这个问题在这里已有答案:
我刚刚开始学习Haskell,我在ghci
中加载文件时遇到了问题。在一个名为Exercises.hs
的文件中,我有一些预定义的练习我应该解决,这个给我一个错误:
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,当我尝试将此文件加载到像ghci
这样的ghci Exercises.hs
时出现问题。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?当我注释第一行ex112 = showSalary'
时,模块已加载并且运行没有问题。为什么会出现这个错误?
只要给它类型:
ex112 :: (Show a) => a -> Integer -> String
ex112 = showSalary'