How to connect to MySQL using Snap's MysqlSimple snaplet

Question

The Snap Framework has a tutorial on how to write a web-application using snaplets. There exists a snaplet for connecting to MySQL, Snap.Snaplets.MysqlSimple, that depends on the library Database.MySQL.Simple, and it seems the snaplet is mostly a thin wrapper to keep the connection handle implicit.

The only example available in the documentation does not work. As described in a 2011 Haskell-cafe post (!), the example lacks some type annotations before it can derive the type of the result. Is there a simple example of a fully functional Snap application that connects to MySQL and fetches some result?

Additional question: I want to change the MySQL connection settings. Running the application creates a file called snaplets/mysql-simple/devel.cfg. Changing the settings in this file does not seem to affect the application even after recompiling, restarting and/or reloading.

Answer 1

The following example uses the default MySQL connection settings:

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE OverloadedStrings #-}

module Main where

import Snap
import Snap.Snaplet
import Snap.Snaplet.Heist
import Snap.Snaplet.MysqlSimple

import Control.Lens.TH

import Data.Int
import Data.Maybe
import qualified Data.Text as T

data App = App
    { _heist       :: Snaplet (Heist App)
    , _db          :: Snaplet Mysql
    }

makeLenses ''App

instance HasHeist App where
  heistLens = subSnaplet heist

main :: IO ()
main = serveSnaplet defaultConfig appInit

appInit :: SnapletInit App App
appInit = makeSnaplet "myapp" "Some application" Nothing $ do
  h <- nestSnaplet "heist" heist $ heistInit "templates"
  d <- nestSnaplet "db" db mysqlInit
  addRoutes [ ("/", defaultHandler), ("", heistServe) ]
  return $ App h d

defaultHandler :: Handler App App ()
defaultHandler = do
  ns <- q
  forM_ ns $ \ (Only i) -> 
    writeText $ T.concat [ "id = ", T.pack (show i), "\n"]

q :: Handler App App [Only Int64]
q = with db $ query_ ("select id from sometable")