A;atom #;None))]
+    (lambda [_]
+      (case (io;run (A;get cache))
+        (#;Some value)
+        value
+
+        _
+        (let [value (generator [])]
+          (exec (io;run (A;compare-and-swap _ (#;Some value) cache))
+            value))))))
+
+(syntax: #export (freeze expr)
+  (do @
+    [g!arg (compiler;gensym "")]
+    (wrap (list (` (freeze' (lambda [(~ g!arg)] (~ expr))))))))
+
+(def: #export (thaw thunk)
+  (All [a] (-> (Thunk a) a))
+  (thunk []))
diff --git a/stdlib/test/test/lux/codata/thunk.lux b/stdlib/test/test/lux/codata/thunk.lux
new file mode 100644
index 000000000..b8ddee578
--- /dev/null
+++ b/stdlib/test/test/lux/codata/thunk.lux
@@ -0,0 +1,29 @@
+##  Copyright (c) Eduardo Julian. All rights reserved.
+##  This Source Code Form is subject to the terms of the Mozilla Public License, v. 2.0.
+##  If a copy of the MPL was not distributed with this file,
+##  You can obtain one at http://mozilla.org/MPL/2.0/.
+
+(;module:
+  lux
+  (lux [io]
+       (control monad)
+       (codata ["&" thunk])
+       pipe
+       ["R" random])
+  lux/test)
+
+(test: "Thunks"
+  [left R;nat
+   right R;nat
+   #let [thunk (&;freeze (n.* left right))
+         expected (n.* left right)]]
+  ($_ seq
+      (assert "Thunking does not alter the expected value."
+              (n.= expected
+                   (&;thaw thunk)))
+      (assert "Thunks only evaluate once."
+              (and (not (is expected
+                            (&;thaw thunk)))
+                   (is (&;thaw thunk)
+                       (&;thaw thunk))))
+      ))
diff --git a/stdlib/test/tests.lux b/stdlib/test/tests.lux
index d6a21cae4..e30a8cf0a 100644
--- a/stdlib/test/tests.lux
+++ b/stdlib/test/tests.lux
@@ -20,6 +20,7 @@
              (codata ["_;" cont]
                      ["_;" env]
                      ["_;" state]
+                     ["_;" thunk]
                      (coll ["_;" stream]))
              (concurrency ["_;" actor]
                           ["_;" atom]