copy from the Haskell code in the "Mathematics for Working Programmer class - Day 3-4"

Maximum Sum Segment problem - Naive Solution.ts

// MSS - Maximum Sum Segment problem
// MSS = maximum . map sum . concat . map inits . tails
// Playground - https://tsplay.dev/mpzE6w
function tail<T>(xs: T[]): T[] {
  return xs.slice(1)
}

function init<T>(xs: T[]): T[] {
  return xs.slice(0, -1)
}

function tails<T>([x, ...xs]: T[]): T[][] {
  if (x === undefined && xs.length === 0) return [[]]
  return [[x, ...xs], ...tails(xs)]
}

function inits<T>([x, ...xs]: T[]): T[][] {
  if (x === undefined) return [[]]
  if (xs.length === 0) return [[], [x]]
  return [...inits([x, ...init(xs)]), [x, ...xs]]
}

function maximum([x, ...xs]: number[]): number {
  if (x === undefined) throw Error('List has no member')
  if (xs.length === 0) return x
  return Math.max(x, maximum(xs))
}

function map<T, R>(f: (x: T) => R): (xs: T[]) => R[] {
  return function forXs(xs) {
    return xs.map(f)
  }
}

// https://zvon.org/other/haskell/Outputprelude/concat_f.html
function concat<T>(xss: T[][]): T[] {
  return xss.flat()
}

function sum([x, ...xs]: number[]): number {
  if (x === undefined) return 0
  if (xs.length === 0) return x
  return x + sum(xs)
}

const input: number[] = [1, 2, -1, 0, 3, -1]
let it
console.log('tails input', it = tails(input))
console.log('map inits it', it = map(inits)(it))
console.log('concat it', it = concat(it))
console.log('map sum it', it = map(sum)(it))
console.log('maximum it', maximum(it))

const mss = compose(maximum, map(sum), concat, map(inits), tails)
console.log('mss = (maximum . map sum . concat . map inits . tails) input', mss(input))