Something similar to what EitherR does?
https://hackage.haskell.org/package/errors-2.2.0/docs/Data-EitherR.html
-------- Original Message --------
Subject: [Haskell-cafe] Error-containing Maybes
From: Ivan Lazar Miljenovic
The Maybe type is often (almost always?) used to indicate that a computation may fail; specifically, `Maybe a` typically indicates that it may not be possible to always produce a value of type `a`.
However, there are times when the inverse of this is desirable (usually within a monadic/side-effectful context): if Nothing is returned then the computation was a success; otherwise, an error message is returned. Whilst this usage definitely fits the types, it breaks the mould of how we think/use Maybe in most cases and may at times cause confusion. As such, it may be advisable (and I know I've done this at least once) to define a `data Result e = Error e | Success` data type to model this behaviour.
Is this a typical enough requirement that would make it worth adding such a datatype to base (despite my usual aversion to piling more things in there) or having a library up on Hackage just for this very simplistic type?
(Another option would be to use `Either error ()`, which could be lifted better into an ExceptT result, albeit with the additional useless parameter/value in there.)