Implementing Clean Architecture with Haskell and Polysemy

tl;dr

This article shows how algebraic effect systems can be used to maintain a clear separation of concerns between different parts of software systems. From a practical programming perspective this improves composability and testability of software components.

I'm demonstrating this idea by using the Polysemy library to implement a multi-layered REST application conforming to the guidelines of the Clean Architecture model.

Motivation

While writing Why Haskell Matters I prepared a little demo application that was meant to showcase a cleanly designed REST application in Haskell. In particular, I wanted to demonstrate how the clear separation of pure and impure code helps to provide strict separation of concerns and state-of-the-art testability of all application layers.

I failed!

I was able to write the domain logic in pure code consisting only of total functions. It was a great pleasure to write unit tests for them!

However, as soon as I started writing controllers that coordinate access to the domain logic as well as to a persistence layer to retrieve and store data, I was stuck in the IO Monad. That is, in test cases I was not able to test the controllers independently of the concrete backend.

Then I tried to apply the final tagless pattern for the persistence layer. This allowed abstracting out the concrete persistence layer and writing controller tests with a mocked persistence backend. But when it came to testing the REST API handlers (written with Servant) I was again stuck in the IO Monad as the Handler type is defined as newtype Handler a = Handler { runHandler' :: ExceptT ServerError IO a }. Maybe it's not a principle issue but just my brain being too small...

I was desperately looking for something that allowed me to combine different types of effects (like persistence, logging, configuration, http handlers, error handling, etc.) in controllers and handlers but still to be able to write tests that allow using mocks or stubs to test components in isolation.

As I reached a dead end, I had a look at some of the algebraic effect systems available in Haskell, like eff, extensible-effects, fused-effects, freer-simple and Polysemy.

In algebraic effect systems, effectful programs are split into two separate parts: the specification of the effects to be performed, and the interpretation (or semantics) given to them.

So my idea was to provide special effect interpretations that would allow building mocked effects for my test suite.

After seeing a presentation on maintainable software architecture with Polysemy which answered many of my questions I rewrote my application based on Polysemy powered algebraic effects.

I'm pretty satisfied with the result, and of course I'm eager to share my approach with you!

The Challenge

A very small boutique restaurant (serving excellent vietnamese food) is looking for a reservation system that allows managing reservations. The restaurant has only twenty seats, they also take only a maximum of twenty reservations per day. (So guests can stay the whole evening and don't have to leave after some time.) (I adopted this scenario from a inspiring talk by Mark Seemann)

They have asked us to write the REST backend for their reservation system.

The chef insists on a scrupulously clean kitchen and is also a lover of clean code. He has read about clean architecture and wants his new software to be a perfect example!

So we cannot just hack away but first have to understand what is expected from us when we are to deliver a clean architecture.

What makes a Clean Architecture ?

I'm following the introduction to clean architecture by Robert C. Martin on his Clean Code blog. He states that his concept builds up on several earlier approaches like hexagonal architecture, ports and adapters or Onion Architecture.

According to him all these approaches share a similar objective: achieve separation of concerns by dividing a software system into different layers. All approaches result in system designs that share a common set of features:

1. The architecture does not depend on any specific software libraries or frameworks. This allows to freely choose such tools according to the actual needs. This avoids "vendor lock in".

2. High testability. The business logic can be tested without any external element like UI, DB, Web Server, etc.

3. The UI is loosely coupled to the core system. So it can be easily changed or replaced without affecting the rest of the system.

4. The Database is also "external" to the core system. It can be easily changed (even from an RDBMS to NonSQL DB) without affecting the business logic.

5. The Business logic is agnostic of the outside world. It has no dependencies to any external systems like DB, ESB, etc.

Layers with clearly separated responsibilities

The architecture consists of four layers, each of which contains components with a specific scope and a limited set of responsibilities.

1. At the centre sits the Domain layer consisting of entities and core business logic.

2. Next comes the Use Cases layer where all resources are coordinated that are required to fulfill a given use case. In particular, it uses entities and logic from the domain layer to implement use cases. But typically it must also interface to a persistent storage to retrieve and store entities.

3. The Interface Adapters layer holds code for UI controllers and presenters as well as adapters to external resources like databases, message queues, configuration, Logging, etc.

4. The External Interfaces layer contains the technical implementation of external interfaces. For example, a concrete REST service assembly, Web and UI infrastructure, databases, etc.

The Dependency Rule

The overriding rule that makes this architecture work is The Dependency Rule. This rule says that source code dependencies can only point inwards. Nothing in an inner circle can know anything at all about something in an outer circle. In particular, the name of something declared in an outer circle must not be mentioned by the code in the an inner circle. That includes, functions, classes. variables, or any other named software entity.

