From a1831f8c53e3d64a661101b7365e83df1de18fa3 Mon Sep 17 00:00:00 2001
From: Patrik Oldsberg <poldsberg@gmail.com>
Date: Sun, 26 Apr 2020 13:16:50 +0200
Subject: [PATCH] packages/core: add helper for inferring ApiFactory types

---
 packages/core/src/api/apis/helpers.ts | 26 ++++++++++++++++++++++++++
 packages/core/src/api/apis/index.ts   |  1 +
 packages/core/src/api/apis/types.ts   |  8 ++++----
 3 files changed, 31 insertions(+), 4 deletions(-)
 create mode 100644 packages/core/src/api/apis/helpers.ts

diff --git a/packages/core/src/api/apis/helpers.ts b/packages/core/src/api/apis/helpers.ts
new file mode 100644
index 0000000000..1a46781c58
--- /dev/null
+++ b/packages/core/src/api/apis/helpers.ts
@@ -0,0 +1,26 @@
+/*
+ * Copyright 2020 Spotify AB
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { ApiFactory } from './types';
+
+// Used to infer types for a standalone ApiFactory that isn't immediately passed
+// to another function.
+// This function doesn't actually do anything, it's only used to infer types.
+export function createApiFactory<Api, Impl, Deps>(
+  factory: ApiFactory<Api, Impl, Deps>,
+): ApiFactory<Api, Impl, Deps> {
+  return factory;
+}
diff --git a/packages/core/src/api/apis/index.ts b/packages/core/src/api/apis/index.ts
index 8dd982264d..9ab9a25e50 100644
--- a/packages/core/src/api/apis/index.ts
+++ b/packages/core/src/api/apis/index.ts
@@ -19,5 +19,6 @@ export { default as ApiRegistry } from './ApiRegistry';
 export { default as ApiTestRegistry } from './ApiTestRegistry';
 export { default as ApiRef } from './ApiRef';
 export * from './types';
+export * from './helpers';
 export * from './definitions';
 export * from './implementations';
diff --git a/packages/core/src/api/apis/types.ts b/packages/core/src/api/apis/types.ts
index 129afdaaa5..c44fd804a5 100644
--- a/packages/core/src/api/apis/types.ts
+++ b/packages/core/src/api/apis/types.ts
@@ -30,8 +30,8 @@ export type ApiHolder = {
   get<T>(api: ApiRef<T>): T | undefined;
 };
 
-export type ApiFactory<A, I, D> = {
-  implements: ApiRef<A>;
-  deps: TypesToApiRefs<D>;
-  factory(deps: D): I extends A ? I : never;
+export type ApiFactory<Api, Impl, Deps> = {
+  implements: ApiRef<Api>;
+  deps: TypesToApiRefs<Deps>;
+  factory(deps: Deps): Impl extends Api ? Impl : never;
 };