Re: Let ReadP carry a failure message

19 Nov 2020


      ReadP was never designed to produce useful error messages. Have you
experimented with your design? Do the messages it produces help find the
problems?

On Thu, Nov 19, 2020, 10:08 PM Dannyu NDos  wrote:
...
Firstly, I propose a new class. This class is like MonadFail, but with
"error bind" operator:
class Monad m => FailMsg m where
    failMsg :: String -> m a
    (>?=) :: m a -> (String -> m a) -> m a
Laws are:
* For every (x :: m a), if (x ≡ fail msg) for some (msg :: String),
msg shall be unique.
 * For (x ≡ fail msg), (x >?= f ≡ f msg). If such msg doesn't exist,
(x >?= _ ≡ x).
Basic instances are:
instance e ~ String => FailMsg (Either e) where
        failMsg = Left
        Left msg >?= f = f msg
        x >?= _ = x
instance FailMsg IO where
        failMsg = throwIO . userError
        action >?= f = catch action (f . ioeGetErrorString)
Now let's focus on ReadP. Let P carry a failure message:
data P a
        = Get (Char -> P a)
        | Look (String -> P a)
        | Fail String
        | Result a (P a)
        | Final (NonEmpty (a,String))
Then we have:
instance FailMsg P where
        fail = Fail
        Fail msg >?= f = f msg
        p >?= _ = p
instance FailMsg ReadP where
        fail msg = R (\_ -> fail msg)
        R m >?= f = R (\k -> case m k of
            Fail msg -> let
                R n = f msg
            in n k
            p -> p
            )
This is incredibly useful. This can be used when there are multiple
types of parse error.
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Re: Let ReadP carry a failure message

David Feuer