ErrorT is deprecated, but ExceptT doesn't fit

问题

I had a monadic computation. At some point it started requiring a MonadFail constraint because of a monadic pattern-match.

My easy fix was to run it with this:

fmap (either error id) . runErrorT

Yet ouch:

Deprecated: "Use Control.Monad.Trans.Except instead"

So it appears that ErrorT is deprecated, and I'm to use ExceptT instead. That sounds fine from the outside, but it doesn't appear like ExceptT is a drop-in replacement at all! Just look at the instance declarations:

instance (Monad m, Error e) => MonadFail (ErrorT e m)
instance MonadFail m => MonadFail (ExceptT e m)

ErrorT provides a MonadFail implementation. ExceptT merely lifts it.

I'm not quite sure where to go from this point.

• my code eventually won't have the refutable bind at all. It's one of those cases where it's statically known to work, yet not proven to the type checker yet. I'll get there, but it'll take a while. (Like, I'm going to need dependent types, so I'm going to need to learn dependent types. It'll take a while.)

• yet in the meantime, my code needs to bind, and really I'm fine with error being called if I mess up.

It seems what I'm looking for is some generic MonadFail implementation that allows me to… well, do what I want with a pattern failing to match. ErrorT did this, and ExceptT doesn't.

What are my non-deprecated options?

来源：https://stackoverflow.com/questions/59537969/errort-is-deprecated-but-exceptt-doesnt-fit