Quoted from Clean Architecture blog post

This dependency rule leads to a very interesting consequence: If a use case interactor needs to access a component from an outer circle, e.g. retrieve data from a database, this must be done in a specific way in order to avoid breaking the dependency rule: In the use case layer we don't have any knowledge about the components of the outer circles. If we require access to a database (or any other external resources), the call interface, as well as the data transfer protocol must be specified in the use case layer.

The components in the outer circles will then implement this interface. Using this kind of interfaces, it is possible to communicate accross the layer boundaries, but still maintain a strict separation of concerns.

If you want to dive deeper into clean architecture I recommend the Clean Architecture blog post as an entry point. Robert C. Martin later also published a whole book Clean Architecture: A Craftsman's Guide to Software Structure and Design on this concept.

In the following sections I'll explain how the clean architecture guidelines can be implemented in a Haskell REST API application by making use of the algebraic effect library Polysemy.

The Domain layer

The ReservationDomain module implements the business logic for seat reservations in a very small boutique restaurant. The restaurant has only one big table with 20 seats. Each day the restaurants accepts only 20 reservations. (There is no limited time-slot for each guest.)

Please note:

• all functions in this module are pure (they don't do any IO) and total (they produce defined results for all possible input values).

• The definitions in this module do not have dependencies to anything from the outer circles.

At the core of our Domain lies the Reservation data type:

-- | a data type representing a reservation
data Reservation = Reservation
    { date     :: Day    -- ^ the date of the reservation
    , name     :: String -- ^ the name of the guest placing the reservation
    , email    :: String -- ^ the email address of the guest
    , quantity :: Natural    -- ^ how many seats are requested
    }
    deriving (Eq, Generic, Read, Show)

This type can be used to express facts like Mr. Miller reserved two seats on 2020-06-01, he can be reached via his email address: [email protected]:

reservation = Reservation {name = "Mr. Miller", quantity = 2, date = read "2020-06-01", email = "[email protected]"}

All reservations of a specific day are represented as a list of reservations: [Reservation].

A ReservationMap is a map from Day to [Reservation]:

-- | a key value map holding a list of reservations for any given day
type ReservationMap = Map Day [Reservation]

That is, we can keep track of all reservations by maintaining them in such a map:

fromList 
  [
    (
      2020-06-01,
        [
          Reservation {date = 2020-06-01, name = "Mr. Miller", email = "[email protected]", quantity = 2}, 
          Reservation {date = 2020-06-01, name = "Andrew M. Jones", email = "[email protected]", quantity = 4}
        ]
    )
  ]

Based on these data types we can define domain logic like computing the used capacity of a list of reservations:

-- | computes the number of reserved seats for a list of reservations
usedCapacity :: [Reservation] -> Natural
usedCapacity [] = 0
usedCapacity (Reservation _ _ _ quantity : rest) = quantity + usedCapacity rest

Based on this we can compute the number of available seats (given a maximum capacity and a list of reservations):

-- | computes the number of available seats from a maximum capacity and a list of reservations.
availableSeats :: Natural-> [Reservation] -> Natural
availableSeats maxCapacity reservations = maxCapacity - usedCapacity reservations

The Reservation data type and some of the domain logic functions are depicted in the in the following diagram:

Testing

As already mentioned: this layer has no knowledge of the world and it's all pure code. Testing domain logic in isolation therefore is straight forward, as you can see from the DomainSpec code.

The data types and functions of the domain layer can be used directly, without any mocking of components:

day = fromGregorian 2020 1 29
res1 = Reservation day "Andrew M. Jones" "[email protected]" 4
res2 = Reservation day "Thomas Miller" "[email protected]" 3
reservations = [res1, res2]
totalCapacity = 20

spec :: Spec
spec =
  describe "Domain Logic" $ do
    it "computes the used capacity for an empty list of reservations" $
      usedCapacity [] `shouldBe` 0

    it "computes the used capacity for a list of reservations" $
      usedCapacity [res1, res2] `shouldBe` 7
      
    it "computes the available seats for a list of reservations" $
      availableSeats totalCapacity [res1, res2] `shouldBe` 13

The Use Case layer

The software in this layer contains application specific business rules. It encapsulates and implements all of the use cases of the system. These use cases orchestrate the flow of data to and from the entities, and direct those entities to use their enterprise wide business rules to achieve the goals of the use case.

Quoted from the Clean Architecture blog post

The module ReservationUseCase specifies the available use cases for the reservation system. It coordinates access to Effects and the actual domain logic. The module exposes service functions that will be used by the REST API in the ExternalInterfaces layer.

Implemented Use Cases:

1. Display the number of available seats for a given day

2. Enter a reservation for a given day and keep it persistent. If the reservation can not be served as all seats are occupies provide a functional error message stating the issue.

