GHC 8.0.1挂起.解释或编译错误？

Question

有人可以解释为什么下面的代码导致G​​HC 8.0.1永远循环编译,或者这是一个错误？

{-# 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

正如jberryman 所说，编译器陷入无限循环，没有UndecidableInstances或是UndecidableSuperclassCycles编译器错误。您应该在GHC Trac网站上报告该情况。

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

{-# 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 _ _ _ = ()