Add branch awareness and tests for the type of 'and'

index.d.ts

@@ -83,7 +83,17 @@ 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 Resolved<unknown> ? S : never): (first: F) => S
+  <F extends AnyFuture, S extends AnyFuture>(second: F extends Rejected<unknown> ? S : never): (first: F) => F
+
+  <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>
+  }
+}
 
 /** Logical or for Futures. See https://github.com/fluture-js/Fluture#alt */
 export const alt: {

test/types/and.test-d.ts

@@ -0,0 +1,34 @@
+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));
+
+// Usage with pipe on Future instances (https://git.io/JLx3F).
+expectType<fl.Never> ((fl.never) .pipe (fl.and (fl.never)));
+const workaround = (fl.resolve ('a')) .pipe (fl.and (fl.never)); expectType<fl.Never> (workaround);
+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)));