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

28 May 2023

      No. If MonadFail did not exist in ghc-8.6.5 then I would get a compiler 
error, right? I have compiled and run it with ghc versions back to 
8.2.2. Furthermore, I did try to define the Monad instance manually, 
along with fail and I do get a compiler error in that case, with every 
ghc since 8.2.2:

test2.hs:15:3: error:
     ‘fail’ is not a (visible) method of class ‘Monad’
    |
15 |   fail = RegexFail . Left

The docs for MonadFail say "Since: base-4.9.0.0" which came with ghc-8.0.

Am 28.05.23 um 16:42 schrieb 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  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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I would rather have questions that cannot be answered, than answers that
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