3. Display the list of reservations for a given day.

4. Delete a given reservation from the system in case of a cancellation. NO functional error is required if the reservation is not present in the system.

5. Display a List of all reservation in the system.

In the Use Case layer we have left the garden Eden of world agnostic code:

In order to compute the number of available seats for a given day, we will have to first look up the actual reservations for that day from a persistent storage, and only then can we call the domain function availableSeats. In addition we also will have to write a Log message when calling the functions to provide an audit trail.

However, the dependency rule of clean architecture bans all direct access to a database or a logging-infrastructure from the use case layer!

How can we define such a use case without violating the dependency rule?

Algebraic Effect systems offer a consistent answer:

1. We declare effects in the use case layer by defining them as an abstract interface.

2. We also specify the actual usage of effects in the use case layer by having calls against the abstract interface.

3. We provide an interpretation of these effects only in the outer layers. This also allows us to provide different implementations. So we can easily swap backends, e.g. migrating from MySQL to PostgreSQL, and it can be used to provide mock implementations for testing purposes.

Let's see how all this looks like when using Polysemy.

Usage of effects

-- | compute the number of available seats for a given day. the result must be a natural number, incl. 0
availableSeats :: (Member Persistence r, Member Trace r) => Day -> Sem r Natural
availableSeats day = do
  trace $ "compute available seats for " ++ show day
  todaysReservations <- fetch day
  return $ Dom.availableSeats maxCapacity todaysReservations

-- | fetch the list of reservations for a given day from the key value store.
--   If no match is found, an empty list is returned.
fetch :: (Member Persistence r, Member Trace r) => Day -> Sem r [Dom.Reservation]
fetch day = do
  trace $ "fetch reservations for " ++ show day
  maybeList <- getKvs day
  return $ fromMaybe [] maybeList

-- | the maximum capacity of the restaurant.
maxCapacity :: Natural
maxCapacity = 20

The type signature of availableSeats contains two constraints on the effect stack type r: (Member Persistence r, Member Trace r) This means that the function may perform two different effects: persistence via the Persistence effect and Logging via the Trace effect.

The type signature also specifies that we need an input of type Day and will return the Natural result wrapped in the Sem r monad.

The Sem monad handles computations of arbitrary extensible effects. A value of type Sem r describes a program with the capabilities of the effect stack r.

The first step of the function body of availableSeats specifies a Log action based on the (Polysemy built-in) Trace effect:

  trace $ "compute available seats for " ++ show day

I repeat: trace does not directly do any logging. The actual logging action - the effect interpretation - will be defined in the application assembly or in a test setup.

The next line specifies a lookup of the reservation list for day from the persistence layer:

  todaysReservations <- fetch day

where fetch is defined as:

fetch :: (Member Persistence r, Member Trace r) => Day -> Sem r [Dom.Reservation]
fetch day = do
  trace $ "fetch reservations for " ++ show day
  maybeList <- getKvs day
  return $ fromMaybe [] maybeList

Declaration of effects

To understand the fetch function, in particular the expression maybeList <- getKvs day we first have to know the definition of the Persistence effect:

type Persistence = KVS Day [Dom.Reservation]

Where KVS (standing for Key/Value Store) is a type that is also defined in the use case layer (KVS.hs):

-- | a key value store specified as a GADT
data KVS k v m a where
  ListAllKvs :: KVS k v m [(k, v)]
  GetKvs     :: k -> KVS k v m (Maybe v)
  InsertKvs  :: k -> v -> KVS k v m ()
  DeleteKvs  :: k -> KVS k v m ()

makeSem ''KVS

The four operations of the key value store are defined in the GADT as type constructors. makeSem ''KVS then uses TemplateHaskell to generate effect functions (or smart Constructors) from the GADT definition. This call results in the definition of the following four functions that represent the specific operations of the key value store:

listAllKvs :: Member (KVS k v) r => Sem r [(k, v)]
getKvs     :: Member (KVS k v) r => k -> Sem r (Maybe v)
insertKvs  :: Member (KVS k v) r => k -> v -> Sem r ()
deleteKvs  :: Member (KVS k v) r => k -> Sem r ()

These functions can be used in the Sem Monad. So now we understand much better what is going on in fetch:

fetch :: (Member Persistence r, Member Trace r) => Day -> Sem r [Dom.Reservation]
fetch day = do
  trace $ "fetch reservations for " ++ show day
  maybeList <- getKvs day
  return $ fromMaybe [] maybeList

As fetch operates in the Sem monad, maybeList is bound to a Maybe [Dom.Reservation] value, which results from the getKVs day action. The function finally uses fromMaybe to return a list of reservations that were retrieved (or [] in case Nothing was found for day).

