Age in context

age.hs

{-# LANGUAGE FlexibleContexts #-}
module Main (main) where

import Control.Monad.Reader

type Year = Int

-- a person record has a birth year
data Person = Person { birthYear :: Year }

-- what we're saying here is that this function executes in a context
-- that contains the current year. This seems like overkill but you
-- could imagine a situation where you have lots of bits of context in
-- a larger program. You can focus a call to age just on the important
-- bit of context floating around, the current year. This function
-- *cannot* know about any other context
age :: MonadReader Year m => Person -> m Int
age person = do
  now <- ask
  return $ now - birthYear person

-- made up function, replace with one that gets the clock from the real world
getSystemYear :: IO Year
getSystemYear = return 2014

main :: IO ()
main = do
  let person = Person 1988
  putStrLn "real talk"
  realContext <- getSystemYear
  let testContext = 2000
  let realAge = runReader (age person) realContext
  let testAge = runReader (age person) testContext
  print realAge
  putStrLn "but in test"
  print testAge

-- runhaskell age.hs
-- real talk
-- 26
-- but in test
-- 12