Developer playground: compare Lambda before and after optimization

[fhammerschmidt]

Add Before, After, and Diff views to the developer playground's Lambda tab. Playground API v9 exposes lambda_optimized from compileWithDebug after the whole-term Lambda passes and before export grouping and JavaScript lowering. Regular compilation does not capture debug output; older compiler bundles retain an availability notice.

Keep the Lambda view mounted and cache the latest rendered comparison by its printed inputs, including line alignment, inline highlights, and context sections. Switching tabs or recompiling identical dumps reuses the diff DOM. Collapsed context renders only when expanded, and unchanged regions retain their expansion state across edits. Hidden Lambda panels keep their dump snapshot in a signal that is updated only while visible. Repeated compile errors leave that snapshot untouched, including with the locked rescript-signals 3.1.1. While Diff is open, changed output is debounced for 200 ms; opening Diff is immediate, and leaving it or unmounting cancels pending work.

Strip shared edges before the line comparison, count those edges once in the output budget, and omit empty visible sections. Token alignment prioritizes shared atoms over punctuation. Preflight all candidate work in a changed block and skip the whole block when it cannot fit the remaining global budget, preserving capacity for later small changes. Actual work on unchosen candidates remains counted so the overall bound still holds.

Validation:

• Fresh ARM Linux validation passes yarn workspace dev-playground test and yarn workspace dev-playground build with Node 22 and the immutable lockfile dependencies.
• Native compiler, runtime, Belt, and browser compiler builds pass.
• The compiler runner (node scripts/test.js -ounit -mocha -build -docstrings) passes: 336 unit cases (one skipped), 622 runtime/Belt tests, all 42 build fixtures, and 744 documentation examples.
• All 37 developer playground tests pass, including 26 Lambda algorithm/rendering tests. New cases cover hidden panels catching up, debounced consecutive edits, cancellation on tab switches, expansion surviving delayed updates, and cached panels surviving repeated compile errors without rebuilding.
• Playground bundle tests cover API v9 optimized output, regular compilation, and master's diagnostic-color regressions.
• Production Vite build, OCaml/ReScript/JavaScript format checks, and changelog validation pass.
• Browser layout and scrolling remain unverified locally; Chromium cannot launch in this macOS environment because system appearance resources are unavailable.

[fhammerschmidt]

Seems I cannot extract optimized lambda for the playground without this addition

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.13%. Comparing base (b7c67d2) to head (8bbffec).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
compiler/core/lam_compile_main.ml	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8636      +/-   ##
==========================================
+ Coverage   81.11%   81.13%   +0.01%     
==========================================
  Files         455      456       +1     
  Lines       63672    63724      +52     
==========================================
+ Hits        51650    51700      +50     
- Misses      12022    12024       +2     

Files with missing lines	Coverage Δ
compiler/core/lam_compile_main.ml	89.41% <0.00%> (-0.32%)	⬇️

... and 5 files with indirect coverage changes

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[pkg-pr-new]

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rescript

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@rescript/belt

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@rescript/darwin-arm64

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@rescript/linux-arm64

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@rescript/win32-x64

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commit: 8bbffec

[github-actions]

Developer playground preview: https://rescript-lang.github.io/rescript/dev-playground/?version=pr-8636

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-09T13:26:25.416528Z	1be49a8	Manual request

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[fhammerschmidt]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Hooray!

Reviewed commit: 1be49a87d6

ℹ️ About Codex in GitHub

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• Open a pull request for review
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• Comment "@codex review".

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[cknitt]

Review notes (no wrong-diff bugs found; mostly performance):

Performance

1. LambdaView.res:78: the diff is recomputed on every compile. LambdaView is rebuilt per compile result, so the Lazy cache only lives for one render, and LambdaInlineDiff.highlight / LambdaDiff.sections aren't cached at all. With the Diff subtab open, each keystroke reruns the line diff (up to 1M cells) and the token diff (up to 250k cells) and rebuilds the diff DOM. Switching Before → Diff → Before → Diff redoes the work too.
2. LambdaView.res:67: collapsed sections still render every hidden line. renderLines runs eagerly inside each closed <details>, so a 10k-line dump with a one-line change still creates ~10k line spans (×4 children). Rendering on toggle would avoid that.
3. LambdaDiff.res:49: the maxLines check uses the untrimmed n + m. Two 10,001-line dumps that differ in one line trim to a 1×1 middle but still show "exceeds the diff work limit", which contradicts the comment and the test.
4. LambdaInlineDiff.res:172: the shared token budget is charged for every candidate pair in a block, including pairs that are never chosen. One early large block (e.g. 16×16 lines of ~100 tokens) can drain the 250k budget, and every later block falls back to whole-line highlighting based on position, not similarity.

Behaviour
5. LambdaInlineDiff.res:711: the token LCS maximizes all matched tokens, punctuation included, but pairing uses the count of shared atoms. An alignment that matches (/) instead of atoms can undercount shared and reject similar pairs via shared*2 < largest.
6. Main.res:1335: because LambdaView is replaced on every compile, opened "Show N unchanged lines" sections collapse again and the scroll position jumps after each edit.
7. LambdaDiff.res:154: sections emits Visible([]) before a leading unchanged run (head === start) and after a trailing one (tail === end_), which renders empty <pre class="lambda-diff-block"> elements. The test's flatMap check doesn't catch it.

Nit
8. package.json:12: stray double space before scripts/lambda-diff.test.mjs.

Items 1, 2 and 6 share one fix: keep the view and its diff results alive across compiles, keyed on the input text, and render collapsed sections only when opened.

🤖 Generated with Claude Code

https://claude.ai/code/session_01RrJ4XshH5etfH87fRThk2B

[cknitt]

Thanks, e33b965 addresses all eight points from my earlier review. Two minor leftovers, neither blocking:

1. Hidden Lambda panels update on every compile. LambdaView and its Before/After panels are now always mounted, so each compile writes both full Lambda dumps into hidden DOM even while another tab (e.g. JS) is active. Before this change the view only existed on the Lambda tab. This costs a little on large modules; skipping those panel updates while visible() is false would avoid it.
2. The Diff tab recomputes on every edit that changes the Lambda output. With Diff open, each such keystroke reruns the full comparison with no debounce and rebuilds the diff DOM. Expanded sections stay open thanks to the per-region state, but the scroll position can still move. The work is bounded now, so this is fine as is; a short debounce would smooth typing on big modules.

🤖 Generated with Claude Code

https://claude.ai/code/session_01RrJ4XshH5etfH87fRThk2B

[cknitt]

Thanks, 8eac328 addresses both follow-ups:

• Hidden panels: panelInput only refreshes while the view is visible. Because lambdaVisible short-circuits on activeTab, nothing in LambdaView subscribes to compileResult while another tab is open, so hidden panels do no work there.
• Debounce: each changed result clears the previous 200 ms timer, opening Diff is immediate, and leaving Diff or unmounting cancels it. Signal.peek(diffInput) keeps the effect from triggering itself.

One tiny nit, not blocking: on the Lambda tab with a compile error, lambdaVisible does read compileResult. Computed.make in rescript-signals has no equality check by default, so panelInput still propagates its unchanged snapshot on every compile. The hidden Before signalText resets its text, and the After View.tracked rebuilds its <pre>. Passing ~equals=(a, b) => a === b to panelInput would skip that.

🤖 Generated with Claude Code

https://claude.ai/code/session_01RrJ4XshH5etfH87fRThk2B

[github-actions]

Semantic diff: https://cdn.rescript-lang.org/semantic-diff/pr-8636/index.html?v=8bbffec3e