Question

我有非常看上去无辜的代码

data Config = Config
    { cInts    :: [Int]
    , cStrings :: [String] }

instance Semigroup Config where
    c1 <> c2 = Config
        { cInts    = andCombiner cInts
        , cStrings = andCombiner cStrings }
      where
        andCombiner field = field c1 <> field c2

它编译并正常工作。但是，如果我添加TypeFamilies或GADTs扩展名，我会看到非常奇怪的错误：

.../Main.hs:19:22: error:
    • Couldn't match type ‘Int’ with ‘[Char]’
      Expected type: [String]
        Actual type: [Int]
    • In the ‘cStrings’ field of a record
      In the expression:
        Config {cInts = andCombiner cInts, cStrings = andCombiner cStrings}
      In an equation for ‘<>’:
          c1 <> c2
            = Config
                {cInts = andCombiner cInts, cStrings = andCombiner cStrings}
            where
                andCombiner field = field c1 <> field c2
   |
19 |         , cStrings = andCombiner cStrings }
   |                      ^^^^^^^^^^^^^^^^^^^^

.../Main.hs:19:34: error:
    • Couldn't match type ‘[Char]’ with ‘Int’
      Expected type: Config -> [Int]
        Actual type: Config -> [String]
    • In the first argument of ‘andCombiner’, namely ‘cStrings’
      In the ‘cStrings’ field of a record
      In the expression:
        Config {cInts = andCombiner cInts, cStrings = andCombiner cStrings}
   |
19 |         , cStrings = andCombiner cStrings }
   |                                  ^^^^^^^^

这个编译器错误的原因是什么？

Answer 1

这归因于-XGADTs和-XTypeFamilies所暗示的-XMonoLocalBinds。您可以通过向andCombiner添加类型签名（或打开-XNoMonoLocalBinds来重新编译代码，但我不建议）：

instance Semigroup Config where
    c1 <> c2 = Config
        { cInts    = andCombiner cInts
        , cStrings = andCombiner cStrings }
      where
        andCombiner :: Semigroup a => (Config -> a) -> a
        andCombiner field = field c1 <> field c2

使用我链接的GHC文档中的术语，andCombiner并未完全推广，因为它提到了未关闭或导入的c1和c2。

TypeFamilies或GADT突然中断了有效代码

1 个答案: