Example of the State and Either Monad used in Purescript

README.md

http://try.purescript.org/?gist=8edc7ff9f3f3bcd347959e9cd2f07376

Main.purs

-- http://try.purescript.org/?gist=8edc7ff9f3f3bcd347959e9cd2f07376
module Main where

import Prelude
import Data.Identity
import Control.Monad.State
import Control.Monad.Except
import Control.Monad.Eff.Console (log, logShow)

type FirstName = String
type LastName = String
type RequirementExplaination = String

type Eligibility = StateT Person (ExceptT String Identity)

data Person = PB {
  firstname :: FirstName,
  lastname :: LastName,
  age :: Int
}

data Requirement
  = AgeAbove Int RequirementExplaination
  | AgeBelow Int RequirementExplaination

instance showPerson :: Show Person where
  show (PB p) = p.firstname <> " " <> p.lastname <> ", age:" <> (show p.age)

defaultPerson :: Person
defaultPerson = PB
  { firstname : "Ted"
  , lastname : "Johnson"
  , age : 65 }
  
person :: FirstName -> LastName -> Int -> Person
person f l a = PB { firstname : f, lastname : l, age : a }

ineligableMessage :: Requirement -> String
ineligableMessage (AgeAbove i m) = "Person is ineligable because their age is below "
                                   <> (show i) <> ". " <> m
ineligableMessage (AgeBelow i m) = "Person is ineligable because their age is above "
                                   <> (show i) <> ". " <> m


meetsRequirement :: Requirement -> Person -> Boolean
meetsRequirement (AgeAbove i m) (PB person) = person.age >= i
meetsRequirement (AgeBelow i m) (PB person) = person.age <= i

eligableWhen :: Requirement -> Eligibility Person
eligableWhen req = do
  pers@(PB person) <- get
  case (meetsRequirement req pers) of
    true -> do
      pure pers
    false -> do
      lift $ throwError (ineligableMessage req)


runEligability p s = runIdentity $ runExceptT $ runStateT p s

main = do
  logShow $
    runEligability (do
      eligableWhen (AgeAbove 63 "The state requires this.")
      eligableWhen (AgeBelow 65 "The state requires this.")
    ) defaultPerson

This shows an error: "Unknown value runIdentity"