模糊类型和重载数值操作的默认值 [英] Ambiguous Types, and Defaults for Overloaded Numeric Operations

问题描述

import Data.Dynamic

default(Integer,Double)

a :: Num a => a
a = 5

-- show :: Show a => a -> String
-- toDyn :: Typeable a => a -> Dynamic

main :: IO ()
-- main = print $ show a -- *** THIS LINE WORKS WELL
main = print $ toDyn a   -- *** THIS LINE LEADS TO AN AMBIGUOUS TYPE ERROR

我不明白为什么第一个主要"版本有效，而不是第二个为什么. 有没有人可以帮助我? 预先感谢您的答复.

I don't understand why the first "main" version works, and not the second. Is there someone who can help me ? Thanks in advance for your reply.

推荐答案

来自 Haskell报告:

在发现模棱两可的类型的情况下，如果出现以下情况，则模棱两可的类型变量 v 是默认的:

• v 仅出现在形式为 C v 的约束中，其中 C 是一个类，并且
• 这些类中的至少一个是数字类(即Num或Num)的子类，并且
• 所有这些类均在Prelude或标准库中定义

• v appears only in constraints of the form C v, where C is a class, and
• at least one of these classes is a numeric class, (that is, Num or a subclass of Num), and
• all of these classes are defined in the Prelude or a standard library

您的示例失败了，因为与Show不同，Typeable不是第三点中指定的类之一，因此不执行默认设置.

Your example fails because unlike Show, Typeable is not one of the classes specified in the third point, so no defaulting is performed.

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