函数定义问题(无实例……产生) [英] Function definition problems (No instance for … arising from)

问题描述

我已经定义了以下函数来查找某物(整数，字符串...)列表的倒数第二个元素

I have defined the following function to find the penultimate element of a list of something (Int, string...)

myButLast :: [a] -> a
myButLast [] = error "myButLast: empty list"
myButLast [x, _] = x
myButLast (_:xs) = myButLast xs

当我用hspec对其进行测试

When I test it with hspec

  it "returns an error for list of one element" $ do
   myButLast [42] `shouldThrow` anyException

我收到以下错误

没有实例用于(Num(IO a0)) 由文字42' Possible fix: add an instance declaration for (Num (IO a0)) In the expression: 42 In the first argument of myButLast'产生，即[42]' In the first argument of shouldThrow'，即'myButLast [42]'

No instance for (Num (IO a0)) arising from the literal 42' Possible fix: add an instance declaration for (Num (IO a0)) In the expression: 42 In the first argument ofmyButLast', namely [42]' In the first argument ofshouldThrow', namely `myButLast [42]'

这是什么意思，以及如何解决?可能是需要上课的限制吗?

What does it mean and how to fix it ? May be a constraint of class needed?

我想处理String和myButLast中任何内容的列表.我所有其他带有倍数元素的测试都可以正常工作.

I want to handle String and list of anything in myButLast. All my other tests with multiples elements works.

推荐答案

shouldThrow具有类型Exception e => IO a -> Selector e -> Expectation.这意味着第一个参数应该在IO monad中.为了使用纯函数，可以使用evaluate函数:

shouldThrow has the type Exception e => IO a -> Selector e -> Expectation. This means that the first argument should be in the IO monad. In order to use pure functions, you can use the evaluate function:

evaluate (myButLast [42]) `shouldThrow` anyException

顺便说一句，您可能要测试特定的错误，以确保在某个时候不会被错误地更改:

Incidentally, you might want to test for the specific error to make sure it doesn't get mistakenly changed at some point:

evaluate (myButLast [42]) `shouldThrow` errorCall "myButLast: empty list"

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