Monday 28 May 2012

Fixing scala.Either - left/ returns projection

[2012-06-06 Update: support for 'if' in for-comprehensions now removed - result must be either Left or Right, and never empty!]
[2012-06-11 Update: added link to 'tests involving Option'.]

That Either can, after all, participate in for comprehensions involving multiple generators was a revelation of an earlier post.

To recap, you need to indicate which of the two possible results you're interested in, by using either the left: LeftProjection or right: RightProjection of the Either. For example, multiple checks (each returning an Either[..., Int]) on some a: Int, would be carried out as follows:

What needs fixing

However, it has subsequently been pointed out (in this bug report), that definitions aren't available,

and worse still, I notice that neither can you use if together with multiple generators and yield:


The above bug-report resulted in a debate about a proposal to fix Either by modifying it in such a way that it would behave like its right: RightProjection. This so-called right-biasing would therefore eliminate the need to specify which result you were interested in (and involve left and right being deprecated).

However, not everyone agreed with this, with some calling for an alternative class to be introduced.

My own opinion is that Either should just be patched so as to enable the above two cases to work. (I don't agree with the right-biasing proposal, since the class's very name does not imply any bias; I'd sooner see an unbiased Either alongside a biased alternative, with a name such as Checked.)


So I ended up trying to do the patching myself, and against the odds, now appear to have succeeded.

Rather than have left/ return an Either, it now returns a Left-/RightProjection.

This means that yield will result in a Left-/RightProjection instead of an Either, which just requires that you add .e to obtain the Either:

The patched Either is maintained in a Github project called scala-either-proj-map-returns-proj, including for comprehension tests, tests involving Option, and the test app from which the example above was taken.

Feel free to build the project yourself, and add further tests that try to break something.

Comments welcome.

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