diff --git a/prelude-ls/prelude-ls-tests.ts b/prelude-ls/prelude-ls-tests.ts
index c04fb2277..a92c3c424 100644
--- a/prelude-ls/prelude-ls-tests.ts
+++ b/prelude-ls/prelude-ls-tests.ts
@@ -162,23 +162,25 @@ var table: Array<{
    name: "donald"
 }];
 prelude.sortBy(x => x.name, table);
-//=> [{"id": 3, "name": "donald"}, {"id": 1, "name": "george"}, {"id": 2, "name": "mike"}]
+//=> [{"id": 3, "name": "donald"},
+//    {"id": 1, "name": "george"},
+//    {"id": 2, "name": "mike"}]
 
-prelude.sum([1, 2, 3, 4, 5]); //=> 15
+var sumRes: number = prelude.sum([1, 2, 3, 4, 5]); //=> 15
 prelude.sum([]);              //=> 0
 
-prelude.product([1, 2, 3]); //=> 6
+var productRes: number = prelude.product([1, 2, 3]); //=> 6
 prelude.product([]);        //=> 1
 
-prelude.mean([1, 2, 3, 4, 5]); //=> 3
+var meanRes: number = prelude.mean([1, 2, 3, 4, 5]); //=> 3
 
-prelude.maximum([4, 1, 9, 3]); //=> 9
+var maximumRes: number = prelude.maximum([4, 1, 9, 3]); //=> 9
 
-prelude.minimum(["c", "e", "a", "d", "b"]); //=> "a"
+var minimumRes: string = prelude.minimum(["c", "e", "a", "d", "b"]); //=> "a"
 
-prelude.maximumBy(x => x.length, ["hi", "there", "I", "am", "looooong"]); //=> "looooong"
-
-prelude.maximumBy(x => x.length, ["hi", "there", "I", "am", "looooong"]); //=> "looooong"
+var maximumByRes: string =
+  prelude.maximumBy(x => x.length, ["hi", "there", "I", "am", "looooong"]);
+  //=> "looooong"
 
 prelude.scan(x => y => x + y, 0, [1, 2, 3]); //=> [0, 1, 3, 6]
 
diff --git a/prelude-ls/prelude-ls.d.ts b/prelude-ls/prelude-ls.d.ts
index 26c1890d4..053ed53a4 100644
--- a/prelude-ls/prelude-ls.d.ts
+++ b/prelude-ls/prelude-ls.d.ts
@@ -77,9 +77,9 @@ declare module "prelude-ls" {
 		export function sortWith<A>(f: (x: A) => (y: A) => number, xs: A[]): A[];
 		export function sortBy<A, B>(f: (x: A) => B): (xs: A[]) => A[];
 		export function sortBy<A, B>(f: (x: A) => B, xs: A[]): A[];
-		export function sum(xs: number[]): number[];
-		export function product(xs: number[]): number[];
-		export function mean(xs: number[]): number[];
+		export function sum(xs: number[]): number;
+		export function product(xs: number[]): number;
+		export function mean(xs: number[]): number;
 		export function maximum<A>(xs: A[]): A;
 		export function minimum<A>(xs: A[]): A;
 		export function maximumBy<A, B>(f: (x: A) => B): (xs: A[]) => A;