Add sequenceEitherT

[thumphries]

These are combinators for running a batch of Either expressions independently, folding the error messages together if any of them fail.

I have been pasting foldEither around the place quite a bit lately, and recently needed foldEitherT, so I thought they might be general enough to propose here.

You can generalise to Semigroup but that would require reimplementing Errors, which is a handy validation type we get for free from transformers.

! @jystic @erikd-ambiata

[erikd-ambiata]

I'm ok with this. I can see that being useful. Hell I've even worked around this issue (twice) in Blowfish by returning an Either a b list, splitting it with partitionEithers and then testing that I have an empty list on the left.

However, since it is going into one of our base libraries, I'd like to get some more eyes on it.

[markhibberd]

I am good with code and implementation, but would rather naming reflect sequence then fold, I would expect foldEither = either.

[jacobstanley]

This name doesn't really match the naming / usage of the other two hoist functions:

hoistEither :: Monad m => Either x a -> EitherT x m a
hoistMaybe :: Monad m => x -> Maybe a -> EitherT x m a

[thumphries]

It's not exported (maybe it should be?), but i'll rename it

[jacobstanley]

Given:

Control.Applicative.Lift.runErrors :: Errors e a -> Either e a

Maybe mkErrors ?

[jacobstanley]

The type of this is similar to sequence:

sequence :: (Traversable t, Monad m) => t (m a) -> m (t a)

Would prefer something like:

sequenceEither :: (Traversable t, Monoid x) => t (Either x a) -> Either x (t a)

(Also x for the Left type variable to be consistent with the rest of the module)

[jacobstanley]

Or perhaps mconcatEither, not sure, but fold I would expect to take an argument

[thumphries]

I've been calling it sequenceErrors in my code, but proposed fold as it's essentially a (monoidal) fold of the left. Names are hard.

[thumphries]

Anyone mind sequenceEither and sequenceEitherT?

[charleso]

sequenceEither sounds good to me 👍

[jacobstanley]

yep, good with this

[thumphries]

Export y/n?

Honestly the Errors type is only mildly useful because it's limited to applicative. It's also a type synonym, so I prefer not to use it directly.

If you happen to want to use it, you'd probably want this function, else you have to learn how Lift works.

[jacobstanley]

n

I'd be happy to not export it, as your other functions don't mention Errors at all

[jacobstanley]

s/f/t/

[jacobstanley]

📻