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Loading the following code in GHCi causes a panic.

Versions affected at least 8.2.2 and 8.0.2

module Main where

import System.IO
import Control.Monad.IO.Class
import Control.Monad.Trans.State
import Text.Printf

putArrayBytes :: Handle       -- ^ output file handle
              -> [String]     -- ^ byte-strings
              -> IO Int -- ^ total number of bytes written
putArrayBytes outfile xs = do
  let writeCount x = modify' (+ length x) >> liftIO (putLine x) :: MonadIO m => StateT Int m ()
  execStateT (mapM_ writeCount xs) 0
  where putLine = hPutStrLn outfile . ("  "++) . concatMap (printf "0x%02X,")

{-
ghci:
:break 12 46
:trace putArrayBytes stdout [['1'..'9'],['2'..'8'],['3'..'7']]
snd $ runStateT _result 0
-}

main = undefined

Configuring GHCi with the following packages:
GHCi, version 8.2.2: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( C:\test\test.hs, interpreted )
Ok, one module loaded.
Loaded GHCi configuration from C:\Users\Andi\AppData\Local\Temp\ghci34988\ghci-script
*Main> :break 12 46
Breakpoint 0 activated at C:\test\test.hs:12:46-63
*Main> :trace putArrayBytes stdout [['1'..'9'],['2'..'8'],['3'..'7']]
Stopped in Main.putArrayBytes.writeCount, C:\test\test.hs:12:46-63
_result :: StateT Int m () = _
putLine :: [Char] -> IO () = _
x :: [Char] = "123456789"
[C:\test\test.hs:12:46-63] [C:\test\test.hs:12:46-63] *Main> snd $ runStateT _result 0

<interactive>:3:7: error:<interactive>: panic! (the 'impossible' happened)
  (GHC version 8.2.2 for x86_64-unknown-mingw32):
        No skolem info:
  m_I5Cm[rt]
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at compiler\utils\Outputable.hs:1133:58 in ghc:Outputable
        callStackDoc, called at compiler\utils\Outputable.hs:1137:37 in ghc:Outputable
        pprPanic, called at compiler\typecheck\TcErrors.hs:2653:5 in ghc:TcErrors

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

[C:\test\test.hs:12:46-63] [C:\test\test.hs:12:46-63] *Main> :r
[1 of 1] Compiling Main             ( C:\test\test.hs, interpreted )
Ok, one module loaded.
*Main> :break 12 46
Breakpoint 1 activated at C:\test\test.hs:12:46-63
*Main> :trace putArrayBytes stdout [['1'..'9'],['2'..'8'],['3'..'7']]
Stopped in Main.putArrayBytes.writeCount, C:\test\test.hs:12:46-63
_result :: StateT Int m () = _
putLine :: [Char] -> IO () = _
x :: [Char] = "123456789"
[C:\test\test.hs:12:46-63] [C:\test\test.hs:12:46-63] *Main> snd $ runStateT _result 0

<interactive>:7:7: error:<interactive>: panic! (the 'impossible' happened)
  (GHC version 8.2.2 for x86_64-unknown-mingw32):
        No skolem info:
  m_I5Nz[rt]
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at compiler\utils\Outputable.hs:1133:58 in ghc:Outputable
        callStackDoc, called at compiler\utils\Outputable.hs:1137:37 in ghc:Outputable
        pprPanic, called at compiler\typecheck\TcErrors.hs:2653:5 in ghc:TcErrors

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

[C:\test\test.hs:12:46-63] [C:\test\test.hs:12:46-63] *Main>

Maybe related to #13393 (closed).

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
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