Build the untagged dispatch as a plan before emitting it by cknitt · Pull Request #9045 · rescript-lang/rescript

cknitt · 2026-10-25T20:37:38Z

The dispatch of a switch on an untagged variant was decided in two places while printing JavaScript: the literal side in compile_switch, and the payload side in compile_untagged_cases, each with only part of the facts. Constant folding in Lambda.switch had a third copy of the rules.

This PR builds the dispatch as data first. The new module compiler/ml/untagged_dispatch.ml turns the variant's facts and the match's arms into a tree of tests (x === lit, typeof x, instanceof, x !== null, &&, ||, !) and statements (if, switch (x), switch (typeof x)). It makes exactly today's decisions, step by step. Lam_compile.compile_untagged_plan prints the tree, and compile_untagged_cases and the literal closure in compile_switch are gone.

The output is byte-identical across the tests, the stdlib and gentype, with one deliberate exception. A body of an if is no longer compiled in a switch context, so a loop left from such an arm loses a needless label (loop_0: … break loop_0; becomes break;). Bodies are compiled in the order they are printed, so loop labels in arms are numbered top to bottom; in the arms of the literal ifs that reverses master's numbering, which no output in the tests shows.

New tests:

ounit_untagged_dispatch_tests.ml pins plans as data, one per dispatch rule.
untagged_loop_control_test.res (scanIf) covers the break from an arm of an if.

The next PRs in the stack fold constants by evaluating the plan (#9046), and decide the literal cases and same-body arms with it (#9047).

🤖 Generated with Claude Code

https://claude.ai/code/session_01CMkrYaktctdKc4M41WF36t

Why: The dispatch of a switch on an untagged variant was decided in two places, the literal side in `compile_switch` and the payload side in `compile_untagged_cases`, each with part of the facts, while printing JavaScript. `Untagged_dispatch.plan` builds the dispatch as a tree of tests and statements from the variant's facts and the match's arms, making today's decisions in one place; `Lam_compile.compile_untagged_plan` prints it. ounit tests pin plans as data. The output is unchanged except that a body of an `if` is no longer compiled in a `switch` context. Generated code changed (a loop left from an arm of an if): Example: @unboxed type t = TextItem(string) | CountItem(float) for i in 0 to Array.length(values) - 1 { switch values->Array.getUnsafe(i) { | TextItem(text) => seen->Array.push(text) | CountItem(_) => break } } Before (JS): loop_0: for (let i = 0, i_finish = values.length; i < i_finish; ++i) { ... } else { break loop_0; } } After (JS): for (let i = 0, i_finish = values.length; i < i_finish; ++i) { ... } else { break; } } Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01CMkrYaktctdKc4M41WF36t Signed-off-by: Christoph Knittel <ck@cca.io>

github-actions · 2026-10-25T20:37:59Z

Changelog

🏠 Internal

Build the dispatch of a switch on an untagged variant as a plan before printing it.

Semantic Diff

2 commits · Full page

Build the untagged dispatch as a plan before emitting it

_{fbfe930 · cknitt + Claude · Oct 25, 2026, 20:21 UTC}

Why

The dispatch of a switch on an untagged variant was decided in two places, the literal side in compile_switch and the payload side in compile_untagged_cases, each with part of the facts, while printing JavaScript.

Untagged_dispatch.plan builds the dispatch as a tree of tests and statements from the variant's facts and the match's arms, making today's decisions in one place; Lam_compile.compile_untagged_plan prints it. ounit tests pin plans as data. The output is unchanged except that a body of an if is no longer compiled in a switch context.

Generated code changed (a loop left from an arm of an if):

@unboxed type t = TextItem(string) | CountItem(float)
for i in 0 to Array.length(values) - 1 {
  switch values->Array.getUnsafe(i) {
  | TextItem(text) => seen->Array.push(text)
  | CountItem(_) => break
  }
}

Before (JS):

loop_0: for (let i = 0, i_finish = values.length; i < i_finish; ++i) {
  ...
  } else {
    break loop_0;
  }
}

After (JS):

for (let i = 0, i_finish = values.length; i < i_finish; ++i) {
  ...
  } else {
    break;
  }
}

✨ Compile the bodies of an if in the order they are printed

_{d764e3d · cknitt + Claude · Oct 26, 2026, 06:09 UTC · new}

Why

The if chain of the payload dispatch compiled its last arm first, so loop labels in its arms were numbered bottom to top. A dispatch on one typeof kind fell back to a switch for an empty case, with bodies compiled outside a switch context.