Then, back in availableSeats we call the domain logic function Dom.availableSeats to compute the number of available seats. The resulting Natural value is lifted into the Sem r monad, thus matching the signature of the return type Sem r Natural.

In the next diagram I'm depicting the layers Use Cases and Domain. The arrow from Use Cases to Domain represents the dependency rule: use case code may only reference domain logic but the domain logic may not reference anything from the use case layer.

On the left side of the diagram we see the use case controllers (aka use case interactors) like availableSeats that coordinate all activities and resources to fulfill a specific use case.

On the right we see the gateway (or interface) code like the KVS abstraction of a key-value store or the fetch operation that wraps the access to the key-value store.

Interpretation of effects / Testing

The key value store functions like getKvs don't perform any concrete operation. They just declare access to an abstract key-value store interface.

The concrete interpretation of these calls will be specified in the application assembly (typically in Main.hs) or in the setup code of test cases. If we provide a pure interpretation then the resulting code will also be pure. This allows writing tests in the same pure way as for the domain logic.

As an example, in UseCasePureSpec I'm providing pure interpretations for all effects.

The runPure function takes a program with effects and handles each effect till it gets reduced to Either ReservationError (ReservationMap‚ a):

runPure :: ReservationMap 
        -> Sem '[UC.Persistence, State ReservationMap, Error UC.ReservationError, Trace] a 
        -> Either UC.ReservationError (ReservationMap, a)
runPure kvsMap program =
  program
     & runKvsPure kvsMap              -- run the key-value store on a simple ReservationMap
     & runError @UC.ReservationError  -- run error handling to produce an Either UC.ReservationError (ReservationMap, a)
     & ignoreTrace                    -- run Trace by simply ignoring all messages 
     & run                            -- run a 'Sem' containing no effects as a pure value

In addition to that I'm providing wrapping functions like runAvailableSeats that use runPure to interprete the effects of the use case functions (eg. UC.availableSeats) and extract the actual result from the [Either UC.ReservationError (ReservationMap, a)] return value:

runAvailableSeats :: ReservationMap -> Day -> Natural
runAvailableSeats kvsMap day = do
  case runPure kvsMap (UC.availableSeats day) of
    Right (_, numSeats) -> numSeats
    Left err            -> error "availableSeats failed"

This is all that it takes to abstract away persistence layer, logging facility and exception handling. We can now write tests in pure code:

-- setting up test fixtures
initReservations :: ReservationMap
initReservations = M.singleton day res

day = read "2020-05-02"
res = [Reservation day "Andrew M. Jones" "[email protected]" 4]

spec :: Spec
spec =
  describe "Reservation Use Case (only pure code)" $ do
  
    it "computes the number of available seats for a given day" $ do
      (runAvailableSeats initReservations day) `shouldBe` 16

The Interface Adapters layer

This layer holds code for adapters to external resources like databases, message queues, configuration, Logging, etc.

The Logging effect Trace ships with Polysemy, so we don't have to implement anything here. (Of course we could overzealously implement our own Graylog adapter here, Hingegen hat unser reservationServer eine Typensignatur but I leave this as an exercise for the reader... )

However, as the KVS type is our own invention we'll have to provide our own implementations. (We could have used the KVStore type from polysemy-zoo, but for didactic purposes we will roll our own.)

The following code is the in-memory implementation from the KVSInMemory module. It defines a key-value store in terms of State (Map k v) that is a Map k v in a State effect context:

runKvsOnMapState :: ( Member (State (M.Map k v)) r, Ord k) 
                 => Sem (KVS k v : r) a 
                 -> Sem r a
runKvsOnMapState = interpret $ \case
  ListAllKvs    -> fmap M.toList get
  GetKvs k      -> fmap (M.lookup k) get
  InsertKvs k v -> modify $ M.insert k v
  DeleteKvs k   -> modify $ M.delete k

So whenever the interpret functions detects a GetKvs k value, that was constructed by a call to getKvs k in the use case layer, it pattern-matches it to a Map lookup of k that is executed against state retrieved by get.

Interestingly get is a smart constructor of the State effect. This means that by interpreting the KVS we have created new effects that in turn have to be interpreted.

The runKvsPure functions (which we already have seen in the use case testing) chains interpretation of the effects KVS and State and thus allows us to work with pure Maps as mocks for a key-value store:

runKvsPure :: Ord k 
           => M.Map k v
           -> Sem (KVS k v : State (M.Map k v) : r) a 
           -> Sem r (M.Map k v, a)
runKvsPure map = runState map . runKvsOnMapState

A key-value store with a SQLite backend.

As we are in the interface adapters layer, we are allowed to get our hands dirty with real world code, like database access. As an example I have provided a SQLite based interpretation of the KVS effect in KVSSqllite.hs.

