4 files changed, 44 insertions, 5 deletions
diff --git a/ui/lib/swatch/derive.js b/ui/lib/swatch/derive.js
index 85547e8..22ceb13 100644
--- a/ui/lib/swatch/derive.js
+++ b/ui/lib/swatch/derive.js
@@ -5,16 +5,12 @@ function tryDerive(args, func, fallback) {
   try {
     return func(...args);
   } catch (e) {
-    console.debug('deriver threw exception', e, func, args);
     return fallback;
   }
 }
 
 // A "deriver" is a function that never raises; if the underlying function would raise, the
 // corresponding deriver instead returns a fallback value (or `undefined`).
-export function makeDeriver(func, fallback) {
+export function makeDeriver(func, fallback = undefined) {
   return (...args) => tryDerive(args, func, fallback);
 }
-
-// Some widely-used derivers, for convenience.
-export const json = makeDeriver(JSON.parse);
diff --git a/ui/lib/swatch/json.js b/ui/lib/swatch/json.js
new file mode 100644
index 0000000..3ebbc0d
--- /dev/null
+++ b/ui/lib/swatch/json.js
@@ -0,0 +1,7 @@
+import { makeDeriver } from '$lib/swatch/derive.js';
+
+export const decode = makeDeriver(JSON.parse);
+
+export function encode(value) {
+  return JSON.stringify(value, /* replacer */ null, /* space */ 2);
+}
diff --git a/ui/lib/swatch/json.test.js b/ui/lib/swatch/json.test.js
new file mode 100644
index 0000000..e01017d
--- /dev/null
+++ b/ui/lib/swatch/json.test.js
@@ -0,0 +1,36 @@
+import * as json from './json.js';
+import { expect, describe, it } from 'vitest';
+
+describe('encode', async () => {
+  it('converts atoms', async () => {
+    expect(json.encode(0)).toStrictEqual('0');
+    expect(json.encode('hello')).toStrictEqual('"hello"');
+    expect(json.encode(null)).toStrictEqual('null');
+    expect(json.encode(true)).toStrictEqual('true');
+  });
+
+  it('converts lists with indentation', async () => {
+    expect(json.encode([])).toStrictEqual('[]');
+    expect(json.encode(['hello', 'world'])).toStrictEqual('[\n  "hello",\n  "world"\n]');
+  });
+
+  it('converts objects with indentation', async () => {
+    expect(json.encode({})).toStrictEqual('{}');
+    expect(json.encode({ field: 'value' })).toStrictEqual('{\n  "field": "value"\n}');
+  });
+});
+
+describe('decode', async () => {
+  it('converts valid json values', async () => {
+    expect(json.decode('0')).toStrictEqual(0);
+    expect(json.decode('null')).toStrictEqual(null);
+    expect(json.decode('{}')).toStrictEqual({});
+    expect(json.decode('[1, 2, "3"]')).toStrictEqual([1, 2, '3']);
+  });
+
+  it('converts invalid json to `undefined`', async () => {
+    expect(json.decode('')).toBeUndefined();
+    expect(json.decode('{"foo":')).toBeUndefined();
+    expect(json.decode('arbitrary nonsense')).toBeUndefined();
+  });
+});
diff --git a/ui/lib/swatch/setup.js b/ui/lib/swatch/setup.js
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/ui/lib/swatch/setup.js