Q: Does this have a problem with empty input?

x:xs needs to be wrapped in parentheses

I've read about a proposal. But it doesn't matter, since my argument was of hypothetical nature. The Show class is used to produce a text representation of a value, which can be easily read back in with the Read class, and nothing more.

And what if they allow 20000 to be written as a new kind of literal 2e4 […]

Isn't 2e4 :: Fractional a => a per section 2.5? Do you have a link for 2e4 :: Num a => a? Also, section 9 specifies instance Show Integer as showsPrec = showSigned showInt, which shows an integral number in base 10.

That's exactly the point. And what if they allow 20000 to be written as a new kind of literal 2e4, then it could also be reflected in the Show instance.

People make assumptions about things, which probably will not hold. Bad code style.

You can even eta reduce that snippet to digits = map digitToInt . show. Either way, this will fail on negative input, such as -1234563.

maybe this?

import Data.Char
digits :: Integer -> [Int]
digits n = (map digitToInt).show n

although it seems useless to convert char to int since char itself can be sorted.