Implement traverseTap

[majk-p]

I'd like to introduce traverseTap. I found it would be useful for me when attempting to perform debug log executing an effect, but dropping the result.

I'd like to add some tests, could someone please advice the best place to add them?

[LukaJCB]

I like it 👍

[johnynek]

my personal opinion is that each function like this should be added to the laws, and the law is just a duplication of the implementation. That way, if someone overrides to make a more optimized version, we have a law that it behaves exactly as the direct implementation.

[majk-p]

my personal opinion is that each function like this should be added to the laws, and the law is just a duplication of the implementation. That way, if someone overrides to make a more optimized version, we have a law that it behaves exactly as the direct implementation.

That make sense. Implemented in https://github.com/typelevel/cats/pull/3647/files#diff-5ee4897e7202dfd0b4fd61f8afac00bbc7a9b28dc9309b3006f3ffc12b2cd36eR55. Couldn't find a better name for this law.

[johnynek]

this is great! Unfortunately, we also need to add a line here:

cats/laws/src/main/scala/cats/laws/discipline/TraverseTests.scala

Line 59 in e0f0a83

"traverse ref zipWithIndex" -> forAll(laws.zipWithIndexRef[A, C] _)

to actually call this code when the tests are run.

[majk-p]

Thank you for pointing this out @johnynek. Sorry I've missed that, I'm not yet familiar enough with the project.
Just fixed that.

[johnynek]

Thanks! This looks useful for logging.

[majk-p]

Had to make one more push to avoid breaking binary compatibility in laws

[codecov-io]

Codecov Report

Merging #3647 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3647   +/-   ##
=======================================
  Coverage   90.24%   90.25%           
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  Files         391      391           
  Lines        8855     8863    +8     
  Branches      256      248    -8     
=======================================
+ Hits         7991     7999    +8     
  Misses        864      864