func声明中的类型错误

Question

我做:

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

产生这个输出:

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解决？

Answer 1

您的类型签名应该是:

addOneToElement :: [Char] -> [Char]

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

a类型签名中的" "是一个通配符 - 它可以匹配任何内容.但是,您尝试将列表连接Char到任何列表 - 并且没有办法做到这一点.