haskell - 当我添加不相关的定义时，为什么会在此程序中出现类型错误？

Question

我给同事看 Free当我遇到一种奇怪的 GHC 行为时，我真的不明白。这是一个简单的、人为的类型检查程序:

import Prelude hiding (readFile, writeFile)
import Control.Monad.Free
import Data.Functor.Sum

data FileSystemF a
  = ReadFile FilePath (String -> a)
  | WriteFile FilePath String a
  deriving (Functor)

data ConsoleF a
  = WriteLine String a
  deriving (Functor)

data CloudF a
  = GetStackInfo String (String -> a)
  deriving (Functor)

type App = Free (Sum FileSystemF (Sum ConsoleF CloudF))

readFile :: FilePath -> App String
readFile path = liftF (InL (ReadFile path id))

writeFile :: FilePath -> String -> App ()
writeFile path contents = liftF (InL (WriteFile path contents ()))

我尝试添加另一个定义，但不太正确:

writeLine :: String -> App ()
writeLine line = liftF (InR (WriteLine line ()))

这里的问题是我错过了另一个 InL .正确的定义应该是这样的:

writeLine :: String -> App ()
writeLine line = liftF (InR (InL (WriteLine line ())))

然而，这不是重点。对我来说奇怪的是 GHC 在我添加第一个不正确的 writeLine 定义时产生的类型错误。 .具体来说，它产生了两种类型的错误:

/private/tmp/free-sandbox/src/FreeSandbox.hs:26:27: error:
    • Couldn't match type ‘ConsoleF’ with ‘Sum ConsoleF CloudF’
        arising from a functional dependency between constraints:
          ‘MonadFree
             (Sum FileSystemF (Sum ConsoleF CloudF))
             (Free (Sum FileSystemF (Sum ConsoleF CloudF)))’
            arising from a use of ‘liftF’ at src/FreeSandbox.hs:26:27-66
          ‘MonadFree
             (Sum FileSystemF ConsoleF)
             (Free (Sum FileSystemF (Sum ConsoleF CloudF)))’
            arising from a use of ‘liftF’ at src/FreeSandbox.hs:29:18-48
    • In the expression: liftF (InL (WriteFile path contents ()))
      In an equation for ‘writeFile’:
          writeFile path contents = liftF (InL (WriteFile path contents ()))
   |
26 | writeFile path contents = liftF (InL (WriteFile path contents ()))
   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

/private/tmp/free-sandbox/src/FreeSandbox.hs:29:18: error:
    • Couldn't match type ‘Sum ConsoleF CloudF’ with ‘ConsoleF’
        arising from a functional dependency between:
          constraint ‘MonadFree
                        (Sum FileSystemF ConsoleF)
                        (Free (Sum FileSystemF (Sum ConsoleF CloudF)))’
            arising from a use of ‘liftF’
          instance ‘MonadFree f (Free f)’ at <no location info>
    • In the expression: liftF (InR (WriteLine line ()))
      In an equation for ‘writeLine’:
          writeLine line = liftF (InR (WriteLine line ()))
   |
29 | writeLine line = liftF (InR (WriteLine line ()))
   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

两个错误中的第二个(第 29 行的那个)是有道理的。这是我预期的错误。但是第 26 行的错误完全让我感到困惑。 writeFile的定义是正确的!添加我对 writeLine 的错误定义应该对 writeFile 没有任何影响， 正确的？这是怎么回事？

我能够在 GHC 8.0.2 和 GHC 8.2.1 上重现这一点。我想知道这是否是 GHC 中的错误(以便我可以报告)，或者这是我不理解的代码问题。

Answer 1

这是一个 GHC 错误。它已在提交 48daaaf0bba279b6e362ee5c632de69ed31ab65d (Don't report fundep wanted/wanted errors) 中修复，根据瑞安斯科特。