Drop leftover conditions and empty branches from if statements by cknitt · Pull Request #9024 · rescript-lang/rescript

cknitt · 2026-10-24T16:35:15Z

Two places in JS generation left an if behind with nothing to run in a branch. Removing those branches negates more conditions, so a negation bug that was already on master is fixed first.

1. Negated float comparisons. Js_exp_make.not negated a comparison by swapping its operator. That's wrong for floats, because a comparison with NaN is always false:

let k = (x: float) => !(x < 0.0)

Before:

function k(x) {
  return x >= 0.0;
}

After:

function k(x) {
  return !(x < 0.0);
}

k(NaN) returned false instead of true. Float comparisons (Pfloatcomp) and comparisons of operands of unknown type (Pjscomp, from %unsafe_lt etc.) now use their own operators, Float_lt, Float_le, Float_gt and Float_ge. They are printed as <, <=, >, >=, and not leaves them as !(…). Int, bigint and string comparisons are unchanged, since their operands can't be NaN. The only existing output this changes is caml_compare_bigint_test.mjs, whose values come from @val external bigint: string => float and so are typed as floats. New test: negated_float_comparison_test.res.

Fixes #9026.

2. A bare condition. Js_stmt_make.if_ turned an if with two empty branches into its condition as a statement. This happened, for example, when the remaining cases of a switch all fall through. The condition is now dropped when no_side_effect_expression accepts it. Empty blocks in the branches are ignored, so an enclosing if can still merge conditions. The both-branches-pure case in Lam_compile now goes through the same path.

let f3 = ({rank: lhs, _}, {rank: rhs}) =>
  switch (lhs, rhs) {
  | (Ranked(x), Ranked(y)) => Pervasives.compare(x, y)
  | _ => assert(false)
  }

Before:

function f3(param, param$1) {
  let lhs = param.rank;
  let rhs = param$1.rank;
  if (typeof lhs !== "object") {
    lhs === "Uninitialized";
  } else {
    if (typeof rhs === "object") {
      return Primitive_int.compare(lhs._0, rhs._0);
    }
    rhs === "Uninitialized";
  }
  throw { ... };
}

After:

function f3(param, param$1) {
  let lhs = param.rank;
  let rhs = param$1.rank;
  if (typeof lhs === "object" && typeof rhs === "object") {
    return Primitive_int.compare(lhs._0, rhs._0);
  }
  throw { ... };
}

3. An empty branch. Js_pass_flatten turns a statement c ? b : e into if (c) { b; } else { e; }. When b or e was pure, a later pass removed it and left {}. A pure branch is now left out: with an empty then-branch the condition is negated, and with both empty the condition is dropped when it is pure.

let f = (x: int) => {
  switch x {
  | 1 => r := 2
  | _ => ()
  }
  Console.log(r)
}

Before:

function f(x) {
  if (x !== 1) {
    
  } else {
    r.contents = 2;
  }
  console.log(r);
}

After:

function f(x) {
  if (x === 1) {
    r.contents = 2;
  }
  console.log(r);
}

In Stdlib_IteratorTests.mjs, a dropped condition was the only use of a let match = next(it), so the call is now a plain statement and the later match$N names are renumbered.

🤖 Generated with Claude Code

https://claude.ai/code/session_015yE4gCmTmJKVecrVt1PU8C

github-actions · 2026-10-24T16:35:37Z

Changelog

🐛 Bug fix

Fix negated float comparisons with NaN: !(x < 0.0) compiled to x >= 0.0, which is false for NaN.

💅 Polish

Stop emitting bare conditions like x === null; and empty if/else branches.

Semantic Diff

3 commits · Full page

✨ Keep the operator of a negated float comparison

_{137367c · cknitt + Claude · Oct 24, 2026, 18:32 UTC · new}

Js_exp_make.not negated a comparison by swapping its operator, turning !(x < 0.0) into x >= 0.0, which is false for NaN. Float comparisons, and comparisons of operands of unknown type, now use their own operators, printed as <, <=, >, >=, which not leaves as !(…). Fixes #9026.

