WIP: and

Author: Avaq

Diff for: index.d.ts

@@ -83,7 +83,24 @@ export interface FutureInstance<L, R> extends Functor<R> {
 export function after(duration: number): <R>(value: R) => Resolved<R>
 
 /** Logical and for Futures. See https://github.com/fluture-js/Fluture#and */
-export function and<L, R>(left: FutureInstance<L, R>): (right: FutureInstance<L, any>) => FutureInstance<L, R>
+export const and: {
+  <F extends AnyFuture, S extends AnyFuture>(second: F extends Never ? S : never): (first: F) => Never
+  <F extends AnyFuture, S extends AnyFuture>(second: F extends Rejected<unknown> ? S : never): (first: F) => F
+  <F extends AnyFuture, S extends AnyFuture>(second: F extends Resolved<unknown> ? S : never): (first: F) => S
+  <F extends AnyFuture, S extends AnyFuture>(second: F extends Uncertain<infer L, unknown> ? S extends Uncertain<L, unknown> ? S : never : never): (first: F) => S
+
+  // <L, R>(second: Uncertain<L, R>): {
+  //   <T>(first: Rejected<T>): Rejected<T>
+  //   (first: Resolved<any>): Uncertain<L, R>
+  //   (first: Uncertain<L, any>): Uncertain<L, R>
+  // }
+
+  // <R>(second: Resolved<R>): {
+  //   <L>(first: Rejected<L>): Rejected<L>
+  //   (first: Resolved<any>): Resolved<R>
+  //   <L>(first: Uncertain<L, any>): Uncertain<L, R>
+  // }
+}
 
 /** Logical or for Futures. See https://github.com/fluture-js/Fluture#alt */
 export const alt: {

Diff for: test/types/and.test-d.ts

@@ -0,0 +1,37 @@
+import {expectType, expectError} from 'tsd';
+
+import * as fl from '../../index.js';
+
+const fsn: fl.Uncertain<string, number> = fl.resolve (42);
+const fns: fl.Uncertain<number, string> = fl.resolve ('a');
+
+// Standard usage on Future instances.
+// expectType<fl.Never> (fl.and (fl.never) (fl.never));
+// expectType<fl.Never> (fl.and (fl.never) (fl.resolve ('a')));
+// expectType<fl.Never> (fl.and (fl.reject ('a')) (fl.never));
+// expectType<fl.Never> (fl.and (fl.resolve ('a')) (fl.never));
+// expectType<fl.Rejected<string>> (fl.and (fl.reject ('a')) (fl.resolve (42)));
+// expectType<fl.Resolved<number>> (fl.and (fl.resolve (42)) (fl.resolve (42)));
+// expectType<fl.Rejected<number>> (fl.and (fl.reject (42)) (fl.reject (42)));
+// expectType<fl.Uncertain<string, number>> (fl.and (fsn) (fsn));
+// expectType<fl.Rejected<string>> (fl.and (fl.never) (fl.reject ('a')));
+// expectType<fl.Rejected<string>> (fl.and (fl.resolve (42)) (fl.reject ('a')));
+// expectType<fl.Rejected<number>> (fl.and (fl.reject ('a')) (fl.reject (42)));
+// expectError (fl.and (fsn) (fns));
+
+const x = fl.resolve ('').pipe(fl.and (fl.never))
+type T = typeof x
+
+// Usage with pipe on Future instances (https://git.io/JLx3F).
+expectType<fl.Never> ((fl.never) .pipe (fl.and (fl.never)));
+expectType<fl.Never> ((fl.resolve ('a')) .pipe (fl.and (fl.never)));
+expectType<fl.Never> ((fl.never) .pipe (fl.and (fl.reject ('a'))));
+expectType<fl.Never> ((fl.never) .pipe (fl.and (fl.resolve ('a'))));
+expectType<fl.Rejected<string>> ((fl.resolve (42)) .pipe (fl.and (fl.reject ('a'))));
+expectType<fl.Resolved<number>> ((fl.resolve (42)) .pipe (fl.and (fl.resolve (42))));
+expectType<fl.Rejected<number>> ((fl.reject (42)) .pipe (fl.and (fl.reject (42))));
+expectType<fl.Uncertain<string, number>> ((fsn) .pipe (fl.and (fsn)));
+expectType<fl.Rejected<string>> ((fl.reject ('a')) .pipe (fl.and (fl.never)));
+expectType<fl.Rejected<string>> ((fl.reject ('a')) .pipe (fl.and (fl.resolve (42))));
+expectType<fl.Rejected<number>> ((fl.reject (42)) .pipe (fl.and (fl.reject ('a'))));
+expectError ((fns) .pipe (fl.and (fsn)));