测试Eq类型的空列表[] [英] Testing empty list [] with Eq type

问题描述

当前，我正在Haskell中编写一个函数来检查列表是否对称.

  isReflexive ::等式=>[(a，a)]->布尔isReflexive [] = TrueisReflexive xs =和[elem(x，x)xs |x<-[fst u |u<-xs] ++ [snd u |u<-xs]]测试=做打印(isReflexive [])主=测试 

该函数在不为空的列表上可以正常工作.但是，当我使用该功能测试空列表时，它引发了错误

由于使用"isReflexive"而产生的模棱两可的类型变量"a2"阻止解决约束(Eq a2)".

可能的解决方法:使用类型注释来指定"a2"应该是什么.

这些潜在的实例存在:

实例方程式排序-在"GHC.Classes"中定义

instance Eq Integer-在"integer-gmp-1.0.2.0:GHC.Integer.Type"中定义

instance Eq a => Eq(也许a)-在"GHC.Maybe"中定义...再加上22个人

...加上涉及范围外类型的7个实例

(使用-fprint-potential-instances查看全部内容)

•在打印"的第一个参数中，即(isReflexive [])"如何解决此错误?

解决方案

问题很简单，为了应用 isReflexive ，GHC需要知道您在哪种类型上使用它.

isReflexive 的类型签名- Eq a =>[(a，a)]->Bool 没有告诉GHC该函数可以使用的具体类型.没关系，这很正常，但是通常，调用该函数的代码使该特定应用程序中的 a 确切是什么.在这里不是这样，因为 [] 本身具有多态(因此模棱两可)的类型， [a] (对于任何 a )./p>

要修复此问题，只需在此处为 [] 提供一个具体类型，该类型与 isReflexive 的签名一致.真的什么都没关系，但是许多可行的例子是:

  test =做print(isReflexive([] :: [(Int，Int)])) 

(请注意，这正是GHC在提示可能的解决方法时告诉您的信息:使用类型注释指定"a2"应为.该消息中的"a2"对应于"a"'在这里，GHC倾向于使用'a1'，'a2'等来引用所有类型变量.)

Currently, I am writing a function in Haskell to check a list is symmetric or not.

isReflexive :: Eq a => [(a, a)] -> Bool 
isReflexive [] = True
isReflexive xs = and [elem (x, x) xs | x <- [fst u | u <- xs] ++ [snd u | u <- xs]]

test = do
    print(isReflexive [])

main = test

The function works fine on the list that is not empty. However, when I test the empty list with the function, it raised an error

Ambiguous type variable ‘a2’ arising from a use of ‘isReflexive’ prevents the constraint ‘(Eq a2)’ from being solved.

Probable fix: use a type annotation to specify what ‘a2’ should be.

These potential instances exist:

instance Eq Ordering -- Defined in ‘GHC.Classes’

instance Eq Integer -- Defined in ‘integer-gmp-1.0.2.0:GHC.Integer.Type’

instance Eq a => Eq (Maybe a) -- Defined in ‘GHC.Maybe’ ...plus 22 others

...plus 7 instances involving out-of-scope types

(use -fprint-potential-instances to see them all)

• In the first argument of ‘print’, namely ‘(isReflexive [])’ How to fix this error?

解决方案

The problem is simply that, in order to apply isReflexive, GHC needs to know which type you are using it on.

The type signature of isReflexive - Eq a => [(a, a)] -> Bool doesn't tell GHC a concrete type that the function works on. That's perfectly fine, and usual, but most often the code that calls the function makes it clear what exactly a is in that particular application. That's not so here, because [] has itself a polymorphic (and therefore ambiguous) type, [a] (for any a).

To fix it you simply have to provide a concrete type for your [] here, which is consistent with the signature of isReflexive. It really doesn't matter what, but an example from many that will work is:

test = do
    print(isReflexive ([] :: [(Int, Int)]))

(Note that this is exactly what GHC is telling you when it says Probable fix: use a type annotation to specify what 'a2' should be. 'a2' in that message corresponds to 'a' here, GHC tends to use 'a1', 'a2' etc to refer to all type variables.)

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