Add `RelationalView.match` (#438)

`match` implements pattern matching over `Entity`

Test plan:
Unit tests included.

src/v3/plugins/github/relationalView.js

@@ -690,6 +690,36 @@ function* getAuthors(
   }
 }
 
+export type MatchHandlers<T> = {|
+  +repo: (x: Repo) => T,
+  +issue: (x: Issue) => T,
+  +pull: (x: Pull) => T,
+  +review: (x: Review) => T,
+  +comment: (x: Comment) => T,
+  +userlike: (x: Userlike) => T,
+|};
+export function match<T>(handlers: MatchHandlers<T>, x: Entity): T {
+  if (x instanceof Repo) {
+    return handlers.repo(x);
+  }
+  if (x instanceof Issue) {
+    return handlers.issue(x);
+  }
+  if (x instanceof Pull) {
+    return handlers.pull(x);
+  }
+  if (x instanceof Review) {
+    return handlers.review(x);
+  }
+  if (x instanceof Comment) {
+    return handlers.comment(x);
+  }
+  if (x instanceof Userlike) {
+    return handlers.userlike(x);
+  }
+  throw new Error(`Unexpected entity ${x}`);
+}
+
 export type Entity = Repo | Issue | Pull | Review | Comment | Userlike;
 export type AuthoredEntity = Issue | Pull | Review | Comment;
 export type TextContentEntity = Issue | Pull | Review | Comment;

src/v3/plugins/github/relationalView.test.js

@@ -138,6 +138,25 @@ describe("plugins/github/relationalView", () => {
     has("url", () => entity.url());
   });
 
+  describe("match", () => {
+    const handlers = {
+      // Return the address so we know it was actually called on the entity,
+      // and a hardcoded string so we know the right function was called.
+      repo: (x: R.Repo) => [x.address(), "REPO"],
+      issue: (x: R.Issue) => [x.address(), "ISSUE"],
+      pull: (x: R.Pull) => [x.address(), "PULL"],
+      review: (x: R.Review) => [x.address(), "REVIEW"],
+      comment: (x: R.Comment) => [x.address(), "COMMENT"],
+      userlike: (x: R.Userlike) => [x.address(), "USERLIKE"],
+    };
+
+    const instances = [repo, issue, pull, review, comment, userlike];
+    for (const instance of instances) {
+      const [actualAddress, functionType] = R.match(handlers, instance);
+      expect(actualAddress.type).toEqual(functionType);
+    }
+  });
+
   describe("comment parent differentiation", () => {
     function hasCorrectParent(name, parent) {
       it(name, () => {