Because the primary purpose of fail is to provide for failed pattern matches, which invoke it with a String representing the pattern match error. On Tue, Nov 30, 2021 at 2:16 PM Daneel Yaitskov 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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-- brandon s allbery kf8nh allbery.b@gmail.com

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 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

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.