如何在Haskell中正确实例化类? [英] How to properly instantiate classes in Haskell?
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问题描述
class(Eq a)=> MyClass a
数据Alpha = Alpha
实例MyClass Alpha其中
Alpha == Alpha = True
我得到错误:
test.hs:5:10:`= ='不是'MyClass'类的(可见)方法
失败,模块加载:无。
解决方案
你必须明确地让Alpha成为Eq的一个实例。这将工作:
pre $ code数据Alpha = Alpha
实例Eq Alpha其中
Alpha == Alpha = True
实例MyClass Alpha
Trying to create a base class from which I can derive different types. What's wrong with the following?
class (Eq a) => MyClass a
data Alpha = Alpha
instance MyClass Alpha where
Alpha == Alpha = True
I get the error:
test.hs:5:10: `==' is not a (visible) method of class `MyClass'
Failed, modules loaded: none.
解决方案
You have to make Alpha an instance of Eq explicitly. This will work:
data Alpha = Alpha
instance Eq Alpha where
Alpha == Alpha = True
instance MyClass Alpha
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