Some fun with Effects in Purescript !

Random.js

exports.random = Math.random;

Random.purs

module Random
  ( RANDOM
  , RandomEff
  , random
  , randomInt
  , randomIndex
  , randomColor
  ) where

import Control.Monad.Eff (Eff)
import Data.Array (length, (!!))
import Data.Int (floor, toNumber)
import Data.Maybe (Maybe(..), fromMaybe)
import Data.String (fromCharArray)
import Prelude (bind, pure, ($), (*), (-), (<=), (<>), (<$>), (<<<))

foreign import data RANDOM :: !

type RandomEff fx a = Eff (random :: RANDOM | fx) a

foreign import random :: forall fx. RandomEff fx Number

randomInt :: forall fx. Int -> RandomEff fx Int
randomInt n = (floor <<< (*) (toNumber n)) <$> random

randomIndex :: forall fx a. Array a -> RandomEff fx (Maybe a)
randomIndex xs = (!!) xs <$> (randomInt $ (length xs) - 1)

letters :: Array Char
letters = ['A', 'B', 'C', 'D', 'E', 'F', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9']

randomColor :: forall fx. RandomEff fx String
randomColor =
  fromCharArray <$> randomLetters ['#'] 6
  where
    randomLetters acc n
      | n <= 0 = pure acc
      | true   = do
        newLetter <- randomIndex letters
        oldLetters <- randomLetters acc (n - 1)
        pure $ oldLetters <> [ '0' `fromMaybe` newLetter ]