#14916: Missing checks when deriving special classes -------------------------------------+------------------------------------- Reporter: kosmikus | Owner: (none) Type: bug | Status: patch Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.2 checker) | Resolution: | Keywords: Deriving Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4501 Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: new => patch * differential: => Phab:D4501 Comment: Luckily, fixing this is quite straightforward by using `checkValidInstHead`, which is what Phab:D4501 accomplishes. One thing that's interesting about `checkValidInstHead` is that is also does validity checks for `FlexibleInstances` and `MultiParamTypeClasses`. This means that using `checkValidInstHead`, unaltered, would result in this program, which is currently accepted by GHC: {{{#!hs {-# LANGUAGE GeneralizedNewtypeDeriving #-} module T where import Control.Monad.Reader newtype MyReader a = MyReader (Int -> a) deriving ( Functor, Applicative, Monad , MonadReader Int ) }}} Being rejected due to not enabling `FlexibleInstances` or `MultiParamTypeClasses`, since it generates `instance MonadReader Int MyReader`. I decided to err on the side of avoiding unnecessary breakage and tweaked `checkValidInstHead` so as to disable these checks for `deriving` clauses (just as we do for `SPECIALISE instance` pragmas today). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14916#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler