Remove decoderErrorString function

I think I added this helper because I read somewhere that you should make all
of your thrown error payloads be strings. This confusion was probably
related to using `Error`. I've changed my mind about that, so let's get
rid of the extra code.

Author: Elias Mulhall

src/decoder.ts

@@ -43,12 +43,6 @@ export type DecoderObject<A> = {[t in keyof A]: Decoder<A[t]>};
 export const isDecoderError = (a: any): a is DecoderError =>
   a.kind === 'DecoderError' && typeof a.at === 'string' && typeof a.message === 'string';
 
-/**
- * `DecoderError` information as a formatted string.
- */
-export const decoderErrorString = (error: DecoderError): string =>
-  `Input: ${JSON.stringify(error.input)}\nFailed at ${error.at}: ${error.message}`;
-
 /*
  * Helpers
  */
@@ -597,8 +591,7 @@ export class Decoder<A> {
    * Run the decoder and return the value on success, or throw an exception
    * with a formatted error string.
    */
-  runWithException = (json: any): A =>
-    Result.withException(Result.mapError(decoderErrorString, this.run(json)));
+  runWithException = (json: any): A => Result.withException(this.run(json));
 
   /**
    * Construct a new decoder that applies a transformation to the decoded

src/index.ts

@@ -1,7 +1,7 @@
 import * as Result from './result';
 export {Result};
 
-export {Decoder, DecoderError, isDecoderError, decoderErrorString, DecoderObject} from './decoder';
+export {Decoder, DecoderError, isDecoderError, DecoderObject} from './decoder';
 
 export {
   string,

test/json-decode.test.ts

@@ -682,9 +682,20 @@ describe('runWithException', () => {
   });
 
   it('throws an exception when the decoder fails', () => {
-    expect(() => decoder.runWithException(42)).toThrowError(
-      'Input: 42\nFailed at input: expected a boolean, got a number'
-    );
+    let thrownError: any;
+
+    try {
+      decoder.runWithException(42);
+    } catch (e) {
+      thrownError = e;
+    }
+
+    expect(thrownError).toEqual({
+      kind: 'DecoderError',
+      input: 42,
+      at: 'input',
+      message: 'expected a boolean, got a number'
+    });
   });
 });