#15026: Type deduction panic -------------------------------------+------------------------------------- Reporter: moomin | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.2.2 Keywords: | Operating System: Windows Architecture: | Type of failure: Poor/confusing Unknown/Multiple | error message Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{#!lhs
{-# LANGUAGE DeriveFunctor #-}
module Bug where
import qualified Data.Set as S
import Data.Foldable (fold, foldMap, toList) import Data.List.Split (splitOn) import Data.Monoid -- ((<>), Sum, mempty, mappend) import Data.Functor.Foldable
newtype Component = Component (Int,Int) deriving (Eq, Ord, Show)
data Day24State v a = Day24State { valueToMatch :: Int, componentsToUse :: S.Set Component, componentMetric :: v, nextStates :: [a] } deriving (Functor, Show)
type Day24StateFix v a = Fix (Day24State v) }}}
Now in GHCI type
:t Data.Functor.Foldable.ana f (undefined :: Day24StateFix Component)
Produces ghc.EXE: panic! (the 'impossible' happened) (GHC version 8.0.2 for x86_64-unknown-mingw32): initTc: unsolved constraints WC {wc_insol = [W] f_akbJ :: t_akbI[tau:3] (CHoleCan: f)} Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15026 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler