To Overcome

servant-persistent updated

Previously, I wrote a blog post on using servant and persistent together. servant has updated to the new 0.7 version, and I felt like it was a good idea to bring my tutorial up to date. I’d also noticed that some folks were using the repository as a starter scaffold for their own apps, which is great! To accommodate that, I’ve beefed up the application a bit to demonstrate some of the features of Servant, including a primitive client, as well as configuration for easy deployment with the keter package. Let’s dive in!

The code for all of this is on the GitHub repository. I’ll be keeping the 0.7 branch up to date with any edits to this post.

Take note: This is less of a tutorial on servant specifically, and more of an exposition on a servant base package that has some convenient defaults for running applications.

Application Structure

The application has three sub-components:

It’s a good idea to extract as much code as you can in the library. This makes it easier to test the code, as you can import it into the tests without having to recompile it every time. Additionally, you can make the library functions available for all kinds of potential executables down the line. We’ll start with Main and dig into the rest.


-- | The 'main' function gathers the required environment information and
-- initializes the application.
main :: IO ()
main = do
    putStrLn "servant-persistent booting up"
    env  <- lookupSetting "ENV" Development
    port <- lookupSetting "PORT" 8081
    pool <- makePool env
    let cfg = Config { getPool = pool, getEnv = env }
        logger = setLogger env
    runSqlPool doMigrations pool
    run port $ logger $ app cfg

main grabs some settings from the environment, creates the database pool, runs migrations, generates JavaScript for querying the API, and finally runs the app. We define lookupSetting a little below:

-- | Looks up a setting in the environment, with a provided default, and
-- 'read's that information into the inferred type.
lookupSetting :: Read a => String -> a -> IO a
lookupSetting env def = do
    maybeValue <- lookupEnv env
    case maybeValue of
        Nothing ->
            return def
        Just str ->
            maybe (handleFailedRead str) return (readMay str))
    handleFailedRead str =
        error $ mconcat 
            [ "Failed to read [["
            , str
            , "]] for environment variable "
            , env

First, we lookup the environment variable. If it’s not present, then we just return the default value. If it is present, then we use the function readMay which we’ve imported from the Safe module. If readMay fails to read the variable, then we throw an error. Consider that readMay "PRoduction" :: Maybe Environment will return Nothing, silently putting us in Development mode. We definitely don’t want that!

Next up is makePool, so let’s check that out. We’ve imported it from Config.


For Development and Test environments, the makePool function is relatively simple:

-- | This function creates a 'ConnectionPool' for the given environment.
-- For 'Development' and 'Test' environments, we use a stock and highly
-- insecure connection string. The 'Production' environment acquires the
-- information from environment variables that are set by the keter
-- deployment application.
makePool :: Environment -> IO ConnectionPool
makePool Test =
    runNoLoggingT (createPostgresqlPool (connStr "_test") (envPool Test))
makePool Development =
    runStdoutLoggingT (createPostgresqlPool (connStr "") (envPool Development))

In Testing, we don’t want it to print anything out, so we use the runNoLoggingT function from Control.Monad.Logger to tell createPostgresqlPool which instance of the MonadLogger type class it’ll use. Likewise, Development will be printing all of the logs to standard out. We create a connStr with a database name suffix of “_test” for testing and no suffix for development.

For production, it gets a bit trickier. We need to get the database environment from keter, so we have to read each bit of the connection string in as environment variables. This part of the function makes heavy use of the MaybeT monad transformer, which might be confusing if you’re not familiar with it. It allows us to combine the effects from ‘IO’ and the effect of Maybe into a single “big effect”, so that when we bind out of MaybeT IO a, we get an a. If we just had IO (Maybe a), then binding out of the IO would give us a Maybe a, which would make the code quite a bit more verbose.

makePool Production = do
    pool <- runMaybeT $ do
        let keys = [ "host="
                   , "port="
                   , "user="
                   , "password="
                   , "dbname="
            envs = [ "PGHOST"
                   , "PGPORT"
                   , "PGUSER"
                   , "PGPASS"
                   , "PGDATABASE"
        envVars <- traverse (MaybeT . lookupEnv) envs
        let prodStr = mconcat . zipWith (<>) keys . fmap BS.pack $ envVars
        runStdoutLoggingT $ createPostgresqlPool prodStr (envPool Production)

traverse is tons of fun. If you know map :: (a -> b) -> [a] -> [b], then mapM shouldn’t be too scary: it’s just mapM :: (a -> m b) -> [a] -> m [b]. As it happens, the m in mapM doesn’t have to a Monad, just Applicative, and it works for more things than just lists. In this case, traverse is taking each String in the envs list, looking it up in the environment and wrapping it in MaybeT, and finally evaluating a value of type MaybeT IO [String].

We now have a list of keys, and a list of values. We zip them together with <> and concatenate them all into a big connection string, with which we create a pool. Finally, we runMaybeT to convert the MaybeT IO ConnectionPool to an IO (Maybe ConnectionPool) and bind that value out.

    case pool of
         Nothing -> error "Database Configuration not present in environment."
         Just a -> return a

If the database configuration isn’t there, then we error out. Otherwise, we return it. This shouldn’t happen, as keter automatically manages the PostgreSQL database information for us on the deployment server.


