haskell - 未定义的显示实例

Question

是否可以为未定义的值定义 Show 实例？也许存在一些 GHC 扩展？我想要这样的东西:

> print (1,undefined)

(1,"undefined")

Answer 1

根据Haskell 2010 report, chapter 9 ，评估 undefined 应该总是导致错误:

-- 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"

由于打印一个值包括评估它，这总是会出错。