In bigger applications there are very often multiple layers of IO caching (Hibernate L1 and L2, Spring cache etc.) which usually are abstracted so that caller needs not to be aware that particular implementation does IO. With some caveats (scope, transactions), it allows for simpler interfaces between components.
For example, if component A needs to query database, it needs not to know whether result is already cached. It might have been retrieved by B or C which A knows nothing about, however they would usually participate in some session or transaction - often implicitly.
Frameworks tend to make this call indistinguishable from simple object method call using techniques like AOP.
Is it possible for Haskell applications to benefit like this? How would client's interface look like?
In Haskell there are many ways to compose computations from components that represent their separate responsibilities. This can be done at the data level with data types and functions (http://www.haskellforall.com/2012/05/scrap-your-type-classes.html) or using type classes. In Haskell you can view every data type, type, function, signature, class, etc as an interface; as long as you have something else of the same type, you can replace a component with something that's compatible.
When we want to reason about computations in Haskell we frequently use the abstraction of a Monad
. A Monad
is an interface for constructing computations. A base computation can be constructed with return
and these can be composed together with functions that produce other computations with >>=
. When we want to add multiple responsibilities to computations represented by monads, we make monad transformers. In the code below, there are four different monad transformers that capture different aspects of a layered system:
DatabaseT s
adds a database with a schema of type s
. It handles data Operation
s by storing data in or retrieving it from the database.
CacheT s
intercepts data Operation
s for a schema s
and retrieves data from memory, if it is available.
OpperationLoggerT
logs the Operation
s to standard output
ResultLoggerT
logs the results of Operation
s to standard output
These four components communicate together using a type class (interface) called MonadOperation s
, which requires that components that implement it provide a way to perform
an Operation
and return its result.
This same type class described what is required to use the MonadOperation s
system. It requires that someone using the interface provide implementations of type classes that the database and cache will rely on. There are also two data types that are part of this interface, Operation
and CRUD
. Notice that the interface doesn't need to know anything about the domain objects or database schema, nor does it need to know about the different monad transformers that will implement it. The monad transformers don't know anything about the schema or domain objects, and the domain objects and example code don't know anything about the monad transformers that build the system.
The only thing the example code knows is that it will have access to a MonadOperation s
due to its type example :: (MonadOperation TableName m) => m ()
.
The program main
runs the example twice in two different contexts. The first time, the program talks to the database, with its Operations
and responses being logged to standard out.
Running example program once with an empty database
Operation Articles (Create (Article {title = "My first article", author = "Cirdec", contents = "Lorem ipsum dolor sit amet."}))
ArticleId 0
Operation Articles (Read (ArticleId 0))
Just (Article {title = "My first article", author = "Cirdec", contents = "Lorem ipsum dolor sit amet."})
Operation Articles (Read (ArticleId 0))
Just (Article {title = "My first article", author = "Cirdec", contents = "Lorem ipsum dolor sit amet."})
The second run logs the responses the program receives, passes Operation
s through the cache, and logs the requests before they reach the database. Due to the new caching, which is transparent to the program, the requests to read the article never happen, but the program still receives a response:
Running example program once with an empty cache and an empty database
Operation Articles (Create (Article {title = "My first article", author = "Cirdec", contents = "Lorem ipsum dolor sit amet."}))
ArticleId 0
Just (Article {title = "My first article", author = "Cirdec", contents = "Lorem ipsum dolor sit amet."})
Just (Article {title = "My first article", author = "Cirdec", contents = "Lorem ipsum dolor sit amet."})
Here's the entire source code. You should think of it as four independent pieces of code: A program written for our domain, starting at example
. An application that is the complete assembly of the program, the domain of discourse, and the various tools that build it, starting at main
. The next two sections, ending with the schema TableName
, describe a domain of blog posts; their only purpose is to illustrate how the other components go together, not to serve as an example for how to design data structures in Haskell. The next section describes a small interface by which components could communicate about data; it's not necessarily a good interface. Finally, the remainder of the source code implements the loggers, database, and caches that are composed together to form the application. In order to decouple the tools and interface from the domain, there are some somewhat hideous tricks with typeable and dynamics in here, this isn't meant to demonstrate a good way to handle casting and generics either.
