ReadonlyArray

How-to

Problem

How can I go from ReadonlyNonEmptyArray<A> to Array<A>?

Solution

import * as RA from 'fp-ts/ReadonlyArray'

const solution: <A>(as: ReadonlyArray<A>) => Array<A> = RA.toArray

Problem

I'm trying to generate every possible object, from a list of possible keys

const COLORS = ['red', 'blue'] as const;
const THINGS = ['ball', 'box'] as const;

interface ColoredThing {
  color: typeof COLORS[number];
  thing: typeof THINGS[number];
}

const ALL_POSSIBLE_COLORED_THINGS = ?

Solution

import * as RA from 'fp-ts/ReadonlyArray'
import { pipe } from 'fp-ts/function'

const COLORS = ['red', 'blue'] as const
const THINGS = ['ball', 'box'] as const

interface ColoredThing {
  color: typeof COLORS[number]
  thing: typeof THINGS[number]
}

const ALL_POSSIBLE_COLORED_THINGS: ReadonlyArray<ColoredThing> = pipe(
  RA.Do,
  RA.apS('color', COLORS),
  RA.apS('thing', THINGS)
)

Alternatively you can use "array comprehension"

const ALL_POSSIBLE_COLORED_THINGS = RA.comprehension(
  [COLORS, THINGS],
  (color, thing) => ({
    color,
    thing
  })
)