combo: convert instance and GitHub configs (#1820)

Summary:
This patch converts the `instanceConfig` and `github/config` parsers
from hand-written traversals to combinator-based structures. The
resulting code is much simpler and easier to read.

Test Plan:
Running the `sc2` CLI for `load` and `graph` still works in the happy
path, with the expected errors if keys in the config files are changed
to the wrong strings.

wchargin-branch: combo-instance-github

src/cli2/instanceConfig.js

@@ -1,5 +1,6 @@
 // @flow
 
+import * as Combo from "../util/combo";
 import {CliPlugin} from "./cliPlugin";
 import {bundledPlugins as getAllBundledPlugins} from "./bundledPlugins";
 
@@ -19,37 +20,25 @@ export type RawInstanceConfig = {|
 // dependencies.
 export type BundledPluginSpec = string;
 
-type JsonObject =
+const parser: Combo.Parser<InstanceConfig> = (() => {
-  | string
+  const C = Combo;
-  | number
+  function makePluginMap(bundledPluginNames) {
-  | boolean
-  | null
-  | JsonObject[]
-  | {[string]: JsonObject};
-
-export function parse(raw: JsonObject): InstanceConfig {
-  if (raw == null || typeof raw !== "object" || Array.isArray(raw)) {
-    throw new Error("bad config: " + JSON.stringify(raw));
-  }
-  const {bundledPlugins: rawBundledPlugins} = raw;
-  if (!Array.isArray(rawBundledPlugins)) {
-    console.warn(JSON.stringify(raw));
-    throw new Error(
-      "bad bundled plugins: " + JSON.stringify(rawBundledPlugins)
-    );
-  }
     const allBundledPlugins = getAllBundledPlugins();
     const bundledPlugins = new Map();
-  for (const name of rawBundledPlugins) {
+    for (const name of bundledPluginNames) {
-    if (typeof name !== "string") {
-      throw new Error("bad bundled plugin: " + JSON.stringify(name));
-    }
       const plugin = allBundledPlugins[name];
       if (plugin == null) {
         throw new Error("bad bundled plugin: " + JSON.stringify(name));
       }
       bundledPlugins.set(name, plugin);
     }
-  const result = {bundledPlugins};
+    return bundledPlugins;
-  return result;
+  }
+  return C.object({
+    bundledPlugins: C.fmap(C.array(C.string), makePluginMap),
+  });
+})();
+
+export function parse(raw: Combo.JsonObject): InstanceConfig {
+  return parser.parseOrThrow(raw);
 }

src/plugins/github/config.js

@@ -1,5 +1,6 @@
 // @flow
 
+import * as Combo from "../../util/combo";
 import {type RepoId, stringToRepoId} from "./repoId";
 
 export type GithubConfig = {|
@@ -20,19 +21,16 @@ type JsonObject =
   | JsonObject[]
   | {[string]: JsonObject};
 
-export function parse(raw: JsonObject): GithubConfig {
+const parser: Combo.Parser<GithubConfig> = (() => {
-  if (raw == null || typeof raw !== "object" || Array.isArray(raw)) {
+  const C = Combo;
-    throw new Error("bad config: " + JSON.stringify(raw));
+  return C.object({
-  }
+    repoIds: C.rename(
-  const {repositories} = raw;
+      "repositories",
-  if (!Array.isArray(repositories)) {
+      C.array(C.fmap(C.string, stringToRepoId))
-    throw new Error("bad repositories: " + JSON.stringify(repositories));
+    ),
-  }
-  const repoIds = repositories.map((x) => {
-    if (typeof x !== "string") {
-      throw new Error("bad repository: " + JSON.stringify(x));
-    }
-    return stringToRepoId(x);
   });
-  return {repoIds};
+})();
+
+export function parse(raw: Combo.JsonObject): GithubConfig {
+  return parser.parseOrThrow(raw);
 }