#13659: Bug report: "AThing evaluated unexpectedly tcTyVar a_alF" -------------------------------------+------------------------------------- Reporter: costaparas | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Linux Architecture: x86_64 | Type of failure: Compile-time (amd64) | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following code: {{{#!hs {-# LANGUAGE GADTs, EmptyDataDecls, TypeFamilies, TypeOperators, DataKinds, FlexibleInstances #-} {- Defines a C-like printf function using DataKinds extensions. -} module Printf where -- format string parameterized by a list of types data Format (fmt :: [*]) where X :: Format '[] -- empty string, i.e. "" L :: a -> String -> Format '[] -- string literal, e.g. "hello" S :: a -> Format '[String] -- "%s" I :: Format a -> Format '[Int, a] -- "%d" }}} produces the following error: GHCi, version 7.10.3: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Printf ( Printf.hs, interpreted ) a.hs:12:27:ghc: panic! (the 'impossible' happened) (GHC version 7.10.3 for x86_64-unknown-linux): AThing evaluated unexpectedly tcTyVar a_alF Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug when run using **ghci Printf.hs** on Liunx Ubuntu 16.04 LTS 64-bit with Intel® Core™ i7-7500U CPU @ 2.70GHz -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13659 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler