From e0a5118f8d97523c60ef424723e0d34d9d6adb93 Mon Sep 17 00:00:00 2001
From: William Chargin <wchargin@gmail.com>
Date: Mon, 23 Apr 2018 13:12:44 -0700
Subject: [PATCH] Switch typed dispatch table to `empty` assertion (#133)
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Summary:
This replaces the implementation of a static check from a somewhat
complicated use of higher-order types to a more simple empty-union
assertion, as suggested by jez in “Case Exhaustiveness in Flow”:
https://blog.jez.io/flow-exhaustiveness/

(I know; we’re not using Reason. One step at a time. :-) )

I adapted the implementation a bit because I prefer explicitly disabling
an ESLint warning over a no-op function call; it is not clear from the
latter that the purpose is to suppress a lint warning.

Test Plan:
In `githubPlugin.js`, add `| "ANOTHER"` to the `NodeType` type, and note
a compile-time Flow error on the appropriate line, with a very readable
error message. Note that all unit tests pass, and running the UI on
`sourcecred/sourcecred` yields correct titles for each node type present
(namely, all node types except for `ORGANIZATION` and `BOT`).

wchargin-branch: empty-union-assertion
---
 .../editor/adapters/githubPluginAdapter.js    | 40 +++++++++----------
 1 file changed, 20 insertions(+), 20 deletions(-)

diff --git a/src/plugins/artifact/editor/adapters/githubPluginAdapter.js b/src/plugins/artifact/editor/adapters/githubPluginAdapter.js
index 201da21..b76d8a1 100644
--- a/src/plugins/artifact/editor/adapters/githubPluginAdapter.js
+++ b/src/plugins/artifact/editor/adapters/githubPluginAdapter.js
@@ -7,7 +7,6 @@ import type {Node} from "../../../../core/graph";
 import type {
   NodePayload,
   NodeType,
-  NodeTypes,
   IssueNodePayload,
   PullRequestNodePayload,
   CommentNodePayload,
@@ -79,26 +78,27 @@ const adapter: PluginAdapter<NodePayload> = {
     function extractAuthorTitle(node: Node<AuthorNodePayload>) {
       return node.payload.login;
     }
-    type TypedNodeToStringExtractor = <T: $Values<NodeTypes>>(
-      T
-    ) => (node: Node<$ElementType<T, "payload">>) => string;
-    const extractors: $Exact<$ObjMap<NodeTypes, TypedNodeToStringExtractor>> = {
-      ISSUE: extractIssueOrPrTitle,
-      PULL_REQUEST: extractIssueOrPrTitle,
-      COMMENT: (node) => extractCommentTitle("comment", node),
-      PULL_REQUEST_REVIEW_COMMENT: (node) =>
-        extractCommentTitle("review comment", node),
-      PULL_REQUEST_REVIEW: extractPRReviewTitle,
-      USER: extractAuthorTitle,
-      ORGANIZATION: extractAuthorTitle,
-      BOT: extractAuthorTitle,
-    };
-    function fallbackAccessor(node: Node<NodePayload>) {
-      throw new Error(`unknown node type: ${node.address.type}`);
+    const anyNode: Node<any> = node;
+    const type: NodeType = (node.address.type: any);
+    switch (type) {
+      case "ISSUE":
+      case "PULL_REQUEST":
+        return extractIssueOrPrTitle(anyNode);
+      case "COMMENT":
+        return extractCommentTitle("comment", anyNode);
+      case "PULL_REQUEST_REVIEW_COMMENT":
+        return extractCommentTitle("review comment", anyNode);
+      case "PULL_REQUEST_REVIEW":
+        return extractPRReviewTitle(anyNode);
+      case "USER":
+      case "ORGANIZATION":
+      case "BOT":
+        return extractAuthorTitle(anyNode);
+      default:
+        // eslint-disable-next-line no-unused-expressions
+        (type: empty);
+        throw new Error(`unknown node type: ${node.address.type}`);
     }
-    return (extractors[node.address.type] || fallbackAccessor)(
-      (node: Node<any>)
-    );
   },
 };