避免出现警告，默认将以下约束键入"Integer" [英] Avoid warning Defaulting the following constraint(s) to type `Integer'

问题描述

我定义了以下函数来反转Int或字符串列表:

I have defined the following function to reverse a list of Int or string:

myReverse :: [a] -> [a]
myReverse [] = []
myReverse (x:xs) = (myReverse xs) ++ [x]

我对hspec的测试:

My test with hspec:

 describe "myReverse" $ do
  it "returns the inversed list of the given list" $ do
   myReverse [1,2,3,4] `shouldBe` [4,3,2,1]

  it "returns the inversed string of the given string" $ do
   myReverse "A man, a plan, a canal, panama!" `shouldBe` "!amanap ,lanac a ,nalp a ,nam A"

像这样我得到警告

tests/OneToTenSpec.hs:69:24:
    Warning: Defaulting the following constraint(s) to type `Integer'
               (Eq a0)
                 arising from a use of `shouldBe' at tests/OneToTenSpec.hs:69:24-33
               (Num a0)
                 arising from the literal `1' at tests/OneToTenSpec.hs:69:15
               (Show a0)
                 arising from a use of `shouldBe' at tests/OneToTenSpec.hs:69:24-33
    In a stmt of a 'do' block:
      myReverse [1, 2, 3, 4] `shouldBe` [4, 3, 2, 1]
    In the second argument of `($)', namely
      `do { myReverse [1, 2, ....] `shouldBe` [4, 3, ....] }'
    In a stmt of a 'do' block:
      it "returns the inversed list of the given list"
      $ do { myReverse [1, 2, ....] `shouldBe` [4, 3, ....] }

所以我对测试进行了以下更改

So I made the following change on the test

myReverse [1 :: Int,2,3,4] `shouldBe` [4,3,2,1]

除了定义列表元素的类型之外，还有另一种避免此警告的方法吗?

Is there is another way to avoid this warning than defining the type of an element of the list ?

推荐答案

不适用于数字文字.由于文字的类型为Num a => a，并且我们将其提供给a中具有多态性的函数，因此没有提示将a解析为什么.

Not with numeric literals. Since literals have the type Num a => a and we're feeding it to a function that's polymorphic in a, there are no hints as to what to resolve a to.

好消息是，这正是默认设置的工作方式，您不必担心！该警告虽然很烦人，但我可以想到两种避免该警告的方法

The good news is that this is exactly how defaulting is meant to work and you have nothing to worry about! The warning is annoying though, I can think of two ways to avoid it

1. 使用显式类型签名
2. 不要使用数字文字

2在您的情况下可能是最好的，从类型上我们知道元素的类型不会影响其功能，因此您可以自由使用Bool

2 will probably be the best in your scenario, we know from the type that the type of element can't impact its function, so you're free to use Bool

 myReverse [True, False] `shouldBe` [False, True]

顺便说一句，您当前的实现是O(n^2)，并且O(n)是可能的，我留给您了解如何实现:)

As an aside your current implementation is O(n^2) and O(n) is possible, I'll leave it to you to figure out how though :)

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