The effect interpreting function is runKvsAsSQLite:

-- | Run a KVStore effect against a SQLite backend. Requires a Config object as input.
runKvsAsSQLite :: (Member (Embed IO) r, Member (Input Config) r, Member Trace r, Show k, Read k, ToJSON v, FromJSON v)
                   => Sem (KVS k v : r) a
                   -> Sem r a
runKvsAsSQLite = interpret $ \case
  GetKvs k      -> getAction k
  ListAllKvs    -> listAction
  InsertKvs k v -> insertAction k v
  DeleteKvs k   -> deleteAction k

The function's type signature introduces a two more constraints on the effect stack type r: Member (Embed IO) r and Member (Input Config) r. (Embed IO) is needed as accessing SQLite will require IO, which can be lifted into the Sem r monad with Embed IO.

SQLite always needs a file name to create a database connection. As we want to be able to keep this name configurable, we use the (Input Config) effect. Config is a data type that I created to represent global application configuration, including the database file name. Input is a Polysemy built-in effect which can provide input to an application, quite similar to a Reader monad.

These effects are introduced by the actual implementations of the KVS constructors, like getAction k, which retrieves a value from the database by looking up the key k:

getAction :: (Member (Input Config) r, Member (Embed IO) r, Member Trace r, Show k, Read k, ToJSON v, FromJSON v) => k -> Sem r (Maybe v)
getAction key = do
  conn <- connectionFrom input
  rows <- embed (SQL.queryNamed conn
                      "SELECT key, value FROM store WHERE key = :key"
                      [":key" := show key] :: IO [KeyValueRow])
  trace $ "get: " ++ show rows
  case rows of
    []                          -> return Nothing
    (KeyValueRow _key value):xs -> return $ (decode . encodeUtf8) value

-- | create a connection based on configuration data, make sure table "store" exists.
connectionFrom :: (Member (Embed IO) r) => Sem r Config -> Sem r SQL.Connection
connectionFrom c = do
  config <- c
  embed (getConnection (dbPath config))
    where
      getConnection :: FilePath -> IO SQL.Connection
      getConnection dbFile = do
        conn <- SQL.open dbFile
        SQL.execute_ conn "CREATE TABLE IF NOT EXISTS store (key TEXT PRIMARY KEY, value TEXT)"
        return conn

Let's have a closer look at what is going on in getAction:

First connectionFrom input is used to create a database connection based on the Config object obtained by input (the smart Constructor of the Input effect). The Config type contains a field dbPath which is read and used to create the connection with getConnection. As this is an IO operation we have to use embed to lift it into the Sem r monad.

In the second step SQL.queryNamed is used to perform the actual select statement against the db connection. Again embed must be used to lift this IO operation.

Finally the resulting [KeyValueRow] list is pattern matched: if the list is empty Nothing is returned. Otherwise Aeson.decode is called to unmarshal a result value from the JSON data retrieved from the database.

The JSON encoding and decoding to and from the DB is the reason for the ToJSON v, FromJSON v constraints on the value type v.

This implementation is inspired by key-value store of a password manager in Polysemy.

Declaring the REST API

Our task was to build the backend for the reservation system. We will have to implement a REST API to allow access to the business logic that we defined in the use case layer.

The overall idea is to provide a REST route for all exposed functions of the ReservationUseCase. The following table shows the mapping of those functions to the REST routes that we want to achieve:

listAll        GET    /reservations
fetch          GET    /reservations/YYYY-MM-DD
tryReservation POST   /reservations
cancel         DELETE /reservations
availableSeats GET    /seats/YYYY-MM-DD

I'm using Servant to define our REST API. The great thing about Servant is that it allows us to define REST APIs in a typesafe manner by using a type level DSL.

Here comes the declaration of our API (please note that we declare our routes to accept and emit data in JSON format):

-- | in order to allow JSON serialization for the Dom.Reservation type, it must instantiate FromJSON and ToJSON.
instance ToJSON Dom.Reservation
instance FromJSON Dom.Reservation

-- | Declaring the routes of the REST API for Restaurant Reservations
type ReservationAPI =
       "reservations" :> Summary "retrieve a map of all reservations (Day -> [Reservation])"
                      :> Get     '[ JSON] Dom.ReservationMap -- GET    /reservations

  :<|> "reservations" :> Summary "retrieve list of reservations for a given day"
                      :> Capture "day" Day
                      :> Get     '[ JSON] [Dom.Reservation]  -- GET    /reservations/YYYY-MM-DD

  :<|> "reservations" :> Summary "place a new reservation"
                      :> ReqBody '[ JSON] Dom.Reservation
                      :> Post    '[ JSON] ()                 -- POST   /reservations