{-# LANGUAGE StandaloneDeriving, GADTs, DeriveDataTypeable, FlexibleInstances, FlexibleContexts, GeneralizedNewtypeDeriving, MultiParamTypeClasses, ScopedTypeVariables, KindSignatures, FunctionalDependencies, UndecidableInstances #-}
module Main (
main
) where
import Data.Typeable
import qualified Data.Map as Map
import Control.Monad.State
import Control.Monad.State.Class
import Control.Monad.Trans
import Data.Dynamic
-- Example
example :: (MonadOperation TableName m) => m ()
example =
do
id <- perform $ Operation Articles $ Create $ Article {
title = "My first article",
author = "Cirdec",
contents = "Lorem ipsum dolor sit amet."
}
perform $ Operation Articles $ Read id
perform $ Operation Articles $ Read id
cid <- perform $ Operation Comments $ Create $ Comment {
article = id,
user = "Cirdec",
comment = "Commenting on my own article!"
}
perform $ Operation Equality $ Create False
perform $ Operation Equality $ Create True
perform $ Operation Inequality $ Create True
perform $ Operation Inequality $ Create False
perform $ Operation Articles $ List
perform $ Operation Comments $ List
perform $ Operation Equality $ List
perform $ Operation Inequality $ List
return ()
-- Run the example twice, changing the cache transparently to the code
main :: IO ()
main = do
putStrLn "Running example program once with an empty database"
runDatabaseT (runOpperationLoggerT (runResultLoggerT example)) Types { types = Map.empty }
putStrLn "\nRunning example program once with an empty cache and an empty database"
runDatabaseT (runOpperationLoggerT (runCacheT (runResultLoggerT example) Types { types = Map.empty })) Types { types = Map.empty }
return ()
-- Domain objects
data Article = Article {
title :: String,
author :: String,
contents :: String
}
deriving instance Eq Article
deriving instance Ord Article
deriving instance Show Article
deriving instance Typeable Article
newtype ArticleId = ArticleId Int
deriving instance Eq ArticleId
deriving instance Ord ArticleId
deriving instance Show ArticleId
deriving instance Typeable ArticleId
deriving instance Enum ArticleId
data Comment = Comment {
article :: ArticleId,
user :: String,
comment :: String
}
deriving instance Eq Comment
deriving instance Ord Comment
deriving instance Show Comment
deriving instance Typeable Comment
newtype CommentId = CommentId Int
deriving instance Eq CommentId
deriving instance Ord CommentId
deriving instance Show CommentId
deriving instance Typeable CommentId
deriving instance Enum CommentId
-- Database Schema
data TableName k v where
Articles :: TableName ArticleId Article
Comments :: TableName CommentId Comment
Equality :: TableName Bool Bool
Inequality :: TableName Bool Bool
deriving instance Eq (TableName k v)
deriving instance Ord (TableName k v)
deriving instance Show (TableName k v)
deriving instance Typeable2 TableName
-- Data interface (Persistance library types)
data CRUD k v r where
Create :: v -> CRUD k v k
Read :: k -> CRUD k v (Maybe v)
List :: CRUD k v [(k,v)]
Update :: k -> v -> CRUD k v (Maybe ())
Delete :: k -> CRUD k v (Maybe ())
deriving instance (Eq k, Eq v) => Eq (CRUD k v r)
deriving instance (Ord k, Ord v) => Ord (CRUD k v r)
deriving instance (Show k, Show v) => Show (CRUD k v r)
data Operation s t k v r where
Operation :: t ~ s k v => t -> CRUD k v r -> Operation s t k v r
deriving instance (Eq (s k v), Eq k, Eq v) => Eq (Operation s t k v r)
deriving instance (Ord (s k v), Ord k, Ord v) => Ord (Operation s t k v r)
deriving instance (Show (s k v), Show k, Show v) => Show (Operation s t k v r)
class (Monad m) => MonadOperation s m | m -> s where
perform :: (Typeable2 s, Typeable k, Typeable v, t ~ s k v, Show t, Ord v, Ord k, Enum k, Show k, Show v, Show r) => Operation s t k v r -> m r
-- Database implementation
