Anti-Pattern

Tight Coupling, low Information Density

createUser :: ApiaryC c =>
    Bool
    -> NewUserData
    -> ReaderT c (ExceptT AppError (LoggingT IO)) User
createUser sendPasswordSet dta = do
    $(logDebug) $ "Creating new user " <> tShow dta
    P.guardEmailAvailable (newUserEmail dta)
    maybe (pure ()) P.guardPhoneAvailable (newUserPhone dta)
    usr <- P.createUser dta
    when sendPasswordSet (genAndSendInitialPasswordSet usr)
    pure usr

                            

Anti-Pattern

What does this signature tell you about the function?

                                
ApairyC c =>
    Bool
    -> NewUserData
    -> ReaderT c (ExceptT Err (LoggingT IO)) Foo
                                
                            

Capabilities?

What can't it do?

What about errors?

Function Capabilities

As APIs

• Guides implementations
• Easy extensibility
• Testing via mocks/ pure implementations
• Improves readability

Function Capabilities

An Example

type ConfCryptM m ctx = ReaderT (ConfCryptFile, ctx) (
    ExceptT ConfCryptError (
        ResourceT m))

class (Monad m, MonadError ConfCryptError m) =>
    MonadDecrypt m k where
    decryptValue :: k -> T.Text -> m T.Text

class Monad m => Command a m where
    evaluate :: a -> m [Text]

data ReadConfCrypt = ReadConfCrypt
instance (Monad m, MonadDecrypt (ConfCryptM m key) key) =>
    Command ReadConfCrypt (ConfCryptM m key) where
                        
                        

Function Capabilities

Guiding our Implementation...

runConfCrypt ::
    ConfCryptFile
    -> ConfCryptM IO () a
    -> IO (Either ConfCryptError [T.Text])

case parsedArguments of ->
    RC KeyAndConf {key, provider} ->
        runConfCrypt parsedConfiguration $
            runWithDecrypt key provider ReadConfCrypt

                        
                        

Extending The Program

Initial RSA Algorithm

Extending The Program

Oh no, KMS Support??!!