An if compiles its then branch before its else branch, so labels are numbered top to bottom, as master numbered them in the class, array and object chain; arms of the literal ifs are now numbered in that order too. A dispatch on one typeof kind is always an if.

Generated code changed (loop labels in literal if arms):

a loop needing a label in each of two literal arms printed as `if`s

Before (JS): the label of the second arm is loop_0, of the first loop_1 After (JS): the label of the first arm is loop_0, of the second loop_1

codecov · 2026-10-25T20:43:36Z

Codecov Report

❌ Patch coverage is 94.80969% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.36%. Comparing base (1659aee) to head (d764e3d).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
compiler/ml/untagged_dispatch.ml	91.72%	11 Missing ⚠️
compiler/core/lam_compile.ml	96.59%	3 Missing ⚠️
tests/ounit_tests/ounit_untagged_dispatch_tests.ml	98.52%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9045      +/-   ##
==========================================
+ Coverage   81.33%   81.36%   +0.02%     
==========================================
  Files         456      458       +2     
  Lines       64273    64366      +93     
==========================================
+ Hits        52275    52369      +94     
+ Misses      11998    11997       -1

Files with missing lines	Coverage Δ
tests/ounit_tests/ounit_tests_main.ml	`100.00% <ø> (ø)`
tests/ounit_tests/ounit_untagged_dispatch_tests.ml	`98.52% <98.52%> (ø)`
compiler/core/lam_compile.ml	`91.63% <96.59%> (+0.45%)`	⬆️
compiler/ml/untagged_dispatch.ml	`91.72% <91.72%> (ø)`

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pkg-pr-new · 2026-10-25T20:49:30Z

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commit: d764e3d

github-actions · 2026-10-25T20:53:06Z

Developer playground preview: https://rescript-lang.github.io/rescript/dev-playground/?version=pr-9045

cknitt · 2026-10-25T21:27:33Z

@codex review

chatgpt-codex-connector · 2026-10-25T21:27:48Z

Codex Review Summary

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

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-26T06:34:22.236225Z	`d764e3d`	Manual request

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chatgpt-codex-connector · 2026-10-25T21:29:15Z

Codex Review: Didn't find any major issues. 👍

Reviewed commit: fbfe93006f

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Why: The `if` chain of the payload dispatch compiled its last arm first, so loop labels in its arms were numbered bottom to top. A dispatch on one `typeof` kind fell back to a `switch` for an empty case, with bodies compiled outside a `switch` context. An `if` compiles its `then` branch before its `else` branch, so labels are numbered top to bottom, as master numbered them in the class, array and object chain; arms of the literal `if`s are now numbered in that order too. A dispatch on one `typeof` kind is always an `if`. Generated code changed (loop labels in literal `if` arms): Example: a loop needing a label in each of two literal arms printed as `if`s Before (JS): the label of the second arm is `loop_0`, of the first `loop_1` After (JS): the label of the first arm is `loop_0`, of the second `loop_1` Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01CMkrYaktctdKc4M41WF36t Signed-off-by: Christoph Knittel <ck@cca.io>

cknitt · 2026-10-26T06:31:47Z

@codex review

chatgpt-codex-connector · 2026-10-26T06:34:20Z

Codex Review: Didn't find any major issues. Can't wait for the next one!

Reviewed commit: d764e3d835

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cknitt mentioned this pull request Oct 25, 2026

Fold untagged matches by evaluating the dispatch plan #9046

Merged

cknitt added this pull request to stack #9048 October 25, 2026 20:38

cknitt marked this pull request as ready for review October 25, 2026 21:27

cknitt requested a review from cristianoc October 26, 2026 06:02

cristianoc approved these changes Oct 26, 2026

View reviewed changes

cknitt merged commit 91c9360 into master Oct 26, 2026
26 checks passed

cknitt deleted the dispatch-plan branch October 26, 2026 08:57

anmonteiro mentioned this pull request Oct 27, 2026

test: cover unboxed dispatch boundaries melange-re/melange#2120

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Build the untagged dispatch as a plan before emitting it - #9045

Build the untagged dispatch as a plan before emitting it#9045
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