Add recho.state(value)

app/docs/api-reference.recho.js

@@ -20,6 +20,7 @@
  * - echo(...values) - Echo values inline with your code as comments (https://recho.dev/notebook/docs/api-echo)
  * - echo.clear() - Clear the output of the current block (https://recho.dev/notebook/docs/api-echo-clear)
  * - invalidation() - Promise that resolves before re-running the current block (https://recho.dev/notebook/docs/api-invalidation)
+ * - recho.state(value) - Create reactive state variables for mutable values (https://recho.dev/notebook/docs/api-state)
  * - recho.inspect(value[, options]) - Format values for inspection (https://recho.dev/notebook/docs/api-inspect)
  *
  * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

app/docs/api-state.recho.js

@@ -0,0 +1,72 @@
+/**
+ * @title recho.state(value)
+ */
+
+/**
+ * ============================================================================
+ * =                           recho.state(value)                            =
+ * ============================================================================
+ *
+ * Creates a reactive state variable that can be mutated over time. This is
+ * similar to React's useState hook and enables mutable reactive values that
+ * automatically trigger re-evaluation of dependent blocks when changed.
+ *
+ * @param {any} value - The initial state value.
+ * @returns {[any, Function, Function]} A tuple containing:
+ *   - state: The reactive state value that can be read directly
+ *   - setState: Function to update the state (accepts value or updater function)
+ *   - getState: Function to get the current state value
+ */
+
+// Basic counter that increments after 1 second
+const [count1, setCount1] = recho.state(0);
+
+setTimeout(() => {
+  setCount1(count1 => count1 + 1);
+}, 1000);
+
+//➜ 1
+echo(count1);
+
+// Timer that counts down from 10
+const [timer, setTimer] = recho.state(10);
+
+{
+  const interval = setInterval(() => {
+    setTimer(t => {
+      if (t <= 0) {
+        clearInterval(interval);
+        return 0;
+      }
+      return t - 1;
+    });
+  }, 1000);
+
+  invalidation.then(() => clearInterval(interval));
+}
+
+//➜ 8
+echo(`Time remaining: ${timer}s`);
+
+// State can be updated with a direct value
+const [message, setMessage] = recho.state("Hello");
+
+setTimeout(() => {
+  setMessage("Hello, World!");
+}, 2000);
+
+//➜ "Hello, World!"
+echo(message);
+
+// Multiple states can be used together
+const [firstName, setFirstName] = recho.state("John");
+const [lastName, setLastName] = recho.state("Doe");
+
+setTimeout(() => {
+  setFirstName("Jane");
+  setLastName("Smith");
+}, 1500);
+
+//➜ "Jane Smith"
+echo(`${firstName} ${lastName}`);
+

app/docs/nav.config.js

@@ -59,6 +59,10 @@ export const docsNavConfig = [
         type: "page",
         slug: "api-invalidation",
       },
+      {
+        type: "page",
+        slug: "api-state",
+      },
       {
         type: "page",
         slug: "api-inspect",

runtime/stdlib/index.js

@@ -3,3 +3,4 @@ export {now} from "./now.js";
 export {interval} from "./interval.js";
 export {inspect, Inspector} from "./inspect.js";
 export * from "../controls/index.js";
+export {state} from "./state.js";

runtime/stdlib/observe.js

@@ -0,0 +1,37 @@
+// Derived from Observable Notebook Kit's observe.
+// https://github.com/observablehq/notebook-kit/blob/main/src/runtime/stdlib/generators/observe.ts
+
+export async function* observe(initialize) {
+  let resolve = undefined;
+  let value = undefined;
+  let stale = false;
+
+  const dispose = initialize((x) => {
+    value = x;
+    if (resolve) {
+      resolve(x);
+      resolve = undefined;
+    } else {
+      stale = true;
+    }
+    return x;
+  });
+
+  if (dispose != null && typeof dispose !== "function") {
+    throw new Error(
+      typeof dispose === "object" && "then" in dispose && typeof dispose.then === "function"
+        ? "async initializers are not supported"
+        : "initializer returned something, but not a dispose function",
+    );
+  }
+
+  try {
+    while (true) {
+      yield stale ? ((stale = false), value) : new Promise((_) => (resolve = _));
+    }
+  } finally {
+    if (dispose != null) {
+      dispose();
+    }
+  }
+}

runtime/stdlib/state.js

@@ -0,0 +1,26 @@
+// Derived from Observable Notebook Kit's mutable and mutator.
+// https://github.com/observablehq/notebook-kit/blob/main/src/runtime/stdlib/mutable.ts
+import {observe} from "./observe.js";
+
+// Mutable returns a generator with a value getter/setting that allows the
+// generated value to be mutated. Therefore, direct mutation is only allowed
+// within the defining cell, but the cell can also export functions that allows
+// other cells to mutate the value as desired.
+function Mutable(value) {
+  let change = undefined;
+  const mutable = observe((_) => {
+    change = _;
+    if (value !== undefined) change(value);
+  });
+  return Object.defineProperty(mutable, "value", {
+    get: () => value,
+    set: (x) => ((value = x), void change?.(value)),
+  });
+}
+
+export function state(value) {
+  const state = Mutable(value);
+  const setState = (x) => (typeof x === "function" ? (state.value = x(state.value)) : (state.value = x));
+  const getState = () => state.value;
+  return [state, setState, getState];
+}

test/js/index.js

@@ -14,6 +14,7 @@ export {mandelbrotSet} from "./mandelbrot-set.js";
 export {matrixRain} from "./matrix-rain.js";
 export {jsDocString} from "./js-doc-string.js";
 export {commentLink} from "./comment-link.js";
+export {mutable} from "./mutable.js";
 export {syntaxError3} from "./syntax-error3.js";
 export {syntaxError4} from "./syntax-error4.js";
 export {nonCallEcho} from "./non-call-echo.js";

test/js/mutable.js

@@ -0,0 +1,8 @@
+export const mutable = `const [a, setA] = recho.state(0);
+
+setTimeout(() => {
+  setA((a) => a + 1);
+}, 1000);
+
+echo(a);
+`;

test/stdlib.spec.js

@@ -8,4 +8,5 @@ it("should export expected functions from stdlib", () => {
   expect(stdlib.toggle).toBeDefined();
   expect(stdlib.number).toBeDefined();
   expect(stdlib.radio).toBeDefined();
+  expect(stdlib.state).toBeDefined();
 });