See https://hackage.haskell.org/package/errors-2.2.0/docs/Data-EitherR.html for somewhat related type. How about `type Result e = Either e ()` ? 2017-06-15 11:38 GMT+02:00 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.)

-- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com http://IvanMiljenovic.wordpress.com _______________________________________________ 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.

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 To: Haskell Cafe Date: 15/06/17 11:38

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.)