has

This is a library which provides generic accessors for records.

-- This example is based on fclabel's one: http://hackage.haskell.org/package/fclabels

import Data.Has

import Control.Arrow

type Person = Name :*: Age :*: Sex :*: Place
type Place  = City :*: Country

newtype Name = Name { unName :: String }
newtype Age  = Age  { unAge  :: Int    }
data    Sex  = Male | Female
newtype City = City { unCity :: String }
newtype Country = Country { unCountry :: String }

nonowarn :: Person
nonowarn =   Name "nonowarn"
         :*: Age 17
         :*: Male
         :*: City "Yokohama"
         :*: Country "Japan"

getAge :: (Has Age a) => a -> Int
getAge = unAge . prj

moveToKyoto :: (Has City a) => a -> a
moveToKyoto = inj (City "Kyoto")

getCity :: (Has City a) => a -> String
getCity = unCity . prj

spendFourYears :: (Has Age a) => a -> a
spendFourYears = upd (Age . (+4) . unAge)

test = (21,"Kyoto") == ((getAge &&& getCity) . spendFourYears . moveToKyoto $ nonowarn)