[Haskell-cafe] regex problem with ghc-8.6 and older

[Brandon Allbery]

Isn't this just `MonadFail` not existing in older GHC versions? You
need to override `fail` in the `Monad` instance instead of using GND.

On Sun, May 28, 2023 at 10:36 AM Ben Franksen <ben.franksen at online.de> wrote:
>
> Hi Everyone
>
> I was trying to fix a bug in a large program (darcs) and stumbled over a
> problem that I cooked down to the minimal test program below. The issue
> here is that with ghc-8.6 and earlier, properly handling an invalid
> regular expression (here: the empty string) does not work: somehow
> something calls "error". With ghc-8.8 and later it works as expected. In
> both cases the latest releases of the regex packages are used.
>
> {-# LANGUAGE GeneralizedNewtypeDeriving #-}
> import Prelude hiding (fail)
> import Control.Monad.Fail
> import Control.Exception
> import Text.Regex.Base
> import Text.Regex.TDFA
>
> newtype RegexFail a = RegexFail { runRegexFail :: Either String a }
>    deriving (Functor, Applicative, Monad)
>
> instance MonadFail RegexFail where
>    fail = RegexFail . Left
>
> test :: RegexFail Regex
> test = makeRegexM ""
>
> main =
>    handle (\(ErrorCall _) -> putStrLn "error call") $
>      case runRegexFail test of
>        Left _ -> putStrLn "clean error handling"
>        Right x -> print (matchM x "" :: Maybe Bool)
>
>  > runghc-8.6 test2.hs                                     √
> error call
>  > runghc-8.8 test2.hs                                     √
> clean error handling
>
> (1) Is this a known problem with ghc < 8.8 / base < 4.13?
> (2) Is there a work-around for older ghc versions?
>
> Cheers
> Ben
> --
> I would rather have questions that cannot be answered, than answers that
> cannot be questioned.  -- Richard Feynman
>
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-- 
brandon s allbery kf8nh
allbery.b at gmail.com