Strange debug output printed: isPredTy Const Int (,) Bool Char
I noticed this when debugging an issue in th-desugar
. If you have this file:
{-# LANGUAGE PolyKinds #-}
module Bug where
type Const a b = b
f :: Const Int (,) Bool Char -> Char
f = snd
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
(here):
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:
- Should we be using
pprTrace
this way? - 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
| ^^^^^^