Add initial ActionState type

Author: mbg

src/action-common.ts

@@ -0,0 +1,37 @@
+import { Env } from "./environment";
+import { FeatureEnablement } from "./feature-flags";
+import { Logger } from "./logging";
+
+/** Describes different state features that an Action may have. */
+export interface FeatureState {
+  Logger: {
+    /** The logger that is in use. */
+    logger: Logger;
+  };
+  Env: {
+    /** Information about environment variables. */
+    env: Env;
+  };
+  FeatureFlags: {
+    /** Information about enabled feature flags. */
+    features: FeatureEnablement;
+  };
+}
+
+/** Identifies a type of state an Action may have. */
+export type StateFeature = keyof FeatureState;
+
+/** Constructs the union of all state types identifies by `Fs`. */
+export type FieldsOf<Fs extends readonly StateFeature[]> = Fs extends [
+  infer Head extends StateFeature,
+]
+  ? FeatureState[Head]
+  : Fs extends [
+        infer Head extends StateFeature,
+        ...infer Tail extends StateFeature[],
+      ]
+    ? FeatureState[Head] & FieldsOf<Tail>
+    : never;
+
+/** Describes the state of an Action that has access to the state corresponding to `Fs`. */
+export type ActionState<Fs extends readonly StateFeature[]> = FieldsOf<Fs>;