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
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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