#12546: GeneralizedNewtypeDeriving produces error messages with incorrect kind signatures -------------------------------------+------------------------------------- Reporter: lexi.lambda | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Given the following program: {{{ {-# LANGUAGE GeneralizedNewtypeDeriving #-} {-# LANGUAGE FlexibleInstances #-} import Control.Monad.Reader newtype AppM a = AppM (ReaderT Int IO a) deriving (Functor, Applicative, Monad, MonadReader) }}} The `MonadReader` deriving declaration should be `MonadReader Int`. GHC produces the following error message: {{{ • Expecting one more argument to ‘MonadReader’ Expected kind ‘* -> Constraint’, but ‘MonadReader’ has kind ‘* -> (* -> *) -> Constraint’ • In the newtype declaration for ‘AppM’ }}} This error message is confusing to me. The kind of `MonadReader` is `* -> (* -> *) -> Constraint`, as the error message states, which makes sense. However, the error message states that it expects kind `* -> Constraint`, despite the fact that `MonadReader Int` is actually of kind `(* -> *) -> Constraint`. ,,(This description is adapted from [http://stackoverflow.com/q/39172590/465378 this Stack Overflow question].),, -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12546 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler