[GHC] #13748: Variables pretty-printed from -ddump-deriv are not scoped properly

23 May 2017

      #13748: Variables pretty-printed from -ddump-deriv are not scoped properly
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (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:
-------------------------------------+-------------------------------------
 This bug is present on GHC 8.0.1, 8.0.2, 8.2.1 and HEAD, and originally
 noted in https://ghc.haskell.org/trac/ghc/ticket/13738#comment:2. Take
 this code:

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

 newtype Wrap f a = Wrap (f a) deriving C

 class C f where
   c :: f a
 }}}

 When you compile it with GHC 8.0.2, you'll get this unsavory output:

 {{{
 GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Works            ( Works.hs, interpreted )

 ==================== Derived instances ====================
 Derived instances:
   instance forall k_a14U (f_a14V :: k_a14U -> *).
            Works.C f_a14V =>
            Works.C (Works.Wrap f_a14V) where
     Works.c
       = GHC.Prim.coerce
           @(forall (a_a13F :: k_a14u). f_a13G a_a13F)
           @(forall (a_a13F :: k_a14u). Works.Wrap f_a13G a_a13F)
           Works.c

 GHC.Generics representation types:
 }}}

 This is wrong, since the quantified variables in the instance head
 (`k_a14U` and `f_a14V`) do not match the occurrences that they bind
 (`k_a14u` and `f_a13G`). This is somewhat easier to see on GHC 8.2.1 or
 HEAD, since the binding sites are printed without uniques:

 {{{
 GHCi, version 8.3.20170516: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Works            ( Works.hs, interpreted )

 ==================== Derived instances ====================
 Derived class instances:
   instance forall k (f :: k -> *).
            Works.C f =>
            Works.C (Works.Wrap f) where
     Works.c
       = GHC.Prim.coerce
           @(forall (a_a1tD :: k_a1uE). f_a1tE a_a1tD)
           @(forall (a_a1tD :: k_a1uE). Works.Wrap f_a1tE a_a1tD)
           Works.c

 Derived type family instances:
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13748
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler