Haskell State Exercises

rand.hs

-- exercices for
-- https://en.wikibooks.org/wiki/Haskell/Understanding_monads/State

import Control.Monad
import Control.Monad.Trans.State
import System.Random

rollDie :: State StdGen Int
rollDie = state $ randomR (1,6)

t1 = evalState rollDie (mkStdGen 0)

rollNDice :: Int -> State StdGen [Int]
-- rollNDice n = sequence $ replicate n rollDie
rollNDice n = replicateM n rollDie

t2 = evalState (rollNDice 5) (mkStdGen 0)

t3 = evalState (fmap (42*) rollDie) (mkStdGen 0)

fmap2 :: (a -> b) -> State s a -> State s b
fmap2 = liftM

t4 = evalState (fmap2 (42*) rollDie) (mkStdGen 0)

fmap3 :: (a -> b) -> State s a -> State s b
fmap3 f m = state $ \s -> let (v, s1) = (runState m) s
                          in  (f v, s1)
t5 = evalState (fmap2 (42*) rollDie) (mkStdGen 0)

stack-state.hs

import Control.Monad.Trans.State

push :: a -> State [a] ()
push x = state $ \s -> ((), x:s)

pop :: State [a] (Maybe a)
pop = state $ \ s ->
  case s of
    []     -> (Nothing, [])
    (x:xs) -> (Just x, xs)

stack = do
  push 42
  push 43
  push 44
  x <- pop
  y <- pop
  z <- pop
  t <- pop
  return (x,y,z,t)

r = evalState stack []