Thursday, 14 June 2012

Fixing scala.Either - unbiased vs biased

[2012-06-18 Update: details of new branch added, where Either gains right-biased capability while retaining its unbiased one.]
[2012-06-25 Update: details of new branch added, where Either gains a withFilter method.]
[2012-06-29 Update: link to SIP candidate added.]

This latest effort is the result of a stalled attempt to write a formal proposal (SIP) explaining exactly how scala.Either should be fixed.

It became apparent that the need to first deprecate things, as opposed to simply removing them, might make the approach I was advocating less attractive.

So, with a view to helping the community to come to an informed decision, I now present a workable set of changes that leaves scala.Either unbiased, together with one that gives it a bias towards its Right subtype, for comparison.


To recap, we wish to make the following changes to Either's Left-/RightProjection inner classes:
  • have map return a Left-/RightProjection instead of an Either
  • have flatMap also do this, and also take an A => Left-/RightProjection instead of an A => Either
  • remove filter
Although we can deprecate filter, we cannot do this to the existing versions of its map and flatMap.

Therefore, we have no choice but to deprecate Left-/RightProjection themselves, and in turn, Either's left and right methods.

The alternative names I came up with are Left-/RightProj, lp and rp. At least they are shorter, and anyway, left and right would no longer be correct, since their map and flatMap now return a Left-/RightProj instead of an Either.

One advantage of this is that we can now just leave out filter, rather than have to deprecate it.


Here we need to do the following:
  • deprecate Either's left and right methods
  • deprecate Left-/RightProjection themselves
  • add the following methods to Either itself
    • get
    • getOrElse
    • foreach
    • forall
    • exists
    • flatMap
    • map
    • toSeq
    • toOption
Note the absence of filter.

Decision time

Please take a look at the unbiased (lp_rp branch) and Right-biased (right-biased branch) end results, together with the respective versions of the accompanying test code:

The reasons to remain unbiased, which occur to me, are as follows,
  • any use-cases demanding the ability to choose from which subtype (Left or Right) the value passed to map, etc. is taken, will not be excluded
  • Either does not imply any bias, although Left and Right do imply a weak one

while those to go biased, would seem to me to be that,
  • there is no need to append .rp (or .lp) in for comprehensions, which must be done for each generator
  • there is no need to append .e in order to obtain the Either from the result yielded.
Please register your preference on this scala-debate thread, or else below.


2012-06-18 Following a highly productive exchange of ideas on the above thread, a new branch entitled add_right-bias_2-10 has been added to the project. In this version of Either, the question of unbiased vs biased goes away! Since these two usage modes turn out not to be mutually exclusive, a right-bias has been added, and the existing unbiased capability (through the lp and rp methods) retained.

2012-06-25 add_right-bias_2-10_withFilter branch added to the project. This version adds a withFilter method to Either, such that right-biased for comprehensions may now use if and involve refutable pattern-matching, provided the appropriate implicit conversions are supplied (as indicated by the compiler), that will be used to create a Left value whenever the expression following if is false, or there is no match.

2012-06-29 SIP candidate now written.

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