From Modules to App

The App type in Network.ABCI.Server is defined as

newtype App m = App
  { unApp :: forall (t :: MessageType). Request t -> m (Response t) }

and ultimately our configuration of modules must be converted to this format. This is probably the most important part of the SDK, to provide the bridge between the list of modules - a heterogeneous list of type ModuleList - and the actual application. The type that provides the input for this bridge is HandlersContext:

data HandlersContext alg ms core = HandlersContext
  { signatureAlgP :: Proxy alg
  , modules       :: M.ModuleList ms (Effs ms core)
  , anteHandler   :: BA.AnteHandler (Effs ms core)
  , compileToCore :: forall a. Sem (BA.BaseAppEffs core) a -> Sem core a


  • alg is the signature schema you would like to use for authentication (e.g. Secp256k1)
  • ms is the type level list of modules
  • r is the global effects list for the application
  • core is the set of core effects that are used to interpret BaseApp to IO.
  • Effs is a type family that gathers the effect dependencies for ms in the appropriate order.

We should say a few words on this compileToCore field. The application developer has to, at the end of the day, specify how the entire effects system for the application will be interpreted to IO. Luckily most of these decisions are abstracted away, but the one that remains is dealing with BaseApp core. The sdk provides two default methods for two different types of core:

  :: forall a.
     Sem (BaseApp CoreEffs) a
  -> Sem CoreEffs a

  :: forall a.
     Sem (BaseApp PureCoreEffs) a
  -> Sem PureCoreEffs a

The difference is that defaultCompileToCore uses the IAVL store external database and also allows for metrics, where defaultCompileToPureCore uses an in-memory db and treats all metrics operations as a no-op.


module Tutorial.Nameservice.Application where

import Data.Proxy
import Nameservice.Modules.Nameservice (Nameservice, nameserviceModule)
import Network.ABCI.Server.App (App)
import Polysemy (Sem)
import Tendermint.SDK.Modules.Auth (Auth, authModule)
import Tendermint.SDK.Application (ModuleList(..), HandlersContext(..), baseAppAnteHandler, makeApp, createIOApp)
import Tendermint.SDK.BaseApp (CoreEffs, Context, defaultCompileToCore, runCoreEffs)
import Tendermint.SDK.Crypto (Secp256k1)
import Tendermint.SDK.Modules.Bank (Bank, bankModule)

At this point we need to simply list the modules we want to use in our application. We only require that if a module is declared as a dependency by another (via the deps type variable in the Module type), then that dependency should be inserted below that module. For example, since Nameservice depends on Bank, we must list Bank after Nameservice. Similarly since Bank depends on Auth, we must list Auth after Bank:

type NameserviceModules =
   '[ Nameservice
    , Bank
    , Auth

We're now ready to define the HandlersContext for our application:

handlersContext :: HandlersContext Secp256k1 NameserviceModules CoreEffs
handlersContext = HandlersContext
  { signatureAlgP = Proxy @Secp256k1
  , modules = nameserviceModules
  , compileToCore  = defaultCompileToCore
  , anteHandler = baseAppAnteHandler
  nameserviceModules =
    :+ bankModule
    :+ authModule
    :+ NilModules

Finally we're able to define our application that runs in the CoreEffs context defined in the SDK:

app :: App (Sem CoreEffs)
app = makeApp handlersContext

Since the ABCI server requires you to pass a value of type App IO, we have one more transformation to perform to get the Sem CoreEffs in our app. We can simple use the createIOApp function:

makeIOApp :: Context -> App IO
makeIOApp ctx = createIOApp (runCoreEffs ctx) app