refs #12 add Future.lift but for now with the pre-TS3.0 api which sup…

…ports only up to 5 parameters.

src/Future.ts

@@ -294,6 +294,22 @@ export class Future<T> {
         return (p1,p2) => p1.flatMap(a1 => p2.map(a2 => fn(a1,a2)));
     }
 
+    /**
+     * Take a function returning a Promise
+     * and lift it to return a [[Future]] instead.
+     */
+    static lift<U>(fn:()=>Promise<U>): ()=>Future<U>
+    static lift<T1,U>(fn:(x:T1)=>Promise<U>): (x:T1)=>Future<U>
+    static lift<T1,T2,U>(fn:(x:T1,y:T2)=>Promise<U>): (x:T1,y:T2)=>Future<U>
+    static lift<T1,T2,T3,U>(fn:(x:T1,y:T2,z:T3)=>Promise<U>): (x:T1,y:T2,z:T3)=>Future<U>
+    static lift<T1,T2,T3,T4,U>(fn:(x:T1,y:T2,z:T3,z1:T4)=>Promise<U>): (x:T1,y:T2,z:T3,z1:T4)=>Future<U>
+    static lift<T1,T2,T3,T4,T5,U>(fn:(x:T1,y:T2,z:T3,z1:T4,z2:T5)=>Promise<U>): (x:T1,y:T2,z:T3,z1:T4,z2:T5)=>Future<U>
+    static lift<L,U>(fn:any): any {
+        return (...args:any[]) => {
+            return Future.of(fn(...args));
+        };
+    }
+
     /**
      * Transform the value contained in a successful Future. Has no effect
      * if the Future was failed. Will turn a successful Future in a failed

tests/Future.ts

@@ -150,6 +150,24 @@ describe("Future.liftA*", () => {
         return ensureFailedWithValue("sorry", computationPromise.toPromise());
     });
 });
+describe("Future.lift", () => {
+    it("lifts a simple promise", async () => {
+        const fn = (x:number) => Promise.resolve(x+1);
+        const fn2 = Future.lift(fn);
+        assert.equal(5, await fn2(4));
+    });
+    it("lifts a failing promise", async () => {
+        const fn = (x:number) => Promise.reject(x+1);
+        const fn2 = Future.lift(fn);
+        try {
+            await fn2(4);
+            assert.ok(false);
+        } catch (ex) {
+            assert.equal(5, ex);
+        }
+
+    });
+});
 describe("Future.traverse", () => {
     it("traverses properly", async () => {
         assert.deepEqual([1,2,3], await Future.traverse([1,2,3], Future.ok).map(v => v.toArray()));