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Show PatternSet throws undefined #37

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Show PatternSet throws undefined#37
#38
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andreasabel
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bugSomething isn't working
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1.3.1.3
@andreasabel

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@andreasabel
andreasabel
opened on Jul 14, 2022

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?

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