  :<|> "reservations" :> Summary "cancel a reservation"
                      :> ReqBody '[ JSON] Dom.Reservation
                      :> Delete  '[ JSON] ()                 -- DELETE /reservations
                      
  :<|> "seats"        :> Summary "retrieve number of free seats for a given day"
                      :> Capture "day" Day
                      :> Get     '[ JSON] Natural                -- GET    /seats/YYYY-MM-DD

Next we have to create the connection between the declared routes and the actual business logic. This will be our REST service implementation. In our case we simply delegate to the use case controller functions. Off course, we might also implement additional functionality here like validation:

import qualified UseCases.ReservationUseCase as UC 

-- | implements the ReservationAPI
reservationServer :: (Member UC.Persistence r, Member (Error UC.ReservationError) r, 
                      Member Trace r, Member (Input Config) r) => ServerT ReservationAPI (Sem r)
reservationServer =
        UC.listAll        -- GET    /reservations
  :<|>  UC.fetch          -- GET    /reservations/YYYY-MM-DD
  :<|>  UC.tryReservation -- POST   /reservations
  :<|>  UC.cancel         -- DELETE /reservations
  :<|>  UC.availableSeats -- GET    /seats/YYYY-MM-DD

I really love how declarative this code is. We don't have to tell how to exchange data between the REST server and the use case controllers.

We just tell what we want: a mapping from the routes to the controller functions. That's all!

In the following diagram, we now see the third layer. Again, the arrow symbolises the dependency rule, which prohibits access from domain or use case layer to the interface adapters layer. To the right we see the ReservationAPI and its reservationServer implementation, which we just explored. They interact with the use case controller functions like availableSeats, listAll, etc.

To the left we see the interpretations of the KVS effect (which was defined in the use case layer): KVSInMemory, KVSSqlite (and a third one KVSFileServer, a file based implementation which you could explore on your own).

Testing the KVS implementations

We'll have a closer look at the test of the SQLite implementation of the KVS effect.

As Polysemy effects are involded we will need to provide an interpretation to actually perform the SQLLite operation.

The test setup looks quite similar to the tests in the use case layer.

We want our test to evaluate the KVS implementation independently of the domain logic and the use case layer. Therefore, we first define an example use case, featuring a data type Memo and a set of typical CRUD operations. The CRUD operations are using the KVS smart constructors and thus exhibit the typical Polysemy effect signatures:

-- | a key value table mapping Natural to a list of Strings
type KeyValueTable = KVS Int [String]

data Memo = Memo Int [String]
    deriving (Show)

persistMemo :: (Member KeyValueTable r)  => Memo -> Sem r ()
persistMemo (Memo id lines ) = insertKvs id lines

fetchMemo :: (Member KeyValueTable r) => Int -> Sem r (Maybe [String])
fetchMemo = getKvs

fetchAll :: (Member KeyValueTable r) => Sem r (M.Map Int [String])
fetchAll = fmap M.fromList listAllKvs

deleteMemo :: (Member KeyValueTable r)  => Int -> Sem r ()
deleteMemo = deleteKvs

Next we define a set of helper functions that allow us to execute the CRUD operations as ordinary IO () actions, which we can use in our test code:

-- Helper functions for interpreting all effects in IO
runPersist :: Memo -> IO ()
runPersist memo = runAllEffects (persistMemo memo)

runFetch :: Int -> IO (Maybe [String])
runFetch k = runAllEffects (fetchMemo k)

runFetchAll :: IO (M.Map Int [String])
runFetchAll = runAllEffects fetchAll

runDelete :: Int -> IO ()
runDelete k = runAllEffects (deleteMemo k)

These wrapper function make use of the runAllEffects function that takes a program with effects and handles each effect till it gets reduced to IO a:

runAllEffects :: Sem '[KeyValueTable, Input Config, Trace, Embed IO] a -> IO a 
runAllEffects program =
  program
    & runKvsAsSQLite       -- use SQLite based interpretation of the (KVS Int [String]) effect
    & runInputConst config -- use the variable config as source for (Input Config) effect
    & ignoreTrace          -- ignore all traces
    & runM                 -- reduce Sem r (Embed IO a) to IO a
  where config = Config {port = 8080, dbPath = "kvs-test.db", backend = SQLite, verbose = False}

-- errors are rethrown as Runtime errors, which can be verified by HSpec.
handleErrors :: IO (Either err a) -> IO a
handleErrors e = do
  either <- e
  case either of
    Right v -> return v
    Left _  -> error "something bad happend"