That covers making the pool. Running migrations was next. This step is neatly handled for us by the persistent library. For further reading on that, check the chapter out. It’s a great resource.

doMigrations :: SqlPersistM ()
doMigrations = runMigration migrateAll

The migrateAll function is generated by the following Persistent Entity Definitions:

share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase|

User json
    name  String
    email String
    deriving Show


The json keyword there means “please generate FromJSON and ToJSON instances for this entity,” which is a really handy tool.

Persistent is smart enough to know if the current database schema is in line with what the entity definitions say. If it is, then it doesn’t do anything. If it can safely make the migrations, then it does so. If it can’t, then it helpfully prints the SQL necessary to the console for you to do yourself.

You’ll probably want to move to something like dbmigrations when your database is a bit more complicated, but Persistent’s migrations are still really useful to verify that your data looks like you expect. You can run printMigration to just print out what Persistent would do.

Easy! Let’s see how we’re generating the JavaScript now. That function was imported from Api.User, which we’ll check out next.


In classic servant manner, we’ve got a little API we’ve defined:

type UserAPI =
         "users" :> Get '[JSON] [Entity User]
    :<|> "users" :> Capture "name" String :> Get '[JSON] (Entity User)
    :<|> "users" :> ReqBody '[JSON] User :> Post '[JSON] Int64

Along with our handlers for the server:

-- | The server that runs the UserAPI
userServer :: ServerT UserAPI App
userServer = allUsers :<|> singleUser :<|> createUser

-- | Returns all users in the database.
allUsers :: App [Entity User]
allUsers =
    runDb (selectList [] [])

It still blows my mind how good Haskell’s type inference is. selectList is a function that accepts a list of filters and a list of options, and returns a list of matching records. Here, we provide nothing other than the inferred return type of Entity User and it knows how to run the query.

-- | Returns a user by name or throws a 404 error.
singleUser :: String -> App (Entity User)
singleUser str = do
    maybeUser <- runDb (selectFirst [UserName ==. str] [])
    case maybeUser of
         Nothing ->
            throwError err404
         Just person ->
            return person

-- | Creates a user in the database.
createUser :: User -> App Int64
createUser p = do
    newUser <- runDb (insert (User (userName p) (userEmail p)))
    return $ fromSqlKey newUser

Here’s a neat trick: App (Entity User) is just a function. We can easily reuse that handler code in the rest of the codebase if we wanted to, and it’d do the right thing.

Finally, the JavaScript generation:

-- | Generates JavaScript to query the User API.
generateJavaScript :: IO ()
generateJavaScript =
    writeJSForAPI (Proxy :: Proxy UserAPI) vanillaJS "./assets/api.js"

Well, what does that look like?


The generated code isn’t super pretty, but it gets the job done.

var getUsers = function(onSuccess, onError)
  var xhr = new XMLHttpRequest();'GET', '/users', true);
  xhr.onreadystatechange = function (e) {
    if (xhr.readyState == 4) {
      if (xhr.status == 204 || xhr.status == 205) {
      } else if (xhr.status >= 200 && xhr.status < 300) {
        var value = JSON.parse(xhr.responseText);
      } else {
        var value = JSON.parse(xhr.responseText);

This is just the vanillaJS option. There’s also jQuery and AngularJS options available.

The same machinery that generates client JavaScript code can also be used to generate Ruby clients, if you need them.

Now, we still need to serve up some static files. We do that in the app function, imported from Api.


This is the function we export to run our UserAPI. Given a Config, we return a WAI Application which any WAI compliant server can run.

userApp :: Config -> Application
userApp cfg = serve (Proxy :: Proxy UserAPI) (appToServer cfg)

This functions tells Servant how to run the App monad with the Servant provided server function.

appToServer :: Config -> Server UserAPI
appToServer cfg = enter (convertApp cfg) userServer

This function converts our App monad into the ExceptT ServantErr IO monad that Servants enter’ function needs in order to run the application. The :~> type is a natural transformation, or, in non-category theory terms, a function that converts two type constructors without looking at the values in the types.

convertApp :: Config -> App :~> ExceptT ServantErr IO
convertApp cfg = Nat (flip runReaderT cfg . runApp)

Since we also want to provide a minimal front end, we need to give Servant a way to serve a directory with HTML and JavaScript. This function creates a WAI application that just serves the files out of the given directory.

files :: Application
files = serveDirectory "assets"

Just like a normal API type, we can use the :<|> combinator to unify two different APIs and applications. This is a powerful tool for code reuse and abstraction! We need to put the ‘Raw’ endpoint last, since it always succeeds.

type AppAPI = UserAPI :<|> Raw

appApi :: Proxy AppAPI
appApi = Proxy

Finally, this function takes a configuration and runs our UserAPI alongside the Raw endpoint that serves all of our files.

app :: Config -> Application
app cfg =
    serve appApi (readerServer cfg :<|> files)

Now, we can do:

$ stack build
$ stack exec perservant

and open localhost:8081 to see our primitive little UI.

We’re done, right? Well, sort of! There’s also deployment with keter!


keter is a very nice little utility for deploying Haskell applications. Here’s the configuration required for the app:

# config/keter.yaml

exec: ../perservant

    postgres: true

And the deployment script:

#! /bin/bash

set -e
echo "Building Perservant..."
stack build
strip `stack exec -- which perservant`
echo "Creating bundle..."
cp `stack exec -- which perservant` perservant
tar -czvf perservant.keter perservant config ql-ui/assets
rm perservant
scp ./perservant.keter user@host:/opt/keter/incoming/perservant.keter
rm perservant.keter

And that’s all you need to get going!