[Haskell-cafe] MonadFail with type parameter for error message

[David Feuer]

Because that wouldn't actually help. You couldn't optimize it for different
stringy types. Using instance overlap (which is evil), you *could* do
something similar.

class Monad m => MonadFail s m where
  fail :: s -> m a
instance {-# OVERLAPPABLE #-} (MonadFail String m, IsString s) => MonadFail
s m where
  fail = fail . toString

I don't think this is terribly likely to work well in practice.

On Tue, Nov 30, 2021, 2:16 PM Daneel Yaitskov <dyaitskov at gmail.com> wrote:

> Dear Cafe,
>
> MonadFail.fail takes String.
> I wasn't able to find MonadFail for custom error type.
> Is there any proposals to base?
>
> Let's say Data.Text, which gains popularity with OverloadedStrings
> extensions.
>
> class MonadFail m where
>   fail :: String -> m a
>
> Why not ?
> class MonadFail m where
>   fail :: (forall s. IsString s => s) -> m a
>
> class MonadFailWith m s where
>   fail :: s -> m a
>
>
>
>
>
>
>
> --
>
> Best regards,
> Daniil Iaitskov
>
>
>
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