[Haskell-cafe] Error-containing Maybes

Ivan Lazar Miljenovic ivan.miljenovic at gmail.com
Thu Jun 15 10:30:53 UTC 2017

On 15 June 2017 at 20:21, Robin Palotai <palotai.robin at gmail.com> wrote:
> See https://hackage.haskell.org/package/errors-2.2.0/docs/Data-EitherR.html
> for somewhat related type.

Ahah, wasn't aware of that package.

> How about `type Result e = Either e ()` ?

I did mention that at the end of my email ;-)

> 2017-06-15 11:38 GMT+02:00 Ivan Lazar Miljenovic
> <ivan.miljenovic at gmail.com>:
>> 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 at gmail.com
>> http://IvanMiljenovic.wordpress.com
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Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com

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