Merge pull request #30 from vankeisb/feature/fix-value-decoder

fix decoder for any value

core/src/TeaCup/Decode.test.ts

@@ -260,3 +260,35 @@ test('oneOf', () => {
 test('from string', () => {
   expect(num.decodeString('123')).toEqual(ok(123));
 });
+
+test('any value', () => {
+  const anyValue = { foo: 'bar' };
+  expect(Decode.value.decodeValue(anyValue)).toEqual(ok(anyValue));
+});
+
+describe('optional field', () => {
+  test("is present", () => {
+    const value = { foo: 'bar', gnu: 13 };
+    expect(Decode.optionalField('gnu', Decode.num).decodeValue(value)).toEqual(ok(13));
+  })
+  test("is missing", () => {
+    const value = { foo: 'bar' };
+    expect(Decode.optionalField('gnu', Decode.num).decodeValue(value)).toEqual(ok(undefined));
+  })
+
+  test("typical use case", () => {
+    type MyType = {
+      foo: string;
+      gnu?: number;
+    }
+    const value = { foo: 'bar' };
+    const expected: MyType = {
+      foo: 'bar'
+    };
+    expect(Decode.map2(
+      (foo, gnu) => { return { foo, gnu } },
+      Decode.field('foo', Decode.str),
+      Decode.optionalField('gnu', Decode.num)).decodeValue(value)
+    ).toEqual(ok(expected));
+  })
+});

core/src/TeaCup/Decode.ts

@@ -178,6 +178,18 @@ export class Decode {
     return Decode.at([key], d);
   }
 
+  /**
+   * Decoder for optinal object fields
+   * @param key the name of the optinal field
+   * @param d the decoder for the field's value
+   */
+  static optionalField<T>(key: string, d: Decoder<T>): Decoder<T | undefined> {
+    return Decode.andThen(
+      (value) => value.map<Decoder<T | undefined>>(v => new Decoder<T>(() => d.decodeValue(v))).withDefault(Decode.succeed(undefined)),
+      Decode.maybe(Decode.field(key, Decode.value))
+    );
+  }
+
   /**
    * Decoder for navigable object properties
    * @param keys a list of fields to navigate
@@ -439,7 +451,7 @@ export class Decode {
   /**
    * Decoder for any value
    */
-  static value: Decoder<any> = new Decoder<any>((o) => o);
+  static value: Decoder<any> = new Decoder<any>((o) => ok(o));
 
   /**
    * Decoder for null