Let ReadP carry a failure message

[Isaac Elliott]

Looks like `MonadError String` to me.

On Fri, 20 Nov 2020, 1:08 pm Dannyu NDos, <ndospark320 at gmail.com> 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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