data Tables t k v = Tables {
tables :: Map.Map String (Map.Map k v)
}
deriving instance Typeable3 Tables
emptyTablesFor :: Operation s t k v r -> Tables t k v
emptyTablesFor _ = Tables {tables = Map.empty}
data Types = Types {
types :: Map.Map TypeRep Dynamic
}
-- Database emulator
mapOperation :: (Enum k, Ord k, MonadState (Map.Map k v) m) => (CRUD k v r) -> m r
mapOperation (Create value) = do
current <- get
let id = case Map.null current of
True -> toEnum 0
_ -> succ maxId where
(maxId, _) = Map.findMax current
put (Map.insert id value current)
return id
mapOperation (Read key) = do
current <- get
return (Map.lookup key current)
mapOperation List = do
current <- get
return (Map.toList current)
mapOperation (Update key value) = do
current <- get
case (Map.member key current) of
True -> do
put (Map.update (\_ -> Just value) key current)
return (Just ())
_ -> return Nothing
mapOperation (Delete key) = do
current <- get
case (Map.member key current) of
True -> do
put (Map.delete key current)
return (Just ())
_ -> return Nothing
tableOperation :: (Enum k, Ord k, Ord v, t ~ s k v, Show t, MonadState (Tables t k v) m) => Operation s t k v r -> m r
tableOperation (Operation tableName op) = do
current <- get
let currentTables = tables current
let tableKey = show tableName
let table = Map.findWithDefault (Map.empty) tableKey currentTables
let (result,newState) = runState (mapOperation op) table
put Tables { tables = Map.insert tableKey newState currentTables }
return result
typeOperation :: (Enum k, Ord k, Ord v, t ~ s k v, Show t, Typeable2 s, Typeable k, Typeable v, MonadState Types m) => Operation s t k v r -> m r
typeOperation op = do
current <- get
let currentTypes = types current
let empty = emptyTablesFor op
let typeKey = typeOf (empty)
let typeMap = fromDyn (Map.findWithDefault (toDyn empty) typeKey currentTypes) empty
let (result, newState) = runState (tableOperation op) typeMap
put Types { types = Map.insert typeKey (toDyn newState) currentTypes }
return result
-- Database monad transformer (clone of StateT)
newtype DatabaseT (s :: * -> * -> *) m a = DatabaseT {
databaseStateT :: StateT Types m a
}
runDatabaseT :: DatabaseT s m a -> Types -> m (a, Types)
runDatabaseT = runStateT . databaseStateT
instance (Monad m) => Monad (DatabaseT s m) where
return = DatabaseT . return
(DatabaseT m) >>= k = DatabaseT (m >>= \x -> databaseStateT (k x))
instance MonadTrans (DatabaseT s) where
lift = DatabaseT . lift
instance (MonadIO m) => MonadIO (DatabaseT s m) where
liftIO = DatabaseT . liftIO
instance (Monad m) => MonadOperation s (DatabaseT s m) where
perform = DatabaseT . typeOperation
-- State monad transformer can preserve operations
instance (MonadOperation s m) => MonadOperation s (StateT state m) where
perform = lift . perform
-- Cache implementation (very similar to emulated database)
cacheMapOperation :: (Enum k, Ord k, Ord v, t ~ s k v, Show t, Show k, Show v, Typeable2 s, Typeable k, Typeable v, MonadState (Map.Map k v) m, MonadOperation s m) => Operation s t k v r -> m r
cacheMapOperation op@(Operation _ (Create value)) = do
key <- perform op
modify (Map.insert key value)
return key
cacheMapOperation op@(Operation _ (Read key)) = do
current <- get
case (Map.lookup key current) of
Just value -> return (Just value)
_ -> do
value <- perform op
modify (Map.update (\_ -> value) key)
return value
cacheMapOperation op@(Operation _ (List)) = do
values <- perform op
modify (Map.union (Map.fromList values))
current <- get
return (Map.toList current)
cacheMapOperation op@(Operation _ (Update key value)) = do
successful <- perform op
modify (Map.update (\_ -> (successful >>= (\_ -> Just value))) key)
return successful
cacheMapOperation op@(Operation _ (Delete key)) = do
result <- perform op
modify (Map.delete key)
