From 022ce6da0376a6bedd159f6006331907c385b16c Mon Sep 17 00:00:00 2001
From: Daniel Barlow <dan@telent.net>
Date: Sun, 20 Feb 2022 12:02:36 +0000
Subject: [PATCH] savepoint

---
 just/observable-test.fnl | 51 ++++++++++++++++++++++++++++++++++++++++
 just/observable.fnl      | 35 +++++++++++++++++++++++++++
 2 files changed, 86 insertions(+)
 create mode 100644 just/observable-test.fnl
 create mode 100644 just/observable.fnl

diff --git a/just/observable-test.fnl b/just/observable-test.fnl
new file mode 100644
index 0000000..e6e922b
--- /dev/null
+++ b/just/observable-test.fnl
@@ -0,0 +1,51 @@
+(local fennel (require :fennel))
+(local observable (require :observable))
+
+(macro expect [form]
+  `(do
+     (assert ,form ,(view [:failed form]))
+     (print ,(view [:passed form]))))
+
+(macro expect-error [form]
+  `(let [(ok?# msg#) (pcall (fn [] ,form))]
+     (assert (not ok?#) msg#)
+     (print ,(view [:passed (list :error form)]))))
+
+(local app-state
+       (observable.new
+        {
+         :foo 27
+         :bar 54
+         :nest {
+                :name "beatles"
+                :artists [:john :paul: :ringo :george ]
+                }
+         }))
+
+
+(expect (= (app-state:get [:foo]) 27))
+(expect (= (app-state:get [:nest :name]) "beatles"))
+
+(let [s (observable.new {:foo 43})]
+  ;; s:get on a non-leaf returns the subtree. use table.concat
+  ;; for comparison of values
+  (expect (= (table.concat (s:get [])) (table.concat {:foo 43}))))
+
+(let [s (observable.new {:foo 43})]
+  ;; update existing entry
+  (s:update [:foo] 84)
+  (expect (= (s:get [:foo]) 84))
+
+  ;; create new entry
+  (s:update [:baz] 48)
+  (expect (= (s:get [:baz]) 48))
+
+  ;; doesn't create new nested keys
+  (expect-error (s:update [:nonexistent :path] 22)))
+
+(let [s (observable.new {:foo {:bar 43}})]
+  (var win false)
+  ;; observers live on subtrees, not individual nodes
+  (s:observe [:foo] #(set win true))
+  (s:update [:foo :bar] 42)
+  (expect (and win)))
diff --git a/just/observable.fnl b/just/observable.fnl
new file mode 100644
index 0000000..e8edcc3
--- /dev/null
+++ b/just/observable.fnl
@@ -0,0 +1,35 @@
+(local {: view} (require :fennel))
+
+(fn concat [dest src]
+  (table.move dest 1 (# dest) (# src) src))
+
+(fn update [data self path value]
+  (let [[first & rest] path]
+    (if (next rest)
+        (update (. data first) self rest value)
+        (do
+          (if data._subscribers
+              (each [_ f (pairs data._subscribers)] (f)))
+          (tset data first value)))))
+
+(fn get [data self path]
+  (let [[first & rest] path]
+    (if (not first) data
+        (next rest) (get (. data first) self rest)
+        (. data first))))
+
+(fn observe [data self path fun]
+  (let [el (get data self path)]
+    (when el
+      (if el._subscribers
+          (el._subscribers:insert fun)
+          (tset el :_subscribers [fun])))))
+
+(fn new [data]
+  {
+   :observe (partial observe data)
+   :update (partial update data)
+   :get (partial get data)
+   })
+
+{: new }