From 2ee438dece36fd039dcfee291ffcc97aacd78edf Mon Sep 17 00:00:00 2001
From: Aya Morisawa <AyaMorisawa4869@gmail.com>
Date: Wed, 19 Dec 2018 09:54:45 +0900
Subject: [PATCH] Add comments for prelude/array.ts

---
 src/prelude/array.ts | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/src/prelude/array.ts b/src/prelude/array.ts
index aefc7b3a5..00f0dc344 100644
--- a/src/prelude/array.ts
+++ b/src/prelude/array.ts
@@ -1,19 +1,35 @@
+/**
+ * Count the number of elements that satisfy the predicate
+ */
 export function countIf<T>(f: (x: T) => boolean, xs: T[]): number {
 	return xs.filter(f).length;
 }
 
+/**
+ * Count the number of elements that is equal to the element
+ */
 export function count<T>(x: T, xs: T[]): number {
 	return countIf(y => x === y, xs);
 }
 
+/**
+ * Concatenate an array of arrays
+ */
 export function concat<T>(xss: T[][]): T[] {
 	return ([] as T[]).concat(...xss);
 }
 
+/**
+ * Intersperse the element between the elements of the array
+ * @param sep The element to be interspersed
+ */
 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>(x: T, xs: T[]): T[] {
 	return xs.filter(y => x !== y);
 }
@@ -26,6 +42,9 @@ 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
+ */
 export function unique<T>(xs: T[]): T[] {
 	return [...new Set(xs)];
 }
@@ -38,6 +57,10 @@ export function maximum(xs: number[]): number {
 	return Math.max(...xs);
 }
 
+/**
+ * Splits an array based on the equivalence relation.
+ * The concatenation of the result equals to the argument.
+ */
 export function groupBy<T>(f: (x: T, y: T) => boolean, xs: T[]): T[][] {
 	const groups = [] as T[][];
 	for (const x of xs) {
@@ -50,10 +73,17 @@ export function groupBy<T>(f: (x: T, y: T) => boolean, xs: T[]): T[][] {
 	return groups;
 }
 
+/**
+ * Splits an array based on the equivalence relation induced by the function.
+ * The concatenation of the result equals to the argument.
+ */
 export function groupOn<T, S>(f: (x: T) => S, xs: T[]): T[][] {
 	return groupBy((a, b) => f(a) === f(b), xs);
 }
 
+/**
+ * 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;
@@ -62,6 +92,9 @@ export function lessThan(xs: number[], ys: number[]): boolean {
 	return xs.length < ys.length;
 }
 
+/**
+ * Returns the longest prefix of elements that satisfy the predicate
+ */
 export function takeWhile<T>(f: (x: T) => boolean, xs: T[]): T[] {
 	const ys = [];
 	for (const x of xs) {