import Control.Lens hiding ((<|), (|>), (:<), (:>))

import Control.Monad (void)

import Control.Monad.IO.Class (liftIO)

import Control.Monad.State.Lazy (StateT(..), runStateT, get, gets)

import Data.Sequence

-- 'Seq' is a dequeue

type Dequeue a = Seq a

main :: IO ()

main = do

putStrLn "-- pure operations"

let xs = fromList [10]

xs' = 20 <| xs -- push front

xs'' = xs' |> 30 -- push back

mapM_ print [xs, xs', xs'']

--NOTE: these are unsafe operations (Please see 'context' for safe operations)

let (x' :< xs') = viewl xs''

(xs :> x) = viewr xs'

mapM_ print [x', x]

void $ runStateT context xs

context :: StateT (Dequeue Int) IO ()

context = do

liftIO $ putStrLn "-- inpure operations"

printState

simple %= (20 <|)

printState

simple %= (|> 30)

printState

--NOTE: these are safe operations by 'Prism'

maybeHead <- gets $ preview _head

liftIO $ print maybeHead

maybeLast <- gets $ preview _last

liftIO $ print maybeLast

where

printState :: Show s => StateT s IO ()

printState = get >>= liftIO . print