未定义的Show实例 [英] a Show instance for undefined

问题描述

可以做任何事情来为一个未定义的值定义一个Show实例吗？也许有一些GHC扩展存在？我想要这样的东西：

Can anything be done to define a Show instance for an undefined value? Maybe some GHC extensions exist? I want something like this:

> print (1,undefined)

(1,"undefined")

推荐答案

根据 Haskell 2010报告，第9章< a>，评价 undefined 应该总是会导致错误：

According to the Haskell 2010 report, chapter 9, evaluating undefined should always cause an error:

-- It is expected that compilers will recognize this and insert error
-- messages that are more appropriate to the context in which undefined
-- appears.
undefined :: a
undefined = error "Prelude.undefined"

自打印一个值包括评估它，这总会给出一个错误。

Since printing a value includes evaluating it, this will always give an error.

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