diff --git a/packages/backend/src/prelude/array.ts b/packages/backend/src/prelude/array.ts
index 0b2830cb7..4e2088e72 100644
--- a/packages/backend/src/prelude/array.ts
+++ b/packages/backend/src/prelude/array.ts
@@ -84,7 +84,7 @@ export function groupOn<T, S>(f: (x: T) => S, xs: T[]): T[][] {
 	return groupBy((a, b) => f(a) === f(b), xs);
 }
 
-export function groupByX<T>(collections: T[], keySelector: (x: T) => string) {
+export function groupByX<T>(collections: T[], keySelector: (x: T) => string): Record<string, T[]> {
 	return collections.reduce((obj: Record<string, T[]>, item: T) => {
 		const key = keySelector(item);
 		if (!Object.prototype.hasOwnProperty.call(obj, key)) {
diff --git a/packages/backend/src/prelude/math.ts b/packages/backend/src/prelude/math.ts
deleted file mode 100644
index 07b94bec3..000000000
--- a/packages/backend/src/prelude/math.ts
+++ /dev/null
@@ -1,3 +0,0 @@
-export function gcd(a: number, b: number): number {
-	return b === 0 ? a : gcd(b, a % b);
-}
diff --git a/packages/backend/src/prelude/maybe.ts b/packages/backend/src/prelude/maybe.ts
deleted file mode 100644
index df7c4ed52..000000000
--- a/packages/backend/src/prelude/maybe.ts
+++ /dev/null
@@ -1,20 +0,0 @@
-export interface IMaybe<T> {
-	isJust(): this is IJust<T>;
-}
-
-export interface IJust<T> extends IMaybe<T> {
-	get(): T;
-}
-
-export function just<T>(value: T): IJust<T> {
-	return {
-		isJust: () => true,
-		get: () => value,
-	};
-}
-
-export function nothing<T>(): IMaybe<T> {
-	return {
-		isJust: () => false,
-	};
-}
diff --git a/packages/backend/src/prelude/string.ts b/packages/backend/src/prelude/string.ts
deleted file mode 100644
index b907e0a2e..000000000
--- a/packages/backend/src/prelude/string.ts
+++ /dev/null
@@ -1,15 +0,0 @@
-export function concat(xs: string[]): string {
-	return xs.join('');
-}
-
-export function capitalize(s: string): string {
-	return toUpperCase(s.charAt(0)) + toLowerCase(s.slice(1));
-}
-
-export function toUpperCase(s: string): string {
-	return s.toUpperCase();
-}
-
-export function toLowerCase(s: string): string {
-	return s.toLowerCase();
-}
diff --git a/packages/backend/test/prelude/maybe.ts b/packages/backend/test/prelude/maybe.ts
deleted file mode 100644
index 0f4b00065..000000000
--- a/packages/backend/test/prelude/maybe.ts
+++ /dev/null
@@ -1,18 +0,0 @@
-import * as assert from 'assert';
-import { just, nothing } from '../../src/prelude/maybe.js';
-
-describe('just', () => {
-	it('has a value', () => {
-		assert.deepStrictEqual(just(3).isJust(), true);
-	});
-
-	it('has the inverse called get', () => {
-		assert.deepStrictEqual(just(3).get(), 3);
-	});
-});
-
-describe('nothing', () => {
-	it('has no value', () => {
-		assert.deepStrictEqual(nothing().isJust(), false);
-	});
-});