Question

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")

Answer 1

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"

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