JS modified (negated float comparison)

let k = (x: float) => !(x < 0.0)

js

 function k(x) {
-  return x >= 0.0;
+  return !(x < 0.0);
 }

Review

Do int, bigint and string comparisons change? No, they keep the old operators, so negating them still swaps the operator. The only changed output is in caml_compare_bigint_test, whose bigint values are typed as float.

Drop a pure condition when both branches of an if are empty

_{adde988 · cknitt + Claude · Oct 24, 2026, 16:35 UTC}

A switch whose remaining cases all fall through to the same code left their test behind as a bare statement, such as x === null;, and the empty block it left in an enclosing if blocked merging the conditions.

JS modified (switch with cases that fall through)

type rank = Uninitialized | Visited | Ranked(int)
let f3 = (lhs: rank, rhs: rank) =>
  switch (lhs, rhs) {
  | (Ranked(x), Ranked(y)) => Pervasives.compare(x, y)
  | _ => assert(false)
  }

js · tests/tests/src/fun_pattern_match.res

-if (typeof lhs !== "object") {
-  lhs === "Uninitialized";
-} else {
-  if (typeof rhs === "object") {
-    return Primitive_int.compare(lhs._0, rhs._0);
-  }
-  rhs === "Uninitialized";
+if (typeof lhs === "object" && typeof rhs === "object") {
+  return Primitive_int.compare(lhs._0, rhs._0);
 }
 throw {

Review

Is a condition with side effects still evaluated? Yes, only conditions that no_side_effect_expression accepts are dropped.

Leave no empty branch when flattening a conditional statement

_{6e90587 · cknitt + Claude · Oct 24, 2026, 17:09 UTC}

The flatten pass turned a statement c ? b : e into an if with b; and e; as branches; a pure branch value such as undefined was dropped later and left an empty branch behind.

JS modified (switch with an empty case at statement level)

let r = ref(0)
let f = (x: int) => {
  switch x {
  | 1 => r := 2
  | _ => ()
  }
  Console.log(r)
}

js

-if (x !== 1) {
-
-} else {
+if (x === 1) {
   r.contents = 2;
 }

Review

Why not build the if with S.if_? It folds an if/else of two expression statements back into a conditional expression, which is what this pass undoes.

cknitt · 2026-10-24T16:36:53Z

@codex review

chatgpt-codex-connector · 2026-10-24T16:37:07Z

Codex Review Summary

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

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-24T19:45:03.934796Z	`6e90587`	Manual request

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

Open a pull request for review
Mark a draft as ready
Comment "@codex review" or "@codex security review".

Codex reacts with 👀 while any review is running, comments if it has suggestions, and reacts with 👍 once all reviews finish with no findings.

chatgpt-codex-connector · 2026-10-24T16:38:19Z

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

Reviewed commit: 2f29cd28e5

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

Open a pull request for review
Mark a draft as ready
Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

codecov · 2026-10-24T16:41:24Z

Codecov Report

❌ Patch coverage is 88.88889% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.32%. Comparing base (188e24e) to head (6e90587).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
compiler/core/js_pass_flatten.ml	62.50%	3 Missing ⚠️
compiler/core/lam_compile_util.ml	85.71%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9024   +/-   ##
=======================================
  Coverage   81.32%   81.32%           
=======================================
  Files         456      456           
  Lines       64017    64042   +25     
=======================================
+ Hits        52059    52082   +23     
- Misses      11958    11960    +2

Files with missing lines	Coverage Δ
compiler/core/js_exp_make.ml	`68.46% <100.00%> (+0.18%)`	⬆️
compiler/core/js_op_util.ml	`91.07% <100.00%> (ø)`
compiler/core/js_stmt_make.ml	`71.92% <100.00%> (+0.63%)`	⬆️
compiler/core/lam_compile.ml	`90.25% <100.00%> (ø)`
compiler/core/lam_compile_primitive.ml	`81.25% <100.00%> (+0.11%)`	⬆️
compiler/core/lam_compile_util.ml	`95.23% <85.71%> (-4.77%)`	⬇️
compiler/core/js_pass_flatten.ml	`76.74% <62.50%> (-4.34%)`	⬇️

