fp-ts: Support for lazy evaluation of large/infinite lists

🚀 Feature request

Support for working with generators and iterators without casting them to array to force evaluation.

Current Behavior

Currently fp-ts only supports fully evaluated arrays which means limits it’s use to small arrays. It is often desirable to defined huge structures, while not all the results of that structure are needed.

Desired Behavior

import { pipe } from 'fp-ts/lib/pipeable'
import { map, take } from 'fp-ts/lib/Iterator'
import { enumFrom } from 'fp-ts/lib/Enumeration'

const evenNumbers = pipe(
  enumFrom(0),
  map((x) => 2 * x),
);
expect(pipe(
  evenNumbers,
  take(4),
), [0, 2, 4, 6])

Suggested Solution

Implement new Enumeration and Iterator modules.

Who does this impact? Who is this for?

For everyone. Using iterators makes defining things easier.

Describe alternatives you’ve considered

It is possible to work around this by sacrificing generality. For example one could create a function that specifically calculates the first 4 even numbers without relaying on a general definition of even numbers.

Additional context

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Iterators_and_Generators https://docs.python.org/3/library/itertools.html https://www.futurelearn.com/courses/functional-programming-haskell/0/steps/27237 http://zvon.org/other/haskell/Outputprelude/enumFromThenTo_f.html

Your environment

I tend to use latest versions of fp-ts and TypeScript where possible.