For my part I think we should also deprecate the Functor (and dependent) instances for Either and tuple - but I know there are some who strongly disagree and I don't care all that much.
If you create your own data type for this, you are designing yourself into the Monad transformer hell you mention in another comment.
I use a statically typed language so I have fewer things to remember because the compiler does the bookkeeping for me.
`Either` is a generic variant record, it should not be used to carry errors.
And as a general rule, any programming construct that relies on convention instead of static typing is prone to generating bugs.
The name suggests that, but the Monad instance suggests that Either is an asymmetric data type, where the left and right branches carry different meaning.
> And as a general rule, any programming construct that relies on convention instead of static typing is prone to generating bugs.
If the Either branches were named `Error` and `Success`, using `Error` to carry errors would be just as much of a convention as using `Left` for that purpose (a more sensible convention, but nonetheless a convention). The only way you'd get around that is if you'd somehow encode in the language what is and what isn't an error and use that to statically force correct branch usage.
Is error left or right again?
That's right, you shouldn't have to remember.
Use a more specific type for this, some GADT with clearly named instances (e.g. "Error" and "Value").