tobgu/pyrsistent

Allow multiple transform operations to be performed before checking invariants

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#89 opened on Jul 7, 2016

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enhancementhelp wanted

Description

(I think this is basically the same issue that @tomprince reported in https://github.com/tobgu/pyrsistent/issues/73 but I can't re-open that issue.)

Here's a use case:

In https://github.com/ClusterHQ/flocker/pull/2806 we introduced a mechanism for calculating diffs between Pyrsistent structures.

A diff is itself a Pyrsistent object and comprises a sequence of _Add, _Set, and _Remove operations which translate to transform calls on the root object to which the diff is being applied.

When we perform these discrete operations on a structure containing objects with invariant checks we trigger InvariantErrors because the invariant check is performed after each operation.

Instead it would be better to delay all the invariant checks until the complete sequence of transformations has been performed.

One thought is that it'd be nice to be able to get a "recursive evolver" which provides the transform interface. And which can be recursively persisted so as to perform the invariant checks on the entire structure.

I've had a go at this in:

This may be related to the following issues: https://github.com/tobgu/pyrsistent/issues/73 https://github.com/tobgu/pyrsistent/issues/44

Contributor guide

Tech stack
python
Domain
backend
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
Understanding of Pyrsistent data structuresKnowledge of invariant checksFamiliarity with the transform interface
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35
Research direction
The issue requests a way to batch multiple transform operations before invariant checks. The author references a use case from Flocker (PR #2806) and their own attempt in PR #2839. The discussion points to previous issues #73 and #44. To address this, one would need to understand the current invariant checking mechanism in Pyrsistent's transform method and design a recursive evolver that defers checks. The codebase is at https://github.com/tobgu/pyrsistent. Relevant files include the transform implementation and the evolver module.