Today we're going to look at the idea of using Haskell's type system to specialize our app implementation according to type-level flags. More specifically we're going to look at a fun way to write a monadic action which alters its behaviour based on which version of a system it's embedded in, simultaneously gaining ground on the expression problem and giving us compile-time guarantees that we haven't accidentally mixed up code from different versions of our app!

The concrete example we'll be looking at is a simple web handler which returns a JSON representation of a User; we'll start with a single possible representation of the user, but will the evolve our system to be able to return a different JSON schema depending on which version of the API the user has selected.

Disclaimer; the system I present probably isn't a great idea in a large production app, but is a fun experiment to learn more about higher kinded types and functional dependencies so we're going to do it anyways. Let's dive right in!

Starting App

Let's build a quick starting app so we have something to work with; I'll elide all the web and http related bits, we'll have a simple handler that fetches a user and our main will run the handler and print things out.

-- You'll need to have the 'mtl' and 'aeson' packages in your project {-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE NamedFieldPuns #-} {-# LANGUAGE GeneralizedNewtypeDeriving #-} import Control.Monad.IO.Class import Data.Aeson -- User ----------------------------------- data User = User { name :: String } deriving ( Show ) instance ToJSON ( User ) where User {name}) = object [ "name" .= name] toJSON ({name})object [name] -- App Monad -------------------------------- newtype AppM a = AppM { runApp :: IO a } deriving ( Functor , Applicative , Monad , MonadIO ) -- User Service -------------------------------- class ( Monad m) => m) MonadUserService m where getUser :: m User instance MonadUserService AppM where = return ( User "Bob Johnson" ) getUser -- App ----------------------------------------- userHandler :: ( MonadUserService m) => m Value m) = do userHandler <- getUser usergetUser return $ toJSON user toJSON user app :: ( MonadIO m, MonadUserService m) => m () m,m)m () = do app <- userHandler userJSONuserHandler $ print userJSON liftIOuserJSON main :: IO () () = runApp app mainrunApp app

Hopefully that's not too cryptic 😅

We've defined a simple user object and wrote an Aeson ToJSON instance for it so we can serialize it. Then we wrote a newtype wrapper around IO which we can use to implement various instances on; note that we use GeneralizedNewtypeDeriving to get our Monad and MonadIO instances for free.

Next we define our interface for a User Service as the MonadUserService typeclass; this has a single member: getUser which defines how to get a user within a given monad. In our case we'll write the simplest possible implementation for our service and just return a static "Bob Johnson" user.

Next up we have our handler which gets a user, serializes, then returns it. Lastly we've got an app which calls the user then prints it, and a main which runs the app.

Brilliant, we're all set up; let's run it and see what we get!

> main main Object (fromList [( "name" , String "Bob Johnson" )]) (fromList [()])

Chapter 2; wherein our API evolves

They said we'd never succeed, but damn them all! In spite all of our investor's criticisms our app is doing wonderfully! We have a whole 7 of users and are making tens of dollars! Some users at large have requested the ability to get a user's first and last name separately; but other users have legacy systems built against our v1 API! Clearly it would be far too much work to duplicate our entire user handler and make alterations for our v2 API, let's see if we can parameterize our app over our API version and defer the choice of app version (and implementation) until the last possible minute!

Like most Haskell refactors we can just start building what we want and let the compiler guide the way; let's change our User data type to reflect the needs of our users:

-- First attempt data User = UserV1 { name :: String } | UserV2 { firstName :: String , lastName :: String } deriving ( Show )

This reflects the choice in our app that the user type could be either of the two shapes; but there's a few problems with this approach. First and foremost is that this means that at EVERY stage in the app where we use a User we need to pattern match over the constructors and handle EVERY one; regardless of which version we happen to be working with. Not only is this not what we wanted, but as we add more versions later on the number of possible code paths we need to handle explodes! One way we can avoid this chaos is to let the type system know that our data is versioned. Enter GADT s!

GADT's with Phantom types

Take a gander at this:

{-# LANGUAGE GADTs #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE KindSignatures #-} import GHC.TypeLits data User ( v :: Nat ) where UserV1 :: { name :: String } -> User 1 UserV2 :: { firstName :: String , lastName :: String } -> User 2

If you haven't worked with GADT s before this may all look a bit strange, let's break it down a bit:

First off; in order to even talk about GADT s we need the GADTs language extension; this unlocks the data ... where syntax! There's a bit more to it, but the basic idea is that this allows us to effectively specify our data constructors like we would normal functions in haskell. This means we can specify constaints on our parameters, and in our case we can specialize the types of the resulting value based on which particular constructor was used. So if someone uses the UserV1 constructor they MUST get a User 1 , and similarly with UserV2 . The compiler remembers this info and in our case can actually tell that if we have a function which accepts a User 1 that we only need to match over the UserV1 constructor since any values of User 1 MUST have been constructed using UserV1 .

