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|
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE NoMonomorphismRestriction #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeApplications #-}
-- This version is shorter than T12734, and should yield a
-- type error message. If things go wrong, you get
-- an nfinite loop
module T12734a where
import Prelude
import Control.Applicative
import Control.Monad.Fix
import Control.Monad.Trans.Identity
import Control.Monad.Trans.Class
import Control.Monad.IO.Class
data A
data B
data Net
data Type
data Layer4 t l
data TermStore
data Stack lrs (t :: * -> *) where
SLayer :: t l -> Stack ls t -> Stack (l ': ls) t
SNull :: Stack '[] t
instance ( Con m (t l)
, Con m (Stack ls t)) => Con m (Stack (l ': ls) t)
instance Monad m => Con m (Stack '[] t)
instance ( expr ~ Expr t lrs
, Con m (TStk t lrs)) => Con m (Layer4 expr Type)
newtype Expr t lrs = Expr (TStk t lrs)
type TStk t lrs = Stack lrs (Layer4 (Expr t lrs))
class Con m t
-- HERE IS A FUNNY BEHAVIOR: the commented line raises context reduction stack overflow
test_gr :: forall m t lrs bind.
( Con m (TStk t lrs)
, bind ~ Expr t lrs
-- ) => m (Expr t lrs) -- GHC 8 worked if you said this...
) => m bind -- but not this!
test_gr = undefined
newtype KT (cls :: *) (t :: k) (m :: * -> *) (a :: *)
= KT (IdentityT m a)
test_ghc_err :: KT A '[Type] IO (Expr Net '[Type])
test_ghc_err = test_gr @(KT A '[Type] IO) @_ @'[Type] @(Expr Net '[Type])
{- Works!
test_ghc_err = test_gr @(KT A '[Type] IO)
@Net
@'[Type]
@(Expr Net '[Type])
-}
{- Some notes. See comment:10 on Trac #12734
[W] Con m (TStk t lrs)
[W] Inferable A lrs m
[W] bind ~ Expr t lrs
[W] m bind ~ KT A '[Type] IO (Expr Net '[Type])
==> m := KT A '[Type] IO
bind := Expr Net '[Type]
t := Net
lrs := '[Type]
[W] Con m (TStk t lrs)
= Con m (Stack lrs (Layer4 bind))
--> inline lrs
[W] Con m (Stack '[Type] (Layer4 bind))
--> isntance
[W] Con m (Stack '[] bind)
--> Monad m
+
[W] Con m (Layer4 bind Type)
-->
[W] bind ~ Expr t0 lrs0
[W] Con m (TStk t0 lrs0)
-}
|
