`Show PatternSet` throws `undefined`

[andreasabel]

undefined in a Show instance, congratulations!

$ cabal repl

ghci> :m +Text.Regex.TDFA.ReadRegex

ghci> parseRegex "[[:digit:]]"

Right (POr [PConcat [PAny {getDoPa = #1, getPatternSet = "*** Exception: Prelude.undefined
CallStack (from HasCallStack):
  error, called at libraries/base/GHC/Err.hs:74:14 in base:GHC.Err
  undefined, called at lib/Text/Regex/TDFA/Pattern.hs:110:23 in regex-tdfa-1.3.1.2-inplace:Text.Regex.TDFA.Pattern

regex-tdfa/lib/Text/Regex/TDFA/Pattern.hs

Lines 98 to 110 in 95d47cb

	instance Show PatternSet where
	showsPrec i (PatternSet s scc sce sec) =
	let (special,normal) = maybe ("","") ((partition (`elem` "]-")) . Set.toAscList) s
	charSpec = (if ']' `elem` special then (']':) else id) (byRange normal)
	scc' = maybe "" ((concatMap show) . Set.toList) scc
	sce' = maybe "" ((concatMap show) . Set.toList) sce
	sec' = maybe "" ((concatMap show) . Set.toList) sec
	in shows charSpec
	. showsPrec i scc' . showsPrec i sce' . showsPrec i sec'
	. if '-' `elem` special then showChar '-' else id
	where byRange xAll@(x:xs) | length xAll <=3 = xAll
	| otherwise = groupRange x 1 xs
	byRange _ = undefined

THOU SHALT NOT ABUSE THE Show CLASS!
Show is to print the HASKELL representation of data, not anything nice for the user!

How is one to explore a package if it tampers with Show, esp. in a way that crashes?