With these preliminaries at hand we can now write our test cases:

key = 4711
text = ["In the morning", "I don't drink coffee", "But lots of curcuma chai."]
memo = Memo key text

spec :: Spec
spec =
  describe "The KV Store SQLite Implementation" $ do
    it "returns Nothing if nothing can be found for a given id" $ do
      maybeMatch <- runFetch key
      maybeMatch `shouldBe` Nothing

    it "persists a key-value pair to the SQLite database" $ do
      runPersist memo
      maybeMatch <- runFetch key
      maybeMatch `shouldBe` Just text

    it "fetches a Map of all key-value entries from the KV store" $ do
      map <- runFetchAll
      M.size map `shouldBe` 1

    it "deletes an entry from the key value store" $ do
      runDelete key
      maybeMatch <- runFetch key
      maybeMatch `shouldBe` Nothing

Testing the REST API

The actual code for testing the REST API looks pretty straightforward. We create a WAI Application instance with createApp and execute REST operations like get and postJSON against it:

reservationData :: LB.ByteString
reservationData = "{\"email\":\"[email protected]\",\"quantity\":10,\"date\":\"2020-05-02\",\"name\":\"Amelia Jones\"}"

postJSON   path = request methodPost   path [(hContentType, "application/json")]
deleteJSON path = request methodDelete path [(hContentType, "application/json")]

spec :: Spec
spec =
  with (createApp) $
    describe "Rest Service" $ do
      it "responds with 200 for a call GET /reservations " $
        get "/reservations" `shouldRespondWith` "{\"2020-05-02\":[{\"email\":\"[email protected]\",\"quantity\":4,\"date\":\"2020-05-02\",\"name\":\"Andrew M. Jones\"}]}"
      it "responds with 200 for a valid POST /reservations" $
        postJSON "/reservations" reservationData `shouldRespondWith` 200
      it "responds with 412 if a reservation can not be done on a given day" $
        (postJSON "/reservations" reservationData >> postJSON "/reservations" reservationData) `shouldRespondWith` 412
      it "responds with 200 for a valid DELETE /reservations" $
        deleteJSON "/reservations" reservationData `shouldRespondWith` 200

Please note that these tests don't need a deployment of the WAI application to a web server. ALl testing can be done within a single process. We stick to the dependency rule not to use anything from a more outward layer.

The interesting part is the creation of the Application instance.

If we had a simple implementation myServer of a REST API myApi, not using any Polysemy effects, we could create an Application instance like so:

createSimpleApp :: Application
createSimpleApp ::= serve myApi myServer

In contrast, our reservationServer has a type signature that contains Polysemy effects:

reservationServer :: (Member UC.Persistence r, Member (Error UC.ReservationError) r, 
                      Member Trace r, Member (Input Config) r) => ServerT ReservationAPI (Sem r)

Instead of building the Application instance directly, as in the simple example, we use liftServer to lift reservationServer into the required ServerT ReservationAPI Handler type by running all effects and by lifting the business logic exception ReservationNotPossible into a Servant ServerError. This time we also use the SQLite based interpretation of the KVS effect:

createApp :: Config -> IO Application
createApp config = return $ serve reservationAPI (liftServer config)

liftServer :: Config -> ServerT ReservationAPI Handler
liftServer config = hoistServer reservationAPI (interpretServer config) reservationServer
  where
    interpretServer config sem =
      sem
        & runKvsAsSQLite
        & runInputConst config
        & runError @ReservationError
        & ignoreTrace
        & runM
        & liftToHandler
    liftToHandler = Handler . ExceptT . (fmap handleErrors)
    handleErrors (Left (ReservationNotPossible msg)) = Left err412 {errBody = pack msg}
    handleErrors (Right value) = Right value

The External Interfaces layer

The outermost layer is generally composed of frameworks and tools such as the Database, the Web Framework, etc. Generally you don’t write much code in this layer other than glue code that communicates to the next circle inwards.

This layer is where all the details go. The Web is a detail. The database is a detail. We keep these things on the outside where they can do little harm.

Quoted from Clean Architecture blog post

For the database we are already finished as the SQlite-Simple library includes the SQLLite C runtime library and is thus self-contained.

We will use WARP as our Web Server, which can be used as a library within our Main program. What we still have to do though, is to assemble a Servant web Application so that it can be executed on the warp server.

We have done this step already for the testing of the REST service. The createApp function that we define in the ApplicationAssembly module will look quite familiar, it just provides some more bells and whistles to integrate all the features that we have developed so far.

