データベース
acid-state
Acid-state allows us to build a "database on demand" for arbitrary Haskell datatypes that guarantees atomic transactions. For example, we can build a simple key-value store wrapped around the Map type.
{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE TemplateHaskell #-}
import Data.Acid
import Data.Typeable
import Data.SafeCopy
import Control.Monad.Reader (ask)
import qualified Data.Map as Map
import qualified Control.Monad.State as S
type Key = String
type Value = String
data Database = Database !(Map.Map Key Value)
deriving (Show, Ord, Eq, Typeable)
$(deriveSafeCopy 0 'base ''Database)
insertKey :: Key -> Value -> Update Database ()
insertKey key value
= do Database m <- S.get
S.put (Database (Map.insert key value m))
lookupKey :: Key -> Query Database (Maybe Value)
lookupKey key
= do Database m <- ask
return (Map.lookup key m)
deleteKey :: Key -> Update Database ()
deleteKey key
= do Database m <- S.get
S.put (Database (Map.delete key m))
allKeys :: Int -> Query Database [(Key, Value)]
allKeys limit
= do Database m <- ask
return $ take limit (Map.toList m)
$(makeAcidic ''Database ['insertKey, 'lookupKey, 'allKeys, 'deleteKey])
fixtures :: Map.Map String String
fixtures = Map.empty
test :: Key -> Value -> IO ()
test key val = do
database <- openLocalStateFrom "db/" (Database fixtures)
result <- update database (InsertKey key val)
result <- query database (AllKeys 10)
print result