... and 3 files with indirect coverage changes

🚀 New features to boost your workflow:

❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

pkg-pr-new · 2026-10-24T16:59:20Z

Open in StackBlitz

rescript

npm i https://pkg.pr.new/rescript@9024

@rescript/belt

npm i https://pkg.pr.new/@rescript/belt@9024

@rescript/darwin-arm64

npm i https://pkg.pr.new/@rescript/darwin-arm64@9024

@rescript/linux-arm64

npm i https://pkg.pr.new/@rescript/linux-arm64@9024

@rescript/linux-x64

npm i https://pkg.pr.new/@rescript/linux-x64@9024

@rescript/runtime

npm i https://pkg.pr.new/@rescript/runtime@9024

@rescript/win32-x64

npm i https://pkg.pr.new/@rescript/win32-x64@9024

commit: 6e90587

cknitt · 2026-10-24T17:21:12Z

@codex review

chatgpt-codex-connector

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: f074ff9361

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

Open a pull request for review
Mark a draft as ready
Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

github-actions · 2026-10-24T17:45:14Z

Developer playground preview: https://rescript-lang.github.io/rescript/dev-playground/?version=pr-9024

Js_exp_make.not negated a comparison by swapping its operator, turning !(x < 0.0) into x >= 0.0, which is false for NaN. Float comparisons, and comparisons of operands of unknown type, now use their own operators, printed as <, <=, >, >=, which not leaves as !(…). Fixes #9026. JS modified (negated float comparison): Example: let k = (x: float) => !(x < 0.0) Before (js): function k(x) { return x >= 0.0; } After (js): function k(x) { return !(x < 0.0); } Review: Do int, bigint and string comparisons change? No, they keep the old operators, so negating them still swaps the operator. The only changed output is in caml_compare_bigint_test, whose bigint values are typed as float. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_015yE4gCmTmJKVecrVt1PU8C Signed-off-by: Christoph Knittel <ck@cca.io>

A switch whose remaining cases all fall through to the same code left their test behind as a bare statement, such as `x === null;`, and the empty block it left in an enclosing if blocked merging the conditions. JS modified (switch with cases that fall through): Example: type rank = Uninitialized | Visited | Ranked(int) let f3 = (lhs: rank, rhs: rank) => switch (lhs, rhs) { | (Ranked(x), Ranked(y)) => Pervasives.compare(x, y) | _ => assert(false) } Before (js): tests/tests/src/fun_pattern_match.res if (typeof lhs !== "object") { lhs === "Uninitialized"; } else { if (typeof rhs === "object") { return Primitive_int.compare(lhs._0, rhs._0); } rhs === "Uninitialized"; } throw { After (js): tests/tests/src/fun_pattern_match.res if (typeof lhs === "object" && typeof rhs === "object") { return Primitive_int.compare(lhs._0, rhs._0); } throw { Review: Is a condition with side effects still evaluated? Yes, only conditions that no_side_effect_expression accepts are dropped. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_015yE4gCmTmJKVecrVt1PU8C Signed-off-by: Christoph Knittel <ck@cca.io>

The flatten pass turned a statement `c ? b : e` into an if with `b;` and `e;` as branches; a pure branch value such as `undefined` was dropped later and left an empty branch behind. JS modified (switch with an empty case at statement level): Example: let r = ref(0) let f = (x: int) => { switch x { | 1 => r := 2 | _ => () } Console.log(r) } Before (js): if (x !== 1) { } else { r.contents = 2; } After (js): if (x === 1) { r.contents = 2; } Review: Why not build the if with S.if_? It folds an if/else of two expression statements back into a conditional expression, which is what this pass undoes. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_015yE4gCmTmJKVecrVt1PU8C Signed-off-by: Christoph Knittel <ck@cca.io>

cknitt · 2026-10-24T19:43:41Z

@codex review

chatgpt-codex-connector · 2026-10-24T19:45:02Z

Codex Review: Didn't find any major issues. What shall we delve into next?

