add methods to Left and Right for upcasting the other type parameter

[jozic]

This is another implementation of #6328 and #6329
based on suggestion by @lrytz

motivation: Left and Right constructors are typed as Left and Right respectively, but sometimes we want something like Option.empty which returns None but typed as Option.

The test represents a code that would not compile if plain Left/Right constructors are used

@SethTisue @lrytz please check it again, thanks!

EDIT: updated in #7080 to name the methods withLeft and withRight

[SethTisue]

anyone have a better idea for the method name?

[dwijnand]

LGTM. Thanks @jozic!

[NthPortal]

Is this worth having for None as well?

[jozic]

@NthPortal
there's Option.empty[A] for None and Option.apply[A] is sort of same for Some

[NthPortal]

@jozic good point

[dwijnand]

If anyone has a better idea for the method name, please send a PR. 😄

[jozic]

thanks @dwijnand

[SethTisue]

gah I hate the name up so I guess I'd better try and think of something

pleased to have this landing, regardless

[NthPortal]

I guess coming up with that will be Left.up to you Seth 😉

[dwijnand]

upcast is another option.

[lrytz]

maybe widen

[joroKr21]

widen would clash with a method on Functor in cats/scalaz which goes F[A] => F[B] for A <: B.

[smarter]

I would call it or, Left("empty").or[Int].

[dwijnand]

That would work really well if, given Either is right-biased, it had a companion object apply:

Either("foo").or[Int]

[NthPortal]

another possibility, based on @smarter's suggestion, might be Left#orRight[R] and Right#orLeft[L]

[jozic]

how about withRight[Int] (withLeft[Int])
Left("foo").withRight[Int] (though it look much longer than Left("foo").up[Int] :))

the issue with or or orRight(orLeft) is that "or" implies some possibility of it to be Right where in fact it's a Left

[SethTisue]

since M5 is imminent, I'm going to make an executive decision here and (for M5, anyway) go with @jozic's suggestion of withRight/withLeft: #7080

justification: if we expected this to be very commonly used, up would be nice, but otherwise, explicitness and readability win

as for withLeft versus orLeft, it's a minor difference, but I find @jozic's point convincing.