Monads.hs

{-# LANGUAGE OverloadedStrings #-}

module Main where
    
import Control.Monad.Reader

-- a basic IO function
f :: IO String
f = return "string"

-- you can pass IO functions to a function
-- which can be executed later.
-- the whole function has to stay in IO though
g :: IO String -> IO String
g f' = do
  v <- f'
  return (v ++ "-g")

-- a basic pure function that takes a string
-- and returns a string.
-- it can do nothing else
h :: String -> String
h s = s ++ "-h"

-- this is a pure function using the Reader monad
-- it has a string that gets passed around,
-- and the function itself returns a string
i :: Reader String String
i = do
  s <- ask
  return (s ++ "-reader")

-- while we don't do IO in here, we very well could
-- we also get access to `ask` via ReaderT
j :: ReaderT String IO Int
j = do
  s <- ask
  return (length s)

-- k doesn't actually use the Reader value
-- but the function below it does
k :: Int -> ReaderT String IO Int
k a = do
  v <- j
  return (v + a)

main :: IO ()
main = do
  a <- f
  print a

  b <- g f
  print b

  -- note that pure functions don't get executed
  -- the print statement actually evaluates the function
  let c = h "string"
  print c

  -- this is a pure reader, thus the let
  let d = runReader i "string"
  print d

  -- this is a monad transformer with IO at the bottom
  -- therefore the action has to be executed
  e <- runReaderT j "string"
  print e

  z <- runReaderT (k 10) "string"
  print z