servant/doc/cookbook/db-sqlite-simple/DBConnection.lhs

# SQLite database

Let's see how we can write a simple web application that uses an
[SQLite](https://www.sqlite.org/) database to store simple textual
messages. As usual, we start with a little bit of throat clearing.

``` haskell
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TypeOperators #-}
import Control.Concurrent
import Control.Exception (bracket)
import Control.Monad.IO.Class
import Database.SQLite.Simple
import Network.HTTP.Client (newManager, defaultManagerSettings)
import Network.Wai.Handler.Warp
import Servant
import Servant.Client
```

We will only care about a single type here, the messages. We want to
be able to add a new one and retrieve them all, using two different
endpoints.

``` haskell
type Message = String

type API = ReqBody '[PlainText] Message :> Post '[JSON] NoContent
      :<|> Get '[JSON] [Message]

api :: Proxy API
api = Proxy
```

We proceed with a simple function for creating a table
for holding our messages if it doesn't already exist.

``` haskell
initDB :: FilePath -> IO ()
initDB dbfile = withConnection dbfile $ \conn ->
  execute_ conn
    "CREATE TABLE IF NOT EXISTS messages (msg text not null)"
```

Next, our server implementation. It will be parametrised (take as
argument) by the name of the file that contains our SQLite database.
The handlers are straighforward. One takes care of inserting a new
value in the database while the other fetches all messages and returns
them. We also provide a function for serving our web app given an
SQLite database file, which simply calls servant-server's `serve` function.

``` haskell
server :: FilePath -> Server API
server dbfile = postMessage :<|> getMessages

  where postMessage :: Message -> Handler NoContent
        postMessage msg = do
          liftIO . withConnection dbfile $ \conn ->
            execute conn
                    "INSERT INTO messages VALUES (?)"
                    (Only msg)
          return NoContent

        getMessages :: Handler [Message]
        getMessages = fmap (map fromOnly) . liftIO $
         withConnection dbfile $ \conn ->
            query_ conn "SELECT msg FROM messages"

runApp :: FilePath -> IO ()
runApp dbfile = run 8080 (serve api $ server dbfile)
```

Let's also derive some clients for our API and use them to
insert two messages and retrieve them in `main`.

``` haskell
postMsg :: Message -> ClientM NoContent
getMsgs :: ClientM [Message]
postMsg :<|> getMsgs = client api

main :: IO ()
main = do
  -- you could read this from some configuration file,
  -- environment variable or somewhere else instead.
  let dbfile = "test.db"
  initDB dbfile
  mgr <- newManager defaultManagerSettings
  bracket (forkIO $ runApp dbfile) killThread $ \_ -> do
    ms <- flip runClientM (ClientEnv mgr (BaseUrl Http "localhost" 8080 "")) $ do
      postMsg "hello"
      postMsg "world"
      getMsgs
    print ms
```

This program prints `Right ["hello","world"]` the first time it is executed,
`Right ["hello","world","hello","world"]` the second time and so on.

The entire source for this example is available as a cabal project
[here](https://github.com/haskell-servant/servant/tree/master/doc/cookbook/db-sqlite-simple).