Configuration of a Polysemy application

Posted on July 17, 2022 by Thomas Mahler

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This is yet another sequel to my Clean Architecture with Haskell and Polysemy article.

In my last blog post I demonstrated how the actual hosting of a WAI web app (e.g. with the Warp server or on AWS Lambda) can be defined as a Polysemy effect. The idea was well received on reddit/r/haskell and also stirred some further discussions. One question that came up: why did I explicitly load configuration before starting the Polysemy effect interpreter? Wouldn’t it be much more in line with the overall idea of my Polysemy Clean Architecture to handle this loading also as an effect?

Here is my original code:

main :: IO ()
main = do
  config <- loadConfig       -- load config BEFORE starting the Polysemy effect interpreter
  serveAppFromConfig config  -- create app from config and run it via AppServer effect
    & runWarpAppServer       -- use Warp to run rest application
    & runM

I explained my reasons for this design but promised to look for a better solution.

In the following I’ll explain the new design that I came up with.

The ideas behind my original design

The central reason for explicitely loading the configuration before starting the polysemy interpreter was that the configuration config is needed to select effect interpreter functions in the assembly of the WAI web app. To better understand this we’ll drill down the execution sequence starting from runWarpAppServer effect handler:

runWarpAppServer :: (Member (Embed IO) r) => Sem (AppServer : r) a -> Sem r a
runWarpAppServer = interpret $ \case
  -- serving an application by constructing it from a config
  ServeAppFromConfig config ->
    embed $
      let app = createApp config
       in do 
        putStrLn $ "starting Warp on Port " ++ show (port config) (port config) app

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

liftServer :: Config -> ServerT ReservationAPI Handler
liftServer config = hoistServer reservationAPI (interpretServer config) reservationServer
    interpretServer :: (Show k, Read k, ToJSON v, FromJSON v)
                    => Config -> Sem '[KVS k v, Input Config, Trace, Error ReservationError, Embed IO] a -> Handler a
    interpretServer config sem  =  sem
      & runSelectedKvsBackend config
      & runInputConst config
      & runSelectedTrace config
      & runError @ReservationError
      & runM
      & liftToHandler

    liftToHandler :: IO (Either ReservationError a) -> Handler a
    liftToHandler = Handler . ExceptT . fmap handleErrors

    handleErrors :: Either ReservationError b -> Either ServerError b
    handleErrors (Left (ReservationNotPossible msg)) = Left err412 { errBody = pack msg}
    handleErrors (Right value) = Right value

The interesting point here is the usage of the Config parameter config in interpretServer. It is used to select concrete effect handler functions in runSelectedKvsBackend conf and runSelectedTrace conf:

-- | global application configuration
data Config = Config {
  port        :: Int     -- ^ the port where the server is listening
, backend     :: Backend -- ^ selects the persistence backend for the KV store
, dbPath      :: String  -- ^ the path to the database
, verbose     :: Bool    -- ^ True enables logging
} deriving (Show, Eq, Read)

data Backend = SQLite | FileServer deriving (Show, Eq, Read)

-- | can select between SQLite or FileServer persistence backends.
runSelectedKvsBackend :: (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
runSelectedKvsBackend config = case backend config of
  SQLite     -> runKvsAsSQLite
  FileServer -> runKvsAsFileServer
-- | if the config flag verbose is set to True, trace to Console, else ignore all trace messages
runSelectedTrace :: (Member (Embed IO) r) => Config -> (Sem (Trace : r) a -> Sem r a)
runSelectedTrace config =
  if verbose config
    then traceToStdout
    else ignoreTrace

In addition to this config is also used by the Input effect handler runInputConst conf:

-- | Run an 'Input' effect by always giving back the same value.
runInputConst :: i -> Sem (Input i ': r) a -> Sem r a
runInputConst c = interpret $ \case
  Input -> pure c

This allows effect handlers like runKvsAsSQLite to use config as configuration. For example to obtain the database connection:

import           Polysemy
import           Polysemy.Input                 (Input, input)
import           Polysemy.Trace                 (Trace, trace)

