Add a BindLeft type class

[natefaubion]

class BindLeft m where
  lbind :: forall a b r. m a r -> (a -> m b r) -> m b r

class (Biapplicative m, BindLeft m) <= MonadLeft m

This is a generalized, type-changing catch operation, which is useful for things like Either, and also lets you implement a Monad instance for Flip.

[JordanMartinez]

Wouldn't you also need to define an ApplyLeft and ApplicativeLeft to enable Flip to have a MonadLeft instance?

[natefaubion]

@JordanMartinez Yes, my formulation above is incorrect. Biapplicative isn't appropriate for this.

[mikesol]

Ran across this today as I was looking for a bindLeft in a project. Would be useful!

[JordanMartinez]

Maybe this would be better moved to the 'qualified do' package?

[garyb]

I don't think it necessarily needs qualified do to be useful, just the existence of the operation at all would be good! But it might also be good to provide a qualified do.