typelevel / typelevel/cats

untilM and friends: check condition first

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Scala
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Description

Currently, ef.untilM_(cond) seems to first run the ef effect, and only then run the condition effect and check it.

However, whileM_ runs the condition first. I think that's a surprising inconsistency.

While it would be harmful to any users of the existing methods to change the behavior, I think we should support both do while and while scenarios in the methods, so the remaining ones (variant of untilM that checks the condition first, variant of whileM that runs the first) could be added.

Naming suggestions welcome!

Contributor guide

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First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by comparing the existing untilM_ and whileM_ entry points and their condition/effect ordering. Determine the API names and behavior for both condition-first and effect-first variants, then add coverage for each ordering. Done means both do-while and while scenarios are supported without changing existing methods.

Written by the indexing model from the issue text.

Assessment

Tech stack
scala
Domain
tooling
Issue type
Feature
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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