Reviewed commit: 6e90587623

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

Open a pull request for review
Mark a draft as ready
Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

Brings in the 13 commits on master since the branch started, through rescript-lang#9024. Only compiler/ml/depend.ml and yarn.lock changed on both sides, in separate places: master adds the `finally` of `try` to the dependency walk, and the branch records every prefix of a free path. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: Ethan Cline <7142767+e-cline@users.noreply.github.com>

cknitt marked this pull request as ready for review October 24, 2026 16:36

cknitt requested a review from cristianoc October 24, 2026 16:55

cknitt enabled auto-merge (squash) October 24, 2026 16:55

cknitt changed the title ~~Drop a pure condition when both branches of an if are empty~~ Drop leftover conditions and empty branches from if statements Oct 24, 2026

chatgpt-codex-connector Bot reviewed Oct 24, 2026

View reviewed changes

Comment thread compiler/core/js_pass_flatten.ml

cristianoc approved these changes Oct 24, 2026

View reviewed changes

cknitt mentioned this pull request Oct 24, 2026

Negated float comparison is wrong for NaN: !(x < 0.0) compiles to x >= 0.0 #9026

Closed

cknitt and others added 3 commits October 24, 2026 18:32

cknitt force-pushed the drop-pure-empty-if branch from f074ff9 to 6e90587 Compare October 24, 2026 19:43

cknitt merged commit d48d577 into master Oct 24, 2026
26 checks passed

cknitt deleted the drop-pure-empty-if branch October 24, 2026 19:58

Navigation Menu

Repository navigation

Drop leftover conditions and empty branches from if statements - #9024

Drop leftover conditions and empty branches from if statements#9024
cknitt merged 3 commits into
masterfrom
drop-pure-empty-if

cknitt commented Oct 24, 2026 •

edited

Loading

Uh oh!

github-actions Bot commented Oct 24, 2026 •

edited

Loading

Uh oh!

cknitt commented Oct 24, 2026

Uh oh!

chatgpt-codex-connector Bot commented Oct 24, 2026 •

edited

Loading

Uh oh!

chatgpt-codex-connector Bot commented Oct 24, 2026

Uh oh!

codecov Bot commented Oct 24, 2026 •

edited

Loading

Uh oh!

pkg-pr-new Bot commented Oct 24, 2026 •

edited

Loading

Uh oh!

cknitt commented Oct 24, 2026

Uh oh!

chatgpt-codex-connector Bot left a comment

Uh oh!

Uh oh!

github-actions Bot commented Oct 24, 2026

Uh oh!

cknitt commented Oct 24, 2026

Uh oh!

chatgpt-codex-connector Bot commented Oct 24, 2026

Uh oh!

Uh oh!

Reviewers

Uh oh!

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

Conversation

cknitt commented Oct 24, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

github-actions Bot commented Oct 24, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Changelog

🐛 Bug fix

💅 Polish

Semantic Diff

Uh oh!

cknitt commented Oct 24, 2026

Uh oh!

chatgpt-codex-connector Bot commented Oct 24, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Codex Review Summary

Uh oh!

chatgpt-codex-connector Bot commented Oct 24, 2026

Uh oh!

codecov Bot commented Oct 24, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Codecov Report

Uh oh!

pkg-pr-new Bot commented Oct 24, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

cknitt commented Oct 24, 2026

Uh oh!

chatgpt-codex-connector Bot left a comment

Choose a reason for hiding this comment

💡 Codex Review

Uh oh!

Uh oh!

github-actions Bot commented Oct 24, 2026

Uh oh!

cknitt commented Oct 24, 2026

Uh oh!

chatgpt-codex-connector Bot commented Oct 24, 2026

Uh oh!

Uh oh!

Reviewers

Uh oh!

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

cknitt commented Oct 24, 2026 •

edited

Loading

github-actions Bot commented Oct 24, 2026 •

edited

Loading

chatgpt-codex-connector Bot commented Oct 24, 2026 •

edited

Loading

codecov Bot commented Oct 24, 2026 •

edited

Loading

pkg-pr-new Bot commented Oct 24, 2026 •

edited

Loading