Is it safe to derive MonadThrow, MonadCatch, MonadBaseControl, MonadUnliftIO, etc?

问题

I'm refactoring some old code, which is in a polymorphic, but type-class constrained, monad:

class ( MonadIO m
      , MonadLogger m
      , MonadLoggerIO m
      , MonadThrow m
      , MonadCatch m
      , MonadMask m
      , MonadBaseControl IO m
      , MonadUnliftIO) => HasLogging m where

In the older code the application's main monad was...

type AppM = ReaderT Env IO

...which will now change to...

newtype AppM (features :: [FeatureFlag]) a = AppM (ReaderT Env IO a)
  deriving (Functor, Applicative, Monad, MonadReader Env, MonadIO)

Given this context, is it safe to derive the following, automatically:

MonadThrow
MonadCatch
MonadMask
MonadBaseControl
MonadUliftIO

Without getting into GHC internals, what's the best way to develop intuition about what's actually happening when the compiler derives things automagically?

回答1:

The user manual has documentation about every extension, and it keeps getting better; here's the section on deriving, that should be sufficient to know what's actually happening: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#extensions-to-the-deriving-mechanism

In this case, all those classes are handled by GeneralizedNewtypeDeriving.

{-# LANGUAGE GeneralizedNewtypeDeriving, UndecidableInstances #-}

module M where

import Control.Monad.IO.Unlift
import Control.Monad.Catch
import Control.Monad.Trans.Control
import Control.Monad.Base
import Control.Monad.Reader

newtype Foo a = Foo (ReaderT () IO a)
  deriving (Functor, Applicative, Monad, MonadIO, MonadUnliftIO, MonadThrow, MonadCatch, MonadMask, MonadBase IO, MonadBaseControl IO)

In general, the three relevant extensions for user-defined classes are GeneralizedNewtypeDeriving, DerivingVia, and DeriveAnyType. And it's also worth enabling DerivingStrategies to make it explicit which is being used.

来源：https://stackoverflow.com/questions/56863771/is-it-safe-to-derive-monadthrow-monadcatch-monadbasecontrol-monadunliftio-et

标签

haskell

monads

monad-transformers

deriving