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GeneralizedNewtypeDeriving produces error messages with incorrect kind signatures

    • Closed Issue created by Alexis King @lexi.lambda

    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 this Stack Overflow question.),,

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