[GHC] #13187: Strange debug output printed: isPredTy Const Int (, ) Bool Char

[GHC]

#13187: Strange debug output printed: isPredTy Const Int (,) Bool Char
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I noticed this when debugging an issue in `th-desugar`. If you have this
 file:

 {{{#!hs
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TemplateHaskell #-}
 module Bug where

 type Const a b = b
 test36_expand = $([| let f :: Const Int (,) Bool Char -> Char
                          f = snd in
                      f |])
 }}}

 And compile it on GHC HEAD with `-ddump-tc-trace` enabled, even if you
 redirect the output of `-ddump-tc-trace` to a file with `-ddump-to-file`,
 GHC will still print what appears to be debugging messages!

 {{{
 $ ~/Software/ghc3/inplace/bin/ghc-stage2 Bug.hs -ddump-to-file -fforce-
 recomp -ddump-tc-trace
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
 isPredTy Const Int (,) Bool Char
 }}}

 The source of that `isPredTy` message is in `Type`
 ([http://git.haskell.org/ghc.git/blob/3eebd1f5fd56689baa63fcc63b7f4bde0ae70d0b:/compiler/types/Type.hs#l1523
 here]):

 {{{#!hs
 go_k :: Kind -> [KindOrType] -> Bool
 -- True <=> ('k' applied to 'kts') = Constraint
 go_k k [] = isConstraintKind k
 go_k k (arg:args) = case piResultTy_maybe k arg of
                       Just k' -> go_k k' args
                       Nothing -> pprTrace "isPredTy" (ppr ty)
                                  False
    -- This last case should not happen; but it does if we
    -- we call isPredTy during kind checking, especially if
    -- there is actually a kind error.  Example that showed
    -- this up: polykinds/T11399
 }}}

 Several things stand out here:

 1. Should we be using `pprTrace` this way?
 2. Given that the comment says that "This last case should not happen", is
 this program indicative of a bug? Or perhaps this code really can be
 legitimately reached, and the comment should be updated to reflect this?

 FWIW, `polykinds/T11399` exhibits the same issue:

 {{{
 $ ~/Software/ghc3/inplace/bin/ghc-stage2 Bug2.hs -ddump-to-file -fforce-
 recomp -ddump-tc-trace
 [1 of 1] Compiling T11399           ( Bug2.hs, Bug2.o )
 isPredTy a_aCn[sk:1] *
 isPredTy a_aCn[sk:1] *
 isPredTy a_aCn[sk:1] *
 isPredTy a_aCn[sk:1] *
 isPredTy a_aCn[sk:1] *
 isPredTy a_aCn[sk:1] *
 isPredTy a_aCn[sk:1] *
 isPredTy a_aCn[sk:1] *
 isPredTy a_aCn[sk:1] *
 isPredTy a_aCn[sk:1] *
 isPredTy a_aCn[sk:1] *
 isPredTy a_aCn[sk:1] *
 isPredTy a_aCn[sk:1] *

 Bug2.hs:10:32: error:
     • Couldn't match kind ‘*’ with ‘GHC.Types.RuntimeRep’
       When matching kinds
         a :: * -> *
         TYPE :: GHC.Types.RuntimeRep -> *
       Expected kind ‘* -> *’, but ‘UhOh a’ has kind ‘a * -> *’
     • In the first argument of ‘Functor’, namely ‘(UhOh a)’
       In the instance declaration for ‘Functor (UhOh a)’
    |
 10 | instance Functor a => Functor (UhOh a) where
    |                                ^^^^^^
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13187>
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