Maybe I'm getting ahead of myself; how is it we can suddenly have numbers like 1 and 2 in our types? The answer lies within the (v :: Nat) annotation. This is a Kind Signature, and as such naturally requires the KindSignatures extension. Others have written more exhaustively on the subject, but the basic idea is that Nat is a kind, i.e. a 'type' for types. This means that the v parameter can't take on just any type, but only types that are part of the Nat kind, which corresponds to the natural (aka non-negative) integers. This is handy, because it means people can't create a user with a version number of String or () or something silly like that. Lastly we need the DataKinds extension to allow us to use Data Constructors in our types; once that's enabled we can import GHC.TypeLits and use integer literals in our types and GHC will figure it all out.

The v paramter of our user is also something called a "phantom type". It's a type parameter on a data type that doesn't actually have an associated value in the right hand side of the data definition. These sorts of things are useful for adding additional information at the type level.

Step one done! We've successfully parameterized our datatype over a version number at the type level! At this point your compiler is probably bugging you about the fact that the User constructor no longer exists; we originally implemented the ToJSON class for the base User type, but now User needs an additional type parameter. This is good! It means we can implement a different instance for each version of user we have; which is basically what we wanted to do in the first place!

Let's alter our ToJSON instance so it has a single name parameter for v1 and a separate first and last name for v2!

{-# LANGUAGE FlexibleInstances #-} -- ... instance ToJSON ( User 1 ) where UserV1 {name}) = object [ "name" .= name] toJSON ({name})object [name] instance ToJSON ( User 2 ) where UserV2 {firstName, lastName}) = toJSON ({firstName, lastName}) "firstName" .= firstName, "lastName" .= lastName] object [firstName,lastName]

Here we're specifying different instances of ToJSON for the different members of our User datatype. Note that, as promised, the compiler KNOWS that only the matching constructor needs to be matched on and that a UserV1 won't show up in an instance for User 2 . We'll need FlexibleInstances turned on so GHC can handle complex types like User 1 in an instance definition.

Next it's time to fix up our MonadUserService class, we know that getUser needs to return a user, but which user type should it return? We can imagine someone implementing a MonadUserService for User 1 and also for User 2 , so it would be nice if instances could specify which version they want to work with. To accomplish that we can add an additional parameter to the class:

{-# LANGUAGE MultiParamTypeClasses #-} -- ... class ( Monad m) => m) MonadUserService v m v m where getUser :: m ( User v) m (v) instance MonadUserService 1 AppM where = return ( UserV1 "Bob Johnson" ) getUser instance MonadUserService 2 AppM where = return ( UserV2 "Bob" "Johnson" ) getUser

Just like ToJSON we can now implement the typeclass instance differently for each version of our user. We'll need MultiParamTypeClasses to add the v parameter to our typeclass.

Generalizing the handler and app over version

We're moving along nicely! Next we need our userHandler and app to know about version numbers, however this layer of our app doesn't really care which exact version of user it's working with, mostly it just cares that certain instances exist for that user. Ideally we can write versions of these that work for either of our user versions all at once.

The first step is to introduce our new paramterized typeclasses:

{-# LANGUAGE FlexibleContexts #-} userHandler :: ( ToJSON ( User v), MonadUserService v m) => m Value v),v m) = do userHandler <- getUser usergetUser return $ toJSON user toJSON user app :: ( ToJSON ( User v), MonadIO m, MonadUserService v m) => m () v),m,v m)m () = do app <- userHandler userJSONuserHandler $ print userJSON liftIOuserJSON

Now we run into a bit of a problem;

• Could not deduce (MonadUserService v0 m) from the context: (MonadIO m, MonadUserService v m) bound by the type signature for: app :: forall (m :: * -> *) (v :: Nat). (MonadIO m, MonadUserService v m) => m () at /Users/cpenner/dev/typesafe-versioning/src/Before2.hs:54:8-48 The type variable ‘v0’ is ambiguous • In the ambiguity check for ‘app’ To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In the type signature: app :: (MonadIO m, MonadUserService v m) => m ()

It's telling us it can't tell which user version we want it to use! So there are a few ways we can fix this; one way would be to specify the specific type that we'd like v to be each time it's used; we can enable AllowAmbiguousTypes , TypeApplications and ScopedTypeVariables to try that; but this ends up being pretty verbose and isn't the nicest to work with. We won't dive into that possibility, but I'd recommend you give it a try though if you like a challenge!

The other option we have is to clear up the disambiguity by giving the type system another way to determine what v should be; in our case the monad m is pervasive throughout our app, so if we can somehow infer v from m then we can save ourselves a lot of trouble. We're already associating the two within the typeclass definition class (Monad m) => MonadUserService v m ; however the type system recognizes that there could be an instance for several different values of v ; and of course we've implemented exactly that!

The way to fix this is to tell the type system that there's one-and-only-one v for each m using FunctionalDependencies; and then find a way to encode the v inside the m so we can still run the different versions of our app.

