acid-state-0.5.2: examples/KeyValue.hs

{-# LANGUAGE DeriveDataTypeable, TypeFamilies, TemplateHaskell #-}
module Main (main) where

import Data.Acid

import Control.Monad.State
import Control.Monad.Reader
import Control.Applicative
import System.Environment
import System.IO
import Data.SafeCopy

import Data.Typeable

import qualified Data.Map as Map

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-- The Haskell structure that we want to encapsulate

type Key = String
type Value = String

data KeyValue = KeyValue !(Map.Map Key Value)
    deriving (Typeable)

$(deriveSafeCopy 0 'base ''KeyValue)

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-- The transaction we will execute over the state.

insertKey :: Key -> Value -> Update KeyValue ()
insertKey key value
    = do KeyValue m <- get
         put (KeyValue (Map.insert key value m))

lookupKey :: Key -> Query KeyValue (Maybe Value)
lookupKey key
    = do KeyValue m <- ask
         return (Map.lookup key m)

$(makeAcidic ''KeyValue ['insertKey, 'lookupKey])

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-- This is how AcidState is used:

main :: IO ()
main = do acid <- openAcidState (KeyValue Map.empty)
          args <- getArgs
          case args of
            [key]
              -> do mbKey <- query acid (LookupKey key)
                    case mbKey of
                      Nothing    -> putStrLn $ key ++ " has no associated value."
                      Just value -> putStrLn $ key ++ " = " ++ value
            [key,val]
              -> do update acid (InsertKey key val)
                    putStrLn "Done."
            _ -> do putStrLn "Usage:"
                    putStrLn "  key          Lookup the value of 'key'."
                    putStrLn "  key value    Set the value of 'key' to 'value'."
          closeAcidState acid