Configuring the Dispatcher

The dispatcher is a special main function that checks which handler it has to run, and runs it.

This is very easy to do in aws-lambda-haskell-runtime. Just use the runLambdaHaskellRuntime function.

runLambdaHaskellRuntime ::
  RuntimeContext handlerType m context request response error =>
  DispatcherOptions ->
  IO context ->
  (forall a. m a -> IO a) ->
  HandlersM handlerType m context request response error () ->
  IO ()

It may seem like an intimidating type signature, but let's go through each parameter and see what it means.

• DispatcherOptions are the configuration options of the dispatcher function. You can just use defaultDispatcherOptions.
• The IO context action is how you initialize the context object.
• (forall a. m a -> IO a) is used when you have your handler in a custom monad. It transforms that custom monadic action into IO. If your handlers run in IO, just use id.
• HandlersM handlerType m context request response error () is the action that registers the handlers under a given name.

For the person validator example we set up in the previous section, it will look like this:

import Aws.Lambda
import qualified Lib

main :: IO ()
main =
  runLambdaHaskellRuntime
    defaultDispatcherOptions
    (pure ())
    id $ do
      -- You could also register multiple handlers
      addStandaloneLambdaHandler "handler" Lib.handler