Question

我刚刚开始学习Haskell，并且在ghci中加载文件时遇到问题。在名为Exercises.hs的文件中，我需要解决一些预定义的练习，而这给我一个错误：

showSalary amount bonus
   | bonus /= 0 = "Salary is " ++ show amount ++ ", and a bonus " ++ 
                  show bonus 
   | otherwise  = "Salary is " ++ show amount

-- 1.2. Give a simpler definition of 'showSalary', using only one if-then-else construct. 

ex112 = showSalary'
showSalary' amount bonus = "Salary is " ++ show amount ++ 
          (if bonus /= 0 then ", and a bonus " ++ show bonus else "")

现在，当我尝试将此文件像ghci一样加载到ghci Exercises.hs中时，就会出现问题。错误消息如下：

$ ghci Exercises
GHCi, version 8.4.3: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Exercises        ( Exercises.hs, interpreted )

Exercises.hs:31:9: error:
    • Ambiguous type variable ‘a0’ arising from a use of ‘showSalary'’
      prevents the constraint ‘(Show a0)’ from being solved.
      Relevant bindings include
        ex112 :: a0 -> Integer -> [Char] (bound at Exercises.hs:31:1)
      Probable fix: use a type annotation to specify what ‘a0’ should be.
      These potential instances exist:
        instance (Show a, Show b) => Show (Either a b)
          -- Defined in ‘Data.Either’
        instance Show GeneralCategory -- Defined in ‘GHC.Unicode’
        instance Show Ordering -- Defined in ‘GHC.Show’
        ...plus 24 others
        ...plus 49 instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    • In the expression: showSalary'
      In an equation for ‘ex112’: ex112 = showSalary'
   |
31 | ex112 = showSalary'
   |         ^^^^^^^^^^^
Failed, no modules loaded.

我不知道我的代码有什么问题？当我注释掉第一行ex112 = showSalary'时，模块已加载且运行正常。为什么会出现此错误？

Answer 1

只需输入以下类型：

ex112 :: (Show a) => a -> Integer -> String        
ex112 = showSalary'

Haskell在if else else语句上引发错误

1 个答案: