#14301: ghc: panic! (the 'impossible' happened) -------------------------------------+------------------------------------- Reporter: mcapodici | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 Keywords: | Operating System: Unknown/Multiple Architecture: x86_64 | Type of failure: None/Unknown (amd64) | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- **When I build my program I get this:** {{{ play-0.1.0.0: build (exe) Preprocessing executable 'play' for play-0.1.0.0... [1 of 1] Compiling Main ( src/Main.hs, .stack-work/dist/x86_64 -linux-nopie/Cabal-1.24.2.0/build/play/play-tmp/Main.o ) ghc: panic! (the 'impossible' happened) (GHC version 8.0.2 for x86_64-unknown-linux): initTc: unsolved constraints WC {wc_insol = [W] interactD_aL2 :: t_aL1[tau:1] (CHoleCan: interactD) [W] interactD_aLt :: t_aLs[tau:1] (CHoleCan: interactD)} Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} **The code:** {{{ {-# LANGUAGE GADTs #-} module Main where import Control.Monad.Trans.Cont import Control.Monad.Trans.Class --main :: IO () {-- CONTT -- main = evalContT contt contt :: ContT () IO () contt = do k <- do callCC $ \exit -> do lift $ putStrLn "Entry" exit $ \_ -> do putStrLn "Exit" lift $ putStrLn "Inside" lift $ k () --} {-- class Monad m => Interact m where get :: m String say :: String -> m () instance Interact IO where get = getLine say = putStrLn prog :: Interact a => a () prog = do say "Hello what is your name?" name <- get say $ "Hello " ++ name ++ ", I am prog! Goodbye" main :: IO () main = prog --} data InteractD m = InteractD { getD :: m String , sayD :: String -> m () } interactDIO :: InteractD IO interactDIO = interactD getLine putStrLn interactDIO2 :: InteractD IO interactDIO2 = interactD (return "James") putStrLn }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14301 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler