diff --git a/packages/backend/src/prelude/README.md b/packages/backend/src/prelude/README.md
deleted file mode 100644
index bb728cfb1b..0000000000
--- a/packages/backend/src/prelude/README.md
+++ /dev/null
@@ -1,3 +0,0 @@
-# Prelude
-このディレクトリのコードはJavaScriptの表現能力を補うためのコードです。
-Misskey固有の処理とは独立したコードの集まりですが、Misskeyのコードを読みやすくすることを目的としています。
diff --git a/packages/backend/src/prelude/array.ts b/packages/backend/src/prelude/array.ts
index 71a24c89b7..5a206fa396 100644
--- a/packages/backend/src/prelude/array.ts
+++ b/packages/backend/src/prelude/array.ts
@@ -1,4 +1,4 @@
-import type { EndoRelation, Predicate } from "./relation.js";
+import type {Predicate} from "./relation.js";
 
 /**
  * Count the number of elements that satisfy the predicate
@@ -30,21 +30,6 @@ export function intersperse<T>(sep: T, xs: T[]): T[] {
 	return concat(xs.map((x) => [sep, x])).slice(1);
 }
 
-/**
- * Returns the array of elements that is not equal to the element
- */
-export function erase<T>(a: T, xs: T[]): T[] {
-	return xs.filter((x) => x !== a);
-}
-
-/**
- * Finds the array of all elements in the first array not contained in the second array.
- * The order of result values are determined by the first array.
- */
-export function difference<T>(xs: T[], ys: T[]): T[] {
-	return xs.filter((x) => !ys.includes(x));
-}
-
 /**
  * Remove all but the first element from every group of equivalent elements
  */
@@ -60,75 +45,6 @@ export function maximum(xs: number[]): number {
 	return Math.max(...xs);
 }
 
-/**
- * Splits an array based on the equivalence relation.
- * The concatenation of the result is equal to the argument.
- */
-export function groupBy<T>(f: EndoRelation<T>, xs: T[]): T[][] {
-	const groups = [] as T[][];
-	for (const x of xs) {
-		if (groups.length !== 0 && f(groups[groups.length - 1][0], x)) {
-			groups[groups.length - 1].push(x);
-		} else {
-			groups.push([x]);
-		}
-	}
-	return groups;
-}
-
-/**
- * Splits an array based on the equivalence relation induced by the function.
- * The concatenation of the result is equal to the argument.
- */
-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) {
-	return collections.reduce((obj: Record<string, T[]>, item: T) => {
-		const key = keySelector(item);
-		if (!Object.prototype.hasOwnProperty.call(obj, key)) {
-			obj[key] = [];
-		}
-
-		obj[key].push(item);
-
-		return obj;
-	}, {});
-}
-
-/**
- * Compare two arrays by lexicographical order
- */
-export function lessThan(xs: number[], ys: number[]): boolean {
-	for (let i = 0; i < Math.min(xs.length, ys.length); i++) {
-		if (xs[i] < ys[i]) return true;
-		if (xs[i] > ys[i]) return false;
-	}
-	return xs.length < ys.length;
-}
-
-/**
- * Returns the longest prefix of elements that satisfy the predicate
- */
-export function takeWhile<T>(f: Predicate<T>, xs: T[]): T[] {
-	const ys = [];
-	for (const x of xs) {
-		if (f(x)) {
-			ys.push(x);
-		} else {
-			break;
-		}
-	}
-	return ys;
-}
-
-export function cumulativeSum(xs: number[]): number[] {
-	const ys = Array.from(xs); // deep copy
-	for (let i = 1; i < ys.length; i++) ys[i] += ys[i - 1];
-	return ys;
-}
-
 export function toArray<T>(x: T | T[] | undefined): T[] {
 	return Array.isArray(x) ? x : x != null ? [x] : [];
 }
diff --git a/packages/backend/src/prelude/math.ts b/packages/backend/src/prelude/math.ts
deleted file mode 100644
index 07b94bec30..0000000000
--- 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 df7c4ed52a..0000000000
--- 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/relation.ts b/packages/backend/src/prelude/relation.ts
index 1f4703f52f..792aeb7760 100644
--- a/packages/backend/src/prelude/relation.ts
+++ b/packages/backend/src/prelude/relation.ts
@@ -1,5 +1 @@
 export type Predicate<T> = (a: T) => boolean;
-
-export type Relation<T, U> = (a: T, b: U) => boolean;
-
-export type EndoRelation<T> = Relation<T, T>;
diff --git a/packages/backend/src/prelude/string.ts b/packages/backend/src/prelude/string.ts
deleted file mode 100644
index 9588825cb7..0000000000
--- 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();
-}