return result
cacheTableOperation :: (Enum k, Ord k, Ord v, t ~ s k v, Show t, Show k, Show v, Typeable2 s, Typeable k, Typeable v, MonadState (Tables t k v) m, MonadOperation s m) => Operation s t k v r -> m r
cacheTableOperation op@(Operation tableName _) = do
current <- get
let currentTables = tables current
let tableKey = show tableName
let table = Map.findWithDefault (Map.empty) tableKey currentTables
(result,newState) <- runStateT (cacheMapOperation op) table
put Tables { tables = Map.insert tableKey newState currentTables }
return result
cacheTypeOperation :: (Enum k, Ord k, Ord v, t ~ s k v, Show t, Show k, Show v, Typeable2 s, Typeable k, Typeable v, MonadState Types m, MonadOperation s m) => Operation s t k v r -> m r
cacheTypeOperation op = do
current <- get
let currentTypes = types current
let empty = emptyTablesFor op
let typeKey = typeOf (empty)
let typeMap = fromDyn (Map.findWithDefault (toDyn empty) typeKey currentTypes) empty
(result, newState) <- runStateT (cacheTableOperation op) typeMap
put Types { types = Map.insert typeKey (toDyn newState) currentTypes }
return result
-- Cache monad transformer
newtype CacheT (s :: * -> * -> *) m a = CacheT {
cacheStateT :: StateT Types m a
}
runCacheT :: CacheT s m a -> Types -> m (a, Types)
runCacheT = runStateT . cacheStateT
instance (Monad m) => Monad (CacheT s m) where
return = CacheT . return
(CacheT m) >>= k = CacheT (m >>= \x -> cacheStateT (k x))
instance MonadTrans (CacheT s) where
lift = CacheT . lift
instance (MonadIO m) => MonadIO (CacheT s m) where
liftIO = CacheT . liftIO
instance (Monad m, MonadOperation s m) => MonadOperation s (CacheT s m) where
perform = CacheT . cacheTypeOperation
-- Logger monad transform
newtype OpperationLoggerT m a = OpperationLoggerT {
runOpperationLoggerT :: m a
}
instance (Monad m) => Monad (OpperationLoggerT m) where
return = OpperationLoggerT . return
(OpperationLoggerT m) >>= k = OpperationLoggerT (m >>= \x -> runOpperationLoggerT (k x))
instance MonadTrans (OpperationLoggerT) where
lift = OpperationLoggerT
instance (MonadIO m) => MonadIO (OpperationLoggerT m) where
liftIO = OpperationLoggerT . liftIO
instance (MonadOperation s m, MonadIO m) => MonadOperation s (OpperationLoggerT m) where
perform op = do
liftIO $ putStrLn $ show op
lift (perform op)
-- Result logger
newtype ResultLoggerT m a = ResultLoggerT {
runResultLoggerT :: m a
}
instance (Monad m) => Monad (ResultLoggerT m) where
return = ResultLoggerT . return
(ResultLoggerT m) >>= k = ResultLoggerT (m >>= \x -> runResultLoggerT (k x))
instance MonadTrans (ResultLoggerT) where
lift = ResultLoggerT
instance (MonadIO m) => MonadIO (ResultLoggerT m) where
liftIO = ResultLoggerT . liftIO
instance (MonadOperation s m, MonadIO m) => MonadOperation s (ResultLoggerT m) where
perform op = do
result <- lift (perform op)
liftIO $ putStrLn $ "\t" ++ (show result)
return result
To build this example, you'll need the mtl
and containers
libraries.
In Haskell, you do need to (and want to!) be aware of anything that does IO.
That is one of the strong points about it.
You can use the MonadIO
type class to write functions that work in any monad that is allowed to perform IO actions:
myFunctionUsingIO :: (MonadIO m) => ... -> m someReturntype
myFunctionUsingIO = do
-- some code
liftIO $ ... -- some IO code
-- some other code
As many programming interfaces in Haskell are expressed via monads, functions like this might work in more contexts.
You can also use unsafePerformIO
to secretly run IO actions from pure code - however this is not advisable in almost all cases. Being pure allows you to immediately see whether side effects are used or not.
IO caching is a side effect, and you are well off if your types reflect that.
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