1 files changed, 73 insertions, 0 deletions
diff --git a/frontend/ts/src/lib/loadable.ts b/frontend/ts/src/lib/loadable.ts
new file mode 100644
index 0000000..919e0ba
--- /dev/null
+++ b/frontend/ts/src/lib/loadable.ts
@@ -0,0 +1,73 @@
+export type Loadable<T> = Init | Loading | Loaded<T> | Failure
+
+type Init = {
+    key: 'INIT'
+}
+
+export const init: Init = { key: 'INIT' }
+
+export function isInit(l: Loadable<any>): l is Init {
+    return l.key == 'INIT'
+}
+
+type Loading = {
+    key: 'LOADING'
+}
+
+export const loading: Loading = { key: 'LOADING' }
+
+export function isLoading(l: Loadable<any>): l is Loading {
+    return l.key == 'LOADING'
+}
+
+type Loaded<T> = {
+    key: 'LOADED',
+    value: T
+}
+
+export function loaded<T>(value: T): Loaded<T> {
+    return { key: 'LOADED', value }
+}
+
+export function isLoaded<T>(l: Loadable<T>): l is Loaded<T> {
+    return l.key == 'LOADED'
+}
+
+type Failure = {
+    key: 'FAILURE',
+    error: string
+}
+
+export function failure(error: string): Failure {
+    return {
+        key: 'FAILURE',
+        error
+    }
+}
+
+export function isFailure(l: Loadable<any>): l is Failure {
+    return l.key == 'FAILURE'
+}
+
+export function map<A, B>(loadable: Loadable<A>, f: (value: A) => B): Loadable<B> {
+    if (loadable.key == 'LOADED') {
+        return loaded(f(loadable.value))
+    } else {
+        return loadable
+    }
+}
+
+export function map2<A, B, C>(l: [Loadable<A>, Loadable<B>], f: (a: A, b: B) => C): Loadable<C> {
+    const [l1, l2] = l
+    if (l1.key == 'FAILURE') {
+        return l1
+    } else if (l2.key == 'FAILURE') {
+        return l2
+    } else if (l1.key == 'LOADING' || l2.key == 'LOADING') {
+        return loading
+    } else if (l1.key == 'INIT' || l2.key == 'INIT') {
+        return init
+    } else {
+        return loaded(f(l1.value, l2.value))
+    }
+}