Add fold function

projects/ngx-loading-customizer/src/lib/functions/fold.function.spec.ts

@@ -0,0 +1,24 @@
+import { errored, idle, loaded, loading } from '../loadable.constructors';
+import { Loadable } from '../loadable.type';
+import { fold } from './fold.function';
+
+describe('the fold function', () => {
+  const constructors = {
+    idle: () => 2,
+    loading: () => 3,
+    loaded: (v: number) => v * 2,
+    error: (_e: unknown) => 0,
+  };
+
+  it.each([
+    [idle, 2],
+    [loading, 3],
+    [loaded(5), 10],
+    [errored('error'), 0],
+  ])(
+    'returns for the loadable %s the value %s',
+    (loadable: Loadable<number>, expectation: number) => {
+      expect(fold(constructors, loadable)).toEqual(expectation);
+    }
+  );
+});

projects/ngx-loading-customizer/src/lib/functions/fold.function.ts

@@ -0,0 +1,22 @@
+import { hasErrored, isIdle, isLoaded } from '../loadable.functions';
+import { Loadable } from '../loadable.type';
+
+export function fold<T, S>(
+  constructors: {
+    idle: () => S;
+    loading: () => S;
+    loaded: (value: T) => S;
+    error: (error: unknown) => S;
+  },
+  loadable: Loadable<T>
+): S {
+  if (isLoaded(loadable)) {
+    return constructors.loaded(loadable.value);
+  } else if (hasErrored(loadable)) {
+    return constructors.error(loadable.error);
+  } else if (isIdle(loadable)) {
+    return constructors.idle();
+  } else {
+    return constructors.loading();
+  }
+}