-- | delete a value from db identified by key
deleteAction :: (Member (Input Config) r, Member (Embed IO) r, Member Trace r, Show k) => k -> Sem r ()
deleteAction key = do
  trace $ "deleteAction: " ++ show key
  conn <- connectionFrom input
  embed $ SQL.executeNamed conn "DELETE FROM store WHERE key = :key" [":key" := show key]

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

My reasoning was as follows: As I needed config as an explicit parameter to serveAppFromConfig in the main glue code and loadConfigbeing of type IO Config, I thought I had to explicitely execute it in the IO Monad and then handing it into the Polysemy effect chain:

main :: IO ()
main = do
  config <- loadConfig       -- load config
  serveAppFromConfig config  -- create app from config and run it via AppServer effect
    & runWarpAppServer       -- use Warp to run rest application
    & runM

But it turned out that I had just not thought things through deep enough!

Chaining of Config loading and application execution as effects

If we take a step back and look at the code in the UseCases package we’ll see that we already have shown how to combine different effects into a sequence within the Polysemy Sem Monad. Take for example the following use case implementation:

cancel :: (Member Persistence r, Member Trace r)  => Dom.Reservation -> Sem r ()
cancel res@(Dom.Reservation date _ _ _) = do
  trace $ "deleting reservation " ++ show res
  reservations <- fetch date
  trace $ "before: " ++ show reservations
  let after = Dom.cancelReservation res reservations
  trace $ "after: " ++ show after
  insertKvs date after

So instead of glueing stuff together in main :: IO (), wouldn’t it be much more in line with our overall intention to formulate the sequencing of configuration loading and hosting the WAI application as a sequence of Polysemy effects? For example:

-- | load configuration via ConfigProvider effect, then contruct and run app via AppServer effect
configureAndServeApp ::  (Member ConfigProvider r, Member AppServer r)  => Sem r ()
configureAndServeApp = do
  config <- getConfig
  serveAppFromConfig config

In this function we use two effects ConfigProvider and Appserver. I already described the AppServer effect in my previous blog post. So we only have to consider the ConfigProvider effect here.

Defining a ConfigProvider Effect

First we define the ConfigProvider Effect. It provides an effect function getConfig :: Member ConfigProvider r => Sem r Config:

{-# LANGUAGE TemplateHaskell #-}

module ExternalInterfaces.ConfigProvider where

import InterfaceAdapters.Config
import Polysemy

-- | The ConfigProvider effect can be used to provide and application with a Config instance.
data ConfigProvider m a where
  GetConfig :: ConfigProvider m Config

-- makeSem uses TemplateHaskell to generate effect functions (or smart Constructors) from the GADT definition:
-- getConfig :: Member ConfigProvider r => Sem r Config
makeSem ''ConfigProvider

Implementing the ConfigProvider effect

Next we define an implementation of the ConfigProvider effect by defining an effect handler function that loads a Config instance from a file:

module ExternalInterfaces.FileConfigProvider where

import InterfaceAdapters.Config
import ExternalInterfaces.ConfigProvider
import Polysemy (Embed, Member, Sem, embed, interpret)

-- | provides a Config object from a local file path
runFileConfigProvider :: (Member (Embed IO) r) => FilePath -> Sem (ConfigProvider : r) a -> Sem r a
runFileConfigProvider path = interpret $ \case
  GetConfig -> embed $ loadConfig path

-- | load application config from file "application.config"
loadConfig :: FilePath -> IO Config
loadConfig path = do
  input <- readFile path
  pure $ read input

The new and shining main function

Now we have all the ingredients ready to clean up the glue code in main :: IO ():

main :: IO ()
main = do
    & runFileConfigProvider "application.config"  -- provide Config from a file
    & runWarpAppServer                            -- use Warp to run rest application
    & runM

The complete control of the application is now exclusively managed by the Polysemy effect library.


I’m excited about how the comments on my last blog post have helped me develop the Polysemy Clean Architecture idea into a much improved design.

So I’m sure that this post will trigger some more discussions and will help to improve remaining grey spots in the overall concept.