24 Days of Hackage: persistent & esqueleto

Many of the applications we write these days tend to be long running processes that gradually manipulate state. Your average website is really not much more than a function that turns requests into responses, where the response varies depending on various state. The state could be whether you’re logged in, blog entries made within a certain time range, items you’ve saved to your wishlist, etc. This state is very important, so just keeping it in memory isn’t a great idea, as that greatly increases the chance of you losing it for good!

For this reason, we have a lot of great technology for persisting state - PostgreSQL, SQLite, Redis, and so on. Today, we’ll have a look at the persistent library, which gives us a unified API into these persistent data stores.

Schemas and Data with persistent

Programming with persistent is a slightly different experience than you may have had with other Haskell libraries, as it provides its own language for defining schemas. This language isn’t built using Haskell combinators, but has its own syntax and is compiled using a quasiquoter. For example, if we wanted to try and beat Facebook at its own game, here’s how we might start:

share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase|
  name Text
  deriving Show

  producer PersonId
  update Text
  createdAt UTCTime
  mood Text Maybe
  deriving Show

The special [| and |] brackets specify that we’re going to use the persistLowerCase quasiquoter, and within this block we see the definition for our schema. In this case, a Person has a name, and a sequence of StatusUpdates. This schema definition language is simple, and is similar to defining data types in Haskell (for good reason, as we’ll see shortly). We have the name of the entity (Person and Update), and each entity has a list of fields. Each field has a name and a data type. Notice that the createdAt field uses the exactly the same type as provided in Data.Time. persistent uses real Haskell types to define schemas, which makes it fantastic for code reuse. We mark the mood attribute of updates as optional by tagging on the Maybe modifier. Finally, each Entity has a series of deriving clauses, just like a Haskell data type.

When we parse this definition and realise it using mkPersist, persistent introduces various data types and type class instances for us. persistent is going to create (amongst other things) a series of data types for each entity, along with Haskell field accessors (e.g., personName or updateCreatedAt).

Once we have declared our schema and associated Haskell data types, we’re ready to start using our persistent data store. We will need to choose a backend to run queries against, so for brevity we’ll use an in-memory SQLite database, though you’re encouraged to clone this code and try it against other backends! As an example, we can create the schema, insert a person, and attach a single status update:

main :: IO ()
main = runSqlite ":memory:" $ do
  runMigration migrateAll
  ollie <- insert (Person "Oliver Charles")
  now <- liftIO getCurrentTime
    [ StatusUpdate ollie "Writing another blog post!" now Nothing,
    , StatusUpdate ollie "I <3 24 Days of Hackage" now (Just "^.^")
  return ()

Very simple - and we didn’t even have to write a line of SQL! Not only did we avoid writing SQL, we can actually use this exact same schema against MongoDB, or Redis, or any other data store with a persistent backend.

Walking through this code, once we’ve connected to our in memory SQLite database, we need to make sure it has the correct relational schema - which we can do with runMigration migrateAll. When we introduced the schema earlier, we asked for a migrateAll action to be created as well. When we call this action, persistent makes sure that our data store has the schema we are expecting - adding tables and columns as necessary.

Next, we create Haskell values (with Person and StatusUpdate) and insert them into our database with insert or insertMany. At this point, persistent is going to mint these entities with new identifiers, so we can refer to them in the future.

Querying with esqueleto

We’ve seen how to put data into a data store, but what about getting it out? While persistent does provide a query API, it can be quite limited. esqueleto builds on top of persistent by providing a richer SQL-like query syntax. For example, we can query for 5 people in our database sorted by name:

sortedNames =
  select $
  from $ \person -> do
  orderBy [asc (person ^. PersonName)]
  limit 5
  return $ person ^. PersonName

Tacking sortedNames >>= mapM_ print onto the end of main above produces the following output:

Value "Oliver Charles"

As we would expect.

esqueleto is a Haskell DSL that works with all of the things that persistent defined for us. In this case, we can see that we are using the PersonName field on the person variable to define our ordering. This lets Haskell know that person must be a Person record, so esqueleto knows how to pull the data out we requested we used from. In this sense, we really benefit from type inference - by operating on the types, we also inform, esqueleto as to which tables to pull data from.

We don’t have to limited to just one entity though, we could just as easily find the latest 5 status updates from all people on the system:

latestUpdates =
  select $
  from $ \(person `InnerJoin` update) -> do
  on (person ^. PersonId ==. update ^. StatusUpdateProducer)
  orderBy [asc (update ^. StatusUpdateCreatedAt)]
  limit 5
  return (person ^. PersonName, update ^. StatusUpdateUpdate)

In this query we specify that we’re going to be performing an inner join on Person and StatusUpdate, and we tell esqueleto which predicate to use in order to perform this join. We then have both the Person and StatusUpdate in scope, so we can sort by the timestamp of the update. We then return a pair of both the name of the person who created the status update, and the contents of the status update itself.

As we saw yesterday with scotty, Haskell can really shine when it comes to prototyping small applications, and I think persistent follows in this tradition. It’s a little opionated, with the mkPersist syntax and automatic creation of surrogate keys, which can make fitting persistent onto existing schemas a challeng. However, if you don’t have that requirement, persistent might be one of the easiest routes to getting data into a persistent store with the minimal amount of code duplication. You don’t have to worry about defining isomorphisms between rows and Haskell values - you can just let persistent do that for you, while you carry on doing the interesting work.

You can contact me via email at ollie@ocharles.org.uk or tweet to me @acid2. I share almost all of my work at GitHub. This post is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

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