SimpleState.hs

-- file: ch14/SimpleState.hs
type SimpleState s a = s -> (a, s)

-- file: ch14/SimpleState.hs
type StringState a = SimpleState String a

-- file: ch14/SimpleState.hs
returnSt :: a -> SimpleState s a
returnSt a = \s -> (a, s)

-- file: ch14/SimpleState.hs
returnAlt :: a -> SimpleState s a
returnAlt a s = (a, s)

-- file: ch14/SimpleState.hs
bindSt :: (SimpleState s a) -> (a -> SimpleState s b) -> SimpleState s b
bindSt m k = \s -> let (a, s') = m s
                   in (k a) s'

-- file: ch14/SimpleState.hs
-- m == step
-- k == makeStep
-- s == oldState

bindAlt step makeStep oldState =
    let (result, newState) = step oldState
    in  (makeStep result) newState

-- file: ch14/SimpleState.hs
bindAlt :: (s -> (a, s))        -- step
        -> (a -> s -> (b, s))   -- makeStep
        -> (s -> (b, s))        -- (makeStep result) newState

-- file: ch14/SimpleState.hs
getSt :: SimpleState s s
getSt = \s -> (s, s)

putSt :: s -> SimpleState s ()
putSt s = \_ -> ((), s)