haskell - GHC 8.0.1 挂起。解释或编译器错误？

Question

有人可以解释为什么下面的代码会导致 GHC 8.0.1 在编译时永远循环，还是这是一个错误？

{-# LANGUAGE TypeFamilyDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE AllowAmbiguousTypes #-}

main = return $ f (Just 'c')

data D1 x
data D2

type family TF x = t | t -> x
type instance TF (D1 x, a) = Maybe (TF (x, a))
type instance TF (D2, ()) = Char

f :: TF (x, a) -> ()
f _ = ()

Answer 1

编译器在没有 UndecidableInstances 的情况下陷入无限循环或 UndecidableSuperclassCycles是一个编译器错误，如 jberryman stated .您应该在 GHC Trac 上报告它地点。

我稍微简化了您的示例，这可能会或可能不会澄清事情，并且可能会使您的错误报告更有效。

{-# LANGUAGE TypeFamilyDependencies #-}

module TFLoop where

import Data.Proxy

main :: IO ()
main = return $ f Proxy Proxy (Just 'c')

data D1 x
data D2

type family TF d a = t | t -> d a
type instance TF (D1 x) a = Maybe (TF x a)
type instance TF D2 () = Char

f :: proxy1 x -> proxy2 a -> TF x a -> ()
f _ _ _ = ()