Add logged decoder failures test example

Author: Elias Mulhall

test/logged-failures.test.ts

@@ -0,0 +1,46 @@
+import {Decoder, DecoderError, Result, number, array, result} from '../src/index';
+
+type Logger<E> = (err: E) => void;
+
+const logError = <T>(logger: Logger<DecoderError>, decoder: Decoder<T>): Decoder<T> =>
+  decoder.mapError(e => {
+    logger(e);
+    return e;
+  });
+
+const loggedArrayDecoder = <T>(decoder: Decoder<T>, logger: Logger<DecoderError>): Decoder<T[]> => {
+  const logged = <U>(d: Decoder<U>) => logError(logger, d);
+  return logged(array(result(logged(decoder)))).map(Result.successes);
+};
+
+const makeLogger = () => {
+  const log: string[] = [];
+  const logger = ({message}: DecoderError) => log.push(message);
+  return {log, logger};
+};
+
+describe('decode valid array members and log invalid members', () => {
+  it('succeeds on valid input', () => {
+    const {log, logger} = makeLogger();
+    const r = loggedArrayDecoder(number(), logger).run([1, 2, 3, 4]);
+    expect(r).toEqual({ok: true, result: [1, 2, 3, 4]});
+    expect(log).toEqual([]);
+  });
+
+  it('succeeds on valid array members while logging and filtering invalid ones', () => {
+    const {log, logger} = makeLogger();
+    const r = loggedArrayDecoder(number(), logger).run([true, [], 999]);
+    expect(r).toEqual({ok: true, result: [999]});
+    expect(log).toEqual(['expected a number, got a boolean', 'expected a number, got an array']);
+  });
+
+  it('failes on non-array json and logs the failure', () => {
+    const {log, logger} = makeLogger();
+    const r = loggedArrayDecoder(number(), logger).run(5);
+    expect(r).toMatchObject({
+      ok: false,
+      error: {at: 'input', message: 'expected an array, got a number'}
+    });
+    expect(log).toEqual(['expected an array, got a number']);
+  });
+});