#15552: Infinite loop/panic with an existential type. -------------------------------------+------------------------------------- Reporter: howtonotwin | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: TypeInType, | TypeFamilies Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: #14723 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): It's not #15473. Here's a slightly cut down version {{{ {-# LANGUAGE DataKinds, ExistentialQuantification, GADTs, PolyKinds, TypeOperators #-} {-# LANGUAGE TypeInType, TypeFamilies #-} module T15552 where import Data.Kind data Elem :: k -> [k] -> Type data EntryOfVal (v :: Type) (kvs :: [Type]) where EntryOfVal :: forall (v :: Type) (kvs :: [Type]) (k :: Type). Elem (k, v) kvs -> EntryOfVal v kvs type family EntryOfValKey (eov :: EntryOfVal v kvs) :: Type type family GetEntryOfVal (eov :: EntryOfVal v kvs) :: Elem (EntryOfValKey eov, v) kvs type family FirstEntryOfVal (v :: Type) (kvs :: [Type]) :: EntryOfVal v kvs where FirstEntryOfVal v (_ : kvs) = 'EntryOfVal (GetEntryOfVal (FirstEntryOfVal v kvs)) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15552#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler