haskell - 数据类型列表 : "could not deduce (a ~ SomeType) from the context (SomeTypeclass a)"

Question

我对 Haskell 的类型系统有以下问题:我试图声明一个数据类型并从函数返回一个包含该类型元素的列表。不幸的是，即使是最小的测试用例，例如

data SampleType = SampleTypeConstructor

instance Show SampleType where
    show x = "(SampleType)"

stList :: (Show a) => [a]
stList = [(SampleTypeConstructor)]

main = do {
    putStrLn (show stList)
}

失败并显示来自 ghc-7.0.2 和 ghc-7.1.20110327 的以下错误消息:

tcase.hs:7:12:
    Could not deduce (a ~ SampleType)
    from the context (Show a)
      bound by the type signature for stList :: Show a => [a]
      at tcase.hs:7:1-34
      `a' is a rigid type variable bound by
          the type signature for stList :: Show a => [a] at tcase.hs:7:1
    In the expression: (SampleTypeConstructor)
    In the expression: [(SampleTypeConstructor)]
    In an equation for `stList': stList = [(SampleTypeConstructor)]

Answer 1

有问题的行是 stList::(Show a) => [a]。您声明 stList 是一个多态列表，其中包含满足显示约束的任何元素。但 stList 不是多态列表!它是 SampleType 的列表。因此，删除签名并查看 ghci 推断出的内容，或者直接提供正确的签名:::[SampleType]。