Add Spacing to stringify output. (#3202)

* add spacing to stringify output.

* fix test in combo.js by adding replacer to stringify.

packages/sourcecred/src/util/combo.js

@@ -3,6 +3,8 @@
 // Simple parser combinator library for structured types rather than
 // bytestring parsing.
 
+import stringify from "json-stable-stringify";
+
 export type JsonObject =
   | string
   | number
@@ -25,9 +27,11 @@ export class Parser<+T> {
   constructor(f: (JsonObject) => ParseResult<T>) {
     this._f = f;
   }
+
   parse(raw: JsonObject): ParseResult<T> {
     return this._f(raw);
   }
+
   parseOrThrow(raw: JsonObject): T {
     const result = this.parse(raw);
     if (result.ok) {
@@ -201,10 +205,23 @@ export function fmap<T, U>(p: Parser<T>, f: (T) => U): Parser<U> {
   });
 }
 
+// replacer for stringify.JSON, in order to replace empty array with just `[]`.
+//
+// this replacer function can be extended and universally be used with other
+// aspect like empty objects.
+function emptyArrayReplacer(key: string, value: any) {
+  // check if the array is empty.
+  if (Array.isArray(value) && value.length === 0) {
+    return [];
+  }
+
+  return value;
+}
+
 // Create a parser that tries each of the given parsers on the same
 // input, taking the first successful parse or failing if all parsers
 // fail. In the failure case, the provided `errorFn` will be called with
-// the error messages from all the subparsers to form the resulting
+// the error messages from all the sub-parsers to form the resulting
 // error; the default error function includes the full text of all the
 // error messages, but a user-supplied error function may act with
 // domain-specific precision.
@@ -231,7 +248,10 @@ export function fmap<T, U>(p: Parser<T>, f: (T) => U): Parser<U> {
 export function orElse<T: $ReadOnlyArray<Parser<mixed>>>(
   parsers: T,
   errorFn?: (string[]) => string = (errors) =>
-    `no parse matched: ${JSON.stringify(errors)}`
+    `no parse matched: ${stringify(errors, {
+      replacer: emptyArrayReplacer,
+      spacer: 4,
+    })}`
 ): Parser<$ElementType<$TupleMap<T, ExtractParserOutput>, number>> {
   return new Parser((x) => {
     const errors = [];
@@ -284,6 +304,7 @@ export function rename<T>(oldKey: string, parser: Parser<T>): RenameField<T> {
 
 class RenameFieldImpl<+T> extends Parser<T> {
   +oldKey: string;
+
   constructor(oldKey: string, parser: Parser<T>) {
     super(parser._f);
     this.oldKey = oldKey;