Add unknownJson decoder

Previously `anyJson` was referred to as the identity decoder, but that's no
longer accurate with the update to using `unknown`. I'm not sure how useful
this is as a decoder, but it seems best to include for completeness sake.

Author: Elias Mulhall

src/combinators.ts

@@ -14,6 +14,9 @@ export const boolean = Decoder.boolean;
 /** See `Decoder.anyJson` */
 export const anyJson = Decoder.anyJson;
 
+/** See `Decoder.unknownJson` */
+export const unknownJson: () => Decoder<unknown> = Decoder.unknownJson;
+
 /** See `Decoder.constant` */
 export const constant = Decoder.constant;
 

src/decoder.ts

@@ -193,6 +193,13 @@ export class Decoder<A> {
    */
   static anyJson = (): Decoder<any> => new Decoder<any>((json: any) => Result.ok(json));
 
+  /**
+   * Decoder identity function which always succeeds and types the result as
+   * `unknown`.
+   */
+  static unknownJson = (): Decoder<unknown> =>
+    new Decoder<unknown>((json: unknown) => Result.ok(json));
+
   /**
    * Decoder primitive that only matches on exact values.
    *

src/index.ts

@@ -8,6 +8,7 @@ export {
   number,
   boolean,
   anyJson,
+  unknownJson,
   constant,
   object,
   array,

test/json-decode.test.ts

@@ -6,6 +6,7 @@ import {
   number,
   boolean,
   anyJson,
+  unknownJson,
   constant,
   object,
   array,
@@ -95,13 +96,42 @@ describe('boolean', () => {
 });
 
 describe('anyJson', () => {
-  it('can be used in other decoders', () => {
-    const json: any = [1, true, 2, 3, 'five', 4, []];
-    const jsonArray: any[] = Result.withDefault([], array(anyJson()).run(json));
-    const successes: Result.Ok<number>[] = jsonArray.map(number().run).filter(Result.isOk);
-    const numbers: number[] = successes.map(ok => ok.result);
+  it('bypasses type validation', () => {
+    // in a real use case this could be a deeply nested object
+    type ComplexType = number;
 
-    expect(numbers).toEqual([1, 2, 3, 4]);
+    interface User {
+      name: string;
+      complexUserData: ComplexType;
+    }
+
+    const userDecoder: Decoder<User> = object({
+      name: string(),
+      complexUserData: anyJson()
+    });
+
+    expect(userDecoder.run({name: 'Wanda', complexUserData: true})).toEqual({
+      ok: true,
+      result: {name: 'Wanda', complexUserData: true}
+    });
+
+    expect(userDecoder.run({name: 'Willard', complexUserData: 'trash data'})).toEqual({
+      ok: true,
+      result: {name: 'Willard', complexUserData: 'trash data'}
+    });
+
+    expect(userDecoder.run({name: 73, complexUserData: []})).toMatchObject({
+      ok: false,
+      error: {at: 'input.name', message: 'expected a string, got a number'}
+    });
+  });
+});
+
+describe('unknownJson', () => {
+  it('accepts any values', () => {
+    expect(unknownJson().run(1)).toEqual({ok: true, result: 1});
+    expect(unknownJson().run(false)).toEqual({ok: true, result: false});
+    expect(unknownJson().run({boots: 'n cats'})).toEqual({ok: true, result: {boots: 'n cats'}});
   });
 });