Kleisli Composition

KleisliComposition.hs

module KleisliComposition where

import           Data.Map (Map)
import qualified Data.Map as M
import Control.Monad ((<=<))

type UserName = String
type GamerID = Int
type PlayerCredits = Int

users :: [UserName]
users = ["nYarlathoTep", "KINGinYELLOW", "dagon1997", "rcarter1919", "xCTHULHUx", "yogSOThoth"]

ids :: [GamerID]
ids = [1..6]

credits :: [PlayerCredits]
credits = [2000, 15000, 300, 12, 50000, 150000]

userNameDB :: Map GamerID UserName
userNameDB = M.fromList $ zip ids users

creditsDB :: Map UserName PlayerCredits
creditsDB = M.fromList $ zip users credits

userFromID :: GamerID -> Maybe UserName
userFromID = flip M.lookup userNameDB -- notice use of flip so we can partially apply the DB

creditsFromUser :: UserName -> Maybe PlayerCredits
creditsFromUser = flip M.lookup creditsDB -- notice use of flip so we can partially apply the DB

creditsFromId :: GamerID -> Maybe PlayerCredits
creditsFromId = creditsFromUser <=< userFromID -- Kleisli composition; fancy name for composing monad factories