#12639: Inconsistent treatment of FlexibleInstances and MPTCs with standard vs. flexible deriving -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Given `{-# LANGUAGE GeneralizedNewtypeDeriving #-}`, I can write {{{#!hs import Control.Monad.State.Strict newtype Foo s m a = Foo (StateT s m a) deriving (Functor, Applicative, Monad, MonadState s) }}} However, if I want to use `StandaloneDeriving` to make the `MonadState` instance more explicit, {{{#!hs deriving instance Monad m => MonadState s (Foo s m) }}} I suddenly need to add `FlexibleInstances` and `MultiParamTypeClasses`. In my personal opinion, the most sensible way to handle this is to change two things in two different directions: 1. Allow MPTC instance declarations (but not class declarations) without `MultiParamTypeClasses`. 2. Require `FlexibleInstances` for standard deriving clauses when they would be required for standalone deriving declarations. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12639 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler