diff --git a/src/prelude/array.ts b/src/prelude/array.ts
index 7eee87bf8b..778e5e37c3 100644
--- a/src/prelude/array.ts
+++ b/src/prelude/array.ts
@@ -1,9 +1,10 @@
-import { EndoRelation } from "./relation";
+import { EndoRelation, Predicate } from './relation';
 
 /**
  * Count the number of elements that satisfy the predicate
  */
-export function countIf<T>(f: (x: T) => boolean, xs: T[]): number {
+
+export function countIf<T>(f: Predicate<T>, xs: T[]): number {
 	return xs.filter(f).length;
 }
 
@@ -97,7 +98,7 @@ export function lessThan(xs: number[], ys: number[]): boolean {
 /**
  * Returns the longest prefix of elements that satisfy the predicate
  */
-export function takeWhile<T>(f: (x: T) => boolean, xs: T[]): T[] {
+export function takeWhile<T>(f: Predicate<T>, xs: T[]): T[] {
 	const ys = [];
 	for (const x of xs) {
 		if (f(x)) {
diff --git a/src/prelude/relation.ts b/src/prelude/relation.ts
index d3c3ee378a..b3aedc5ffc 100644
--- a/src/prelude/relation.ts
+++ b/src/prelude/relation.ts
@@ -1,3 +1,5 @@
+export type Predicate<T> = (x: T) => boolean;
+
 export type Relation<T, U> = (x: T, y: U) => boolean;
 
 export type EndoRelation<T> = Relation<T, T>;