Summation (i.e., ∑)

summate.py

from functools import reduce

def summate(startingAt, endingWith, callback):
	"""
	Returns the sum of the result values of `callback` starting at `startingAt` and ending with `endingWith`.
	Example: sigma(1, 6, lambda n: 3 * n)
	See: https://replit.com/@jedmao/SuperSigma#main.py
	"""
	return sum([callback(x) for x in range(startingAt - 1, endingWith + 1)])

summate.ts

/**
 * @returns the sum of the result values of `callback` starting at `startingAt` and ending with `endingWith`.
 * @example sigma(1, 6, (n) => 3 * n)
 * @see https://replit.com/@jedmao/SuperSigmaTS#index.ts
 */
export function summate(
  startingAt: number,
  endingWith: number,
  callback: (i: number) => number
) {
  return [...range(startingAt, endingWith)].reduce(
    (accumulator, i) => accumulator + callback(i)
  )
}

function* range(start: number, end: number, step = 1) {
  for (let i = start - 1; i <= end; i += step) {
    yield i;
  }
}