Lets add a new extension and alter our typeclass appropriately:

{-# LANGUAGE FunctionalDependencies #-} class ( Monad m) => MonadUserService v m | m -> v m)v m where getUser :: m ( User v) m (v)

We've added a the | m -> v annotation which reads something like "... where m determines v ". Adding this annotation allows us to avoid the Ambiguous Type errors because we've told the type system that for any given m there's only one v ; so if it knows m , (which in our case it does) then it can safely determine exactly which v to use.

Now you'll probably see something like this:

Functional dependencies conflict between instance declarations: instance MonadUserService 1 AppM -- Defined at ... instance MonadUserService 2 AppM -- Defined at ...

We told the type system there'd only be a single v for every m ; then immediately gave it two instances for AppM ; GHC caught us lying! That's okay, GHC will forgive us if we can somehow make the two m 's different! We can do this by adding a phantom type to the AppM monad which simply denotes which version we're working with; let's try editing our AppM monad like this:

newtype AppM ( v :: Nat ) a = AppM ) a { runApp :: IO a } deriving ( Functor , Applicative , Monad , MonadIO )

We've added the (v :: Nat) type argument here, it doesn't show up any where in our data, meaning it's a Phantom Type which is just there to help use denote something at the type level, in this case we denote which user version we're currently working with. Now we can add that additional info to our MonadUserService instances:

instance MonadUserService 1 ( AppM 1 ) where = return ( UserV1 "Bob Johnson" ) getUser instance MonadUserService 2 ( AppM 2 ) where = return ( UserV2 "Bob" "Johnson" ) getUser

It seems a bit redundant, but it gets us where we're going!

Not done yet! We still need to tell GHC which version of our app we want to run! You can use TypeApplications for this if you like, but the easier way is to just specify with a type annotation:

main :: IO () () = runApp ( app :: AppM 1 ()) mainrunApp (())

Try running the different versions and see what you get!

> runApp ( app :: AppM 1 ()) runApp (()) Object (fromList [( "name" , String "Bob Johnson" )]) (fromList [()]) > runApp ( app :: AppM 2 ()) runApp (()) Object (fromList [( "lastName" , String "Johnson" ),( "firstName" , String "Bob" )]) (fromList [(),()])

That should do it! We can quickly and easily switch between versions of our app by changing the type annotation; if we like we could even write some aliases to help out:

appV1 :: AppM 1 () () = app appV1app appV2 :: AppM 2 () () = app appV2app

Nice! Now we can write our app in such a way that it's generic and polymorphic over the version of user when that part of the app doesn't care which version it is, but we can still specialize to a specific user version when needed by using specific typeclasses or by pattern matching on the User constructor. The type system will guarantee that we never accidentally switch between user versions in the middle of our app; and we can defer the choice of version until the last possible second (at the top level call site). Sounds like a win to me!

Hope you learned something!

Bonus Section: Asserting Version Compatibility

If you take this pattern even further you might end up with multiple versions in your app; something like this:

AppM ( userVersion :: Nat ) ( postVersion :: Nat ) a = AppM ) () a

This works fine of course, but as the number of version parameters grows it gets tough to keep track of which versions are compatible with each other, maybe userVersion == 2 is only compatible with a postVersion >= 3 ? Here's a fun trick using ConstraintKinds and TypeFamilies to let us easily assert that our app is never run with incompatible versions:

{-# LANGUAGE TypeOperators #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE UndecidableInstances #-} import Data.Kind type family Compatible ( userVersion :: Nat ) ( postVersion :: Nat ) :: Constraint where ) ( Compatible 1 1 = () () Compatible 2 3 = () () Compatible 2 4 = () () Compatible a b = TypeError ( Text "userVersion " :<>: ShowType a a b :<>: Text " is not compatible with postVersion " :<>: ShowType b) b)

You may need to dig into this a bit on your own to understand it fully, but the basic idea is that it's a function over types which when given two versions will either result in an empty constraint (i.e. () ) which will allow compilation to continue, or will result in a failing TypeError and will print a nice error message to the user. You can use it like this:

runAppWithCheck :: Compatible userVersion postVersion => AppM userVersion postVersion a -> IO a userVersion postVersionuserVersion postVersion a -- We only really need the additional type information, under the hood we can just call `runApp` = runApp runAppWithCheckrunApp

Now if you try to run your app with incompatible versions you'll get a nice error something like:

error: • userVersion 2 is not compatible with postVersion 1 • In the expression: runAppWithCheck (app :: AppM 2 1 ()) In an equation for ‘main’: main = runAppWithCheck (app :: AppM 2 1 ()) | | main = runAppWithCheck (app :: AppM 2 1 ())

Good stuff! You can even use DataKinds to add a little structure to your version numbers so you can't accidentally mix up your userVersions with your postVersions, but I'll leave that for you to figure out 😉

Hopefully you learned something 🤞! If you did, please consider checking out my book: It teaches the principles of using optics in Haskell and other functional programming languages and takes you all the way from an beginner to wizard in all types of optics! You can get it here. Every sale helps me justify more time writing blog posts like this one and helps me to continue writing educational functional programming content. Cheers!