haskell - fun 声明中的类型错误

Question

我愿意:

Prelude> "sone" ++ "otehr"
"soneotehr"

但是这样的代码:

addOneToElement :: [a] -> [a]
addOneToElement element = element ++ "next"

main = do
   let s = addOneToElement("some")
   putStrLn s

产生以下输出:

all_possible_combinations.hs:22:37:
    Couldn't match expected type `a' against inferred type `Char'
      `a' is a rigid type variable bound by
          the type signature for `addOneToElement'
            at all_possible_combinations.hs:21:20
      Expected type: [a]
      Inferred type: [Char]
    In the second argument of `(++)', namely `"next"'
    In the expression: element ++ "next"

为什么我会收到此错误以及如何修复它？

Answer 1

您的类型签名应该是:

addOneToElement :: [Char] -> [Char]

(或更简单地说，addOneToElement::String -> String)

类型签名中的“a”是一个通配符 - 它可以匹配任何内容。但是，您试图将 Char 列表连接到任何内容的列表 - 但没有办法做到这一点。