[GHC] #14578: Incorrect parenthesization of types in -ddump-deriv

[GHC]

#14578: Incorrect parenthesization of types in -ddump-deriv
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Debugging         |  Unknown/Multiple
  information is incorrect           |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by RyanGlScott:

Old description:

> Compiling this program on GHC 8.2 or later:
>
> {{{#!hs
> {-# LANGUAGE GeneralizedNewtypeDeriving #-}
> {-# OPTIONS_GHC -ddump-deriv #-}
> module Bug where
>
> import Control.Applicative
> import Data.Functor.Compose
> import Data.Semigroup
>
> newtype App f a = MkApp (f a)
>   deriving (Functor, Applicative)
>
> instance (Applicative f, Semigroup a) => Semigroup (App f a) where
>   (<>) = liftA2 (<>)
>
> newtype Wat f g a = MkWat (App (Compose f g) a)
>   deriving Semigroup
> }}}
>
> Will result in some incorrectly pretty-printed types in the `-ddump-
> deriv` output:
>
> {{{
> $ /opt/ghc/8.2.2/bin/ghci Bug.hs -dsuppress-uniques
> GHCi, version 8.2.2: http://www.haskell.org/ghc/  :? for help
> Loaded GHCi configuration from /home/rgscott/.ghci
> [1 of 1] Compiling Bug              ( Bug.hs, interpreted )
>
> ==================== Derived instances ====================
> Derived class instances:
>
>   <elided for brevity>
>
>   instance (Data.Semigroup.Semigroup a, GHC.Base.Applicative g,
>             GHC.Base.Applicative f) =>
>            Data.Semigroup.Semigroup (Bug.Wat f g a) where
>     (Data.Semigroup.<>)
>       = GHC.Prim.coerce
>           @(Bug.App Data.Functor.Compose.Compose f g a
>             -> Bug.App Data.Functor.Compose.Compose f g a
>                -> Bug.App Data.Functor.Compose.Compose f g a)
>           @(Bug.Wat f g a -> Bug.Wat f g a -> Bug.Wat f g a)
>           (Data.Semigroup.<>)
>     Data.Semigroup.sconcat
>       = GHC.Prim.coerce
>           @(Data.List.NonEmpty.NonEmpty Bug.App
> Data.Functor.Compose.Compose f g a
>             -> Bug.App Data.Functor.Compose.Compose f g a)
>           @(Data.List.NonEmpty.NonEmpty Bug.Wat f g a -> Bug.Wat f g a)
>           Data.Semigroup.sconcat
>     Data.Semigroup.stimes
>       = GHC.Prim.coerce
>           @(forall (b :: TYPE GHC.Types.LiftedRep).
>             GHC.Real.Integral b =>
>             b
>             -> Bug.App Data.Functor.Compose.Compose f g a
>                -> Bug.App Data.Functor.Compose.Compose f g a)
>           @(forall (b :: TYPE GHC.Types.LiftedRep).
>             GHC.Real.Integral b => b -> Bug.Wat f g a -> Bug.Wat f g a)
>           Data.Semigroup.stimes
> }}}
>
> Notice that is shows `Bug.App Data.Functor.Compose.Compose f g a`, which
> is wrong. It should be `Bug.App (Data.Functor.Compose.Compose f g) a`.

New description:

 Compiling this program on GHC 8.2 or later:

 {{{#!hs
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# OPTIONS_GHC -ddump-deriv #-}
 module Bug where

 import Control.Applicative
 import Data.Functor.Compose
 import Data.Semigroup

 newtype App f a = MkApp (f a)
   deriving (Functor, Applicative)

 instance (Applicative f, Semigroup a) => Semigroup (App f a) where
   (<>) = liftA2 (<>)

 newtype Wat f g a = MkWat (App (Compose f g) a)
   deriving Semigroup
 }}}

 Will result in some incorrectly pretty-printed types in the `-ddump-deriv`
 output:

 {{{
 $ /opt/ghc/8.2.2/bin/ghci Bug.hs -dsuppress-uniques
 GHCi, version 8.2.2: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 ==================== Derived instances ====================
 Derived class instances:

   <elided for brevity>

   instance (Data.Semigroup.Semigroup a, GHC.Base.Applicative g,
             GHC.Base.Applicative f) =>
            Data.Semigroup.Semigroup (Bug.Wat f g a) where
     (Data.Semigroup.<>)
       = GHC.Prim.coerce
           @(Bug.App Data.Functor.Compose.Compose f g a
             -> Bug.App Data.Functor.Compose.Compose f g a
                -> Bug.App Data.Functor.Compose.Compose f g a)
           @(Bug.Wat f g a -> Bug.Wat f g a -> Bug.Wat f g a)
           (Data.Semigroup.<>)
     Data.Semigroup.sconcat
       = GHC.Prim.coerce
           @(Data.List.NonEmpty.NonEmpty Bug.App
 Data.Functor.Compose.Compose f g a
             -> Bug.App Data.Functor.Compose.Compose f g a)
           @(Data.List.NonEmpty.NonEmpty Bug.Wat f g a -> Bug.Wat f g a)
           Data.Semigroup.sconcat
     Data.Semigroup.stimes
       = GHC.Prim.coerce
           @(forall (b :: TYPE GHC.Types.LiftedRep).
             GHC.Real.Integral b =>
             b
             -> Bug.App Data.Functor.Compose.Compose f g a
                -> Bug.App Data.Functor.Compose.Compose f g a)
           @(forall (b :: TYPE GHC.Types.LiftedRep).
             GHC.Real.Integral b => b -> Bug.Wat f g a -> Bug.Wat f g a)
           Data.Semigroup.stimes
 }}}

 Notice that it shows `Bug.App Data.Functor.Compose.Compose f g a` within
 the visible type applications, which is wrong. It should be `Bug.App
 (Data.Functor.Compose.Compose f g) a`.

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14578#comment:1>
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