typelevel / typelevel/cats

Deprecate the `Parallel` ZipLazyList instance

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bug Source Breaking
Dominant language
Scala
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Description

ZipLazyList and LazyList have different behavior for Applicative#pure and therefore are not isomorphic in pure which is one of the laws of Parallel. We currently don't test this, but it would fail if we could, so we should deprecate and remove the instance.

Contributor guide

Open the contributing guide

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

Locate the Parallel instances for ZipLazyList and LazyList, then inspect the Applicative#pure behavior and existing Parallel law coverage. Confirm the non-isomorphism described in the issue, identify the deprecation and removal path, and verify that the affected instance is no longer available after the change.

Written by the indexing model from the issue text.

Assessment

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

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