# Pastebin JWvF4erR
-- 01. Background:
class AWSRequest req where
  type AWSResponse req :: Type
  -- Other stuff

-- Requests which returns paged results that we know how to stream
class AWSRequest req => AWSPager req where
  -- Stuff

-- Amazonka has a bunch of functions which take an `Env` and use it to
-- make requests:
sendEither :: (AWSRequest req, MonadResource m) => Env -> req -> m (Either Error (AWSResponse req))
paginate :: (AWSPager req, MonadResource m) => Env -> req -> ConduitT Void (AWSResponse req) m ()
within :: Region -> Env -> Env



-- 02. The Problem:
-- Abstract over these operations in a way that allows useful alternative implementations (e.g., mocks for testing)



-- 03. Solution 1: operations inside typeclass:
class Monad m => MonadAmazonka m where
  sendEither :: AWSRequest req => req -> m (Either Error (AWSResponse req))
  paginate :: AWSPager req => req -> ConduitT Void (AWSResponse req) m
  within :: Region -> m a -> m a

newtype AmazonkaT m a = AmazonkaT (ReaderT Env m a)
instance MonadResource m => MonadAmazonka (AmazonkaT m)

-- Problem: Return type of `paginate` prevents GeneralizedNewtypeDeriving from working.



-- 04. Solution 2: Env inside typeclass:
class Monad m => MonadAmazonka m where
  askEnv :: m Env
  localEnv :: (Env -> Env) -> m a -> m a

sendEither :: (AWSRequest req, MonadResource m) => req -> m (Either Error (AWSResponse req))

-- Problem: Mocking doesn't really work as you have all these new constraints on your `m`



-- 05. Solution 3: class provides a lift from a base monad:
class Monad m => MonadAmazona m where
  liftAmazonka :: Amazonka a -> m a

newtype AmazonkaT m a = AmazonkaT (ReaderT Env m a)
type Amazonka = AmazonkaT (ResourceT IO)
instance MonadAmazonka Amazonka

-- Problem: Mocking is completely impossible since you can't inspect the actual computation.



-- 06. Solution 4: use a GADT/free monad/homomorphism typeclass.
-- Inspired by: https://blog.ocharles.org.uk/posts/2017-08-23-extensible-effects-and-transformers.html

data AmazonkaCall a where
  SendEither :: AWSRequest a => a -> AmazonkaCall (Either Error (AWSResponse a))
  -- etc

class MonadAmazonka m where
  liftAmazonka :: Free (Coyoneda AmazonkaCall) a -> m a

-- Problem: It is not possible to write operations like `within` as you get values of type `Free (Coyoneda AmazonkaCall a)` in negative position if you try.