haskell - 幻影和代理 - 导致 ghoSTLy 错误

Question

我正在开发一个项目，在该项目中，我通过幻像类型跟踪我使用的数据库模式 - 它有一个 KnownSymbol - 这是模式名称。

就在前几天，我遇到了以下问题 - 我不明白:

为什么不可能定义 withoutProxy 或重新表述它 - 为什么 GHC 假设 test 是 * 类型而不是 (Proxy::Proxy test) 中的 >Symbol，尽管类型签名另有说明并且启用了 ScopedTypeVariables。

{-# LANGUAGE DataKinds           #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE KindSignatures      #-}
module T where

import GHC.TypeLits
import Data.Proxy

newtype Phantom (x :: Symbol) y = Phantom y

withProxy :: (KnownSymbol test) => Proxy test -> Phantom test ()
withProxy _ = Phantom ()

withoutProxy :: (KnownSymbol test) => Phantom test ()
withoutProxy = withProxy (Proxy :: Proxy test)

我得到的错误是最令人困惑的

> ghci test.hs
GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling T                ( test.hs, interpreted )

test.hs:14:27: error:
    • Couldn't match type ‘*’ with ‘Symbol’
      Expected type: Proxy test
        Actual type: Proxy test
      Use -fprint-explicit-kinds to see the kind arguments
    • In the first argument of ‘withProxy’, namely
        ‘(Proxy :: Proxy test)’
      In the expression: withProxy (Proxy :: Proxy test)
      In an equation for ‘withoutProxy’:
          withoutProxy = withProxy (Proxy :: Proxy test)
Failed, modules loaded: none.

然后启用-fprint-explicit-kinds

Prelude> :set -fprint-explicit-kinds 
Prelude> :r
[1 of 1] Compiling T                ( test.hs, interpreted )

test.hs:15:27: error:
    • Couldn't match type ‘*’ with ‘Symbol’
      Expected type: Proxy Symbol test
        Actual type: Proxy * test
    • In the first argument of ‘withProxy’, namely
        ‘(Proxy :: Proxy test)’
      In the expression: withProxy (Proxy :: Proxy test)
      In an equation for ‘withoutProxy’:
          withoutProxy = withProxy (Proxy :: Proxy test)
Failed, modules loaded: none.

Answer 1

简短回答:添加forall

withoutProxy :: forall test. (KnownSymbol test) => Phantom test ()
withoutProxy = withProxy (Proxy :: Proxy test)

没有它，第二行的test与上面的不同。

---

或者，根本不添加类型注释:

withoutProxy :: (KnownSymbol test) => Phantom test ()
withoutProxy = withProxy Proxy

---

或者您可以添加{-# LANGUAGE PolyKinds #-}，然后事情也会统一，因为Proxy::Proxy test将forall k (测试::k)。代理 k 测试，其中 k 是一种类型。