• createApp accepts a Config parameter which is used to configure application settings.
• selectKvsBackend selects the concrete KVS interpretation.
• selectTraceVerbosity selects the Trace interpretation:

-- | creates the WAI Application that can be executed by Warp.run.
createApp :: Config -> IO Application
createApp config = do
  return (serve reservationAPI $ hoistServer reservationAPI (interpretServer config) reservationServer)
  where
    interpretServer config sem  =  sem
      & selectKvsBackend config
      & runInputConst config
      & runError @ReservationError
      & selectTraceVerbosity config
      & runM
      & liftToHandler
    liftToHandler = Handler . ExceptT . (fmap handleErrors)
    handleErrors (Left (ReservationNotPossible msg)) = Left err412 { errBody = pack msg}
    handleErrors (Right value) = Right value

-- | can select between SQLite or FileServer persistence backends.
selectKvsBackend :: (Member (Input Config) r, Member (Embed IO) r, Member Trace r, Show k, Read k, ToJSON v, FromJSON v)
                 => Config -> Sem (KVS k v : r) a -> Sem r a
selectKvsBackend config = case backend config of
  SQLite     -> runKvsAsSQLite
  FileServer -> runKvsAsFileServer
  InMemory   -> error "not supported"

-- | if the config flag verbose is set to True, trace to Console, else ignore all trace messages
selectTraceVerbosity :: (Member (Embed IO) r) => Config -> (Sem (Trace : r) a -> Sem r a)
selectTraceVerbosity config =
  if verbose config
    then traceToIO
    else ignoreTrace

The application assembly also features a function to load a Config instance. Typically, this would involve loading a configuration file or reading command line arguments. We take a shortcut here and just provide a static instance:

-- | load application config. In real life, this would load a config file or read commandline args.
loadConfig :: IO Config
loadConfig = return Config {port = 8080, backend = SQLite, dbPath = "kvs.db", verbose = True}

With the whole application assembly written as library code, there is not much left to do in the Main module:

import           ExternalInterfaces.ApplicationAssembly (createApp, loadConfig)
import           InterfaceAdapters.Config
import           Network.Wai.Handler.Warp               (run)

main :: IO ()
main = do
  config <- loadConfig
  app    <- createApp config
  putStrLn $ "Starting server on port " ++ show (port config)
  run (port config) app

The following diagram shows the elements added by the External Interface layer:

• On the left we have application assembly code like createApp used by the Warp server or some of the different runPure functions that we used in HSpec tests.
• On the right we have the SQLite runtime library that provides access to the SQLite database and the Haskell runtime in general, which provides access to the filesystem and the OS in general.

Testing

Testing the application assembly is quite straightforward and resembles the testing of the REST service:

loadConfig :: IO Config
loadConfig = return Config {port = 8080, backend = SQLite, dbPath = "kvs-assembly.db", verbose = False}

spec :: Spec
spec =
  with (loadConfig >>= createApp) $
    describe "Rest Service" $ do

      it "responds with 20 for a first call to GET /seats/YYYY-MM-DD" $
        get "/seats/2020-05-02" `shouldRespondWith` "20"

      it "responds with 200 for a valid POST /reservations" $
        postJSON "/reservations" reservationData `shouldRespondWith` 200

      it "responds with 200 for a call GET /reservations " $
        get "/reservations" `shouldRespondWith` "{\"2020-05-02\":[{\"email\":\"[email protected]\",\"quantity\":12,\"date\":\"2020-05-02\",\"name\":\"Amelia Jones\"}]}"

      it "responds with 412 if a reservation can not be done on a given day" $
        (postJSON "/reservations" reservationData >> postJSON "/reservations" reservationData) `shouldRespondWith` 412

      it "responds with 20 for a first call to GET /seats/YYYY-MM-DD" $
        get "/seats/2020-05-02" `shouldRespondWith` "8"

      it "responds with 200 for a valid DELETE /reservations" $
        deleteJSON "/reservations" reservationData `shouldRespondWith` 200

Swagger Documentation

For all those who have been patient enough to stay with me until here, I now have a little bonus.

There is a servant-swagger-ui addon available which allows to serve a SwaggerDoc UI for any Servant API. This UI renders an automatically generated documentation of our Reservation API and even allows to test all API operations directly.

You can launch it by executing stack build --exec PolysemyCleanArchitecture in the root folder of the project.

This will launch the REST service and open up the Swagger UI in your Web browser:

The code for this goody can be found in the SwaggerUI module.

Conclusion

Robert C. Martin concludes his blog post with a brief summary:

Conforming to these simple rules is not hard, and will save you a lot of headaches going forward. By separating the software into layers, and conforming to The Dependency Rule, you will create a system that is intrinsically testable, with all the benefits that implies. When any of the external parts of the system become obsolete, like the database, or the web framework, you can replace those obsolete elements with a minimum of fuss.

Quoted from the Clean Architecture blog post

I have emphasized the testability aspect quite a lot in this article. However, this approach allows switching freely between alternative backends in production environments as well.

As we have seen Polysemy — or algebraic effect systems in general — make this possible by the separation of effect declaration, effect usage and effect interpretation.

Furthermore, Polysemy also allows you to freely combine several effects. This is a huge gain in software composability.