Renamer hangs (because of -XInstanceSigs?)
{-# Language RankNTypes, TypeInType, EmptyCase, GADTs, FlexibleInstances, ConstraintKinds, UndecidableInstances, AllowAmbiguousTypes, InstanceSigs, ScopedTypeVariables #-}
import Data.Kind
import Data.Proxy
type Cat ob = ob -> ob -> Type
data Struct :: (k -> Constraint) -> Type where
S :: Proxy (a::k) -> Struct (cls::k -> Constraint)
type Structured a cls = (S ('Proxy :: Proxy a)::Struct cls)
data AStruct :: Struct cls -> Type where
AStruct :: cls a => AStruct (Structured a cls)
class StructI (structured::Struct (cls :: k -> Constraint)) where
struct :: AStruct structured
instance (Structured xx cls ~ structured, cls xx) => StructI structured where
struct :: AStruct (Structured xx cls)
struct = AStruct
data Hom :: Cat k -> Cat (Struct cls) where
class Category (cat::Cat ob) where
i :: StructI a => ríki a a
instance Category ríki => Category (Hom ríki :: Cat (Struct cls)) where
-- Commenting out this instance signature makes the issue go away
i :: forall a. StructI a => Hom ríki a a
i = case struct :: AStruct (Structured a cls) of
Running on 8.2.1 and 8.5.20180105 both loop until interrupted
$ ghci -ignore-dot-ghci 199.hs
GHCi, version 8.5.20180105: http://www.haskell.org/ghc/ :? for help
[1 of 1] Compiling Main ( 199.hs, interpreted )
^CInterrupted.
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