a Show instance for undefined

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.

Answer 2

The bottom value (of which undefined is one flavor) is a value that is never constructed and hence can't be observed. This implies that you can't print it either. This value can't be compared to null from other languages, which usually can be observed and even checked against.

It is useful to think of undefined as well as error "blah" and all other bottoms as equivalent to results of infinite loops. The result of an infinite loop is never constructed and hence can't be observed.