Make `char` an alias of `int` by cknitt · Pull Request #8892 · rescript-lang/rescript

cknitt · 2026-10-18T07:53:59Z

First step of #7028.

char was a private abbreviation of int: a char could be coerced to an int with :>, but an int could not be used as a char. It is now a plain alias, so char literals and ints mix freely, including in one switch:

let fromInt: char = 65

let mixed = x =>
  switch x {
  | 'a' => "a"
  | 98 => "b"
  | 'c' .. 'e' => "c-e"
  | _ => "other"
  }

// `codePointAt` returns option<int>; emoji literals are full code points
let reaction = s =>
  switch String.codePointAt(s, 0) {
  | Some('😀' .. '😆') => "happy"
  | Some(0x1F44B) => "wave" // '👋'
  | Some(_) | None => "other"
  }

// reaction("😄 hi") == "happy", reaction("👋") == "wave", reaction("hi") == "other"

reaction compiles to:

function reaction(s) {
  let match = s.codePointAt(0);
  if (match !== undefined) {
    if (match >= 128512) {
      if (match >= 128519) {
        return "other";
      } else {
        return "happy";
      }
    } else if (match !== 128075) {
      return "other";
    } else {
      return "wave";
    }
  } else {
    return "other";
  }
}

Pattern matching treats a char constant and an int constant with the same value as the same constant (Parmatch.const_compare / const_key) for exhaustiveness checks, unused-case checks, and switch compilation. Char literals keep their /* 'a' */97 comments in the JS and their char spelling in warning examples.

Switch bounds: char switches use int32 bounds instead of 0..max_int, because a char can now be negative. A switch whose range starts at '\000' therefore gets an extra x >= 0 check in the output. Before, it was skipped because chars were never negative. This costs at most one more comparison per path, and it changes two snapshots (adt_optimize_test.mjs, string_unicode_test.mjs). Other char ranges already checked both bounds, so their output is unchanged. For example, in string_unicode_test.mjs:

// Before                          // After
if (x !== 333) {                   if (x >= 256) {
  if (x >= 256) {                    if (x !== 333) {
    return 3;                          return 3;
  } else {                           } else {
    return 0;                          return 2;
  }                                  }
} else {                           } else if (x >= 0) {
  return 2;                          return 0;
}                                  } else {
                                     return 3;
                                   }

Runtime: Primitive_string_extern.getChar (codePointAt) now returns option<int> instead of option<char>, so the runtime no longer uses char. The generated JS is unchanged.
Editor: pipe completion on a char value now offers Int functions. It previously offered none, because it looked in the removed Stdlib.Char module and its type filter treated char and int as different types.
Not included: integer range patterns (1 .. 5) still report Invalid_interval here. They come later in this stack: Represent char range patterns with a typed-tree node #8893 adds an interval pattern node, and Support int range patterns #8894 accepts int ranges.

New tests:

char_int_alias_test.res: conversions, mixed patterns, codePointAt results, emoji (astral code points and surrogates), negative values.
super_errors: warning 8 and warning 11 on mixed char/int patterns.
Analysis: CompletionChar.res covers pipe completion on a char.

🤖 Generated with Claude Code

https://claude.ai/code/session_01JALXUyHhkDqzcB3WuW9Svo

cknitt · 2026-10-18T07:59:38Z

@codex review

codecov · 2026-10-18T07:59:52Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.05%. Comparing base (1869441) to head (eaf8b90).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8892   +/-   ##
=======================================
  Coverage   81.04%   81.05%           
=======================================
  Files         455      455           
  Lines       63508    63509    +1     
=======================================
+ Hits        51469    51475    +6     
+ Misses      12039    12034    -5

Files with missing lines	Coverage Δ
analysis/src/type_utils.ml	`66.13% <100.00%> (+0.17%)`	⬆️
compiler/ml/matching.ml	`77.34% <100.00%> (-0.04%)`	⬇️
compiler/ml/parmatch.ml	`77.37% <100.00%> (+0.06%)`	⬆️
compiler/ml/predef.ml	`99.28% <100.00%> (-0.01%)`	⬇️

... and 2 files with indirect coverage changes

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chatgpt-codex-connector · 2026-10-18T07:59:58Z

Codex Review Summary

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

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-18T11:09:00.437250Z	`1df38d6`	Manual request

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chatgpt-codex-connector · 2026-10-18T08:04:41Z

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

Reviewed commit: 1df38d69b0

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pkg-pr-new · 2026-10-18T08:18:56Z

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commit: eaf8b90

github-actions · 2026-10-18T08:20:49Z

Developer playground preview: https://rescript-lang.github.io/rescript/dev-playground/?version=pr-8892

cknitt · 2026-10-18T08:27:29Z

@codex review

chatgpt-codex-connector · 2026-10-18T08:31:02Z

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

Reviewed commit: 1df38d69b0

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cknitt · 2026-10-18T11:06:08Z

@codex review

chatgpt-codex-connector · 2026-10-18T11:08:59Z

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

Reviewed commit: 1df38d69b0

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`char` was a private abbreviation of `int`: a char could be coerced to an int with `:>`, but an int could not be used as a char. It is now a plain alias, so char literals and ints mix freely, including in one `switch`. Pattern matching treats a char constant and an int constant with the same value as the same constant, for exhaustiveness, unused cases and switch compilation. Char literals keep their spelling in generated JavaScript comments and in warning examples. First step of #7028. Example (rescript): let f = x => switch x { | '\000' .. '\255' => 0 | 'ō' => 2 | _ => 3 } Before (js): function f(x) { if (x !== 333) { if (x >= 256) { return 3; } else { return 0; } } else { return 2; } } After (js): function f(x) { if (x >= 256) { if (x !== 333) { return 3; } else { return 2; } } else if (x >= 0) { return 0; } else { return 3; } } Source: tests/tests/src/string_unicode_test.mjs in the diff (the range starts at code point 0, written as a literal NUL character in the test). Review: Why keep `Const_char` instead of typing char literals as ints? It keeps the `/* 'a' */97` comments in the JavaScript and the char spelling in warnings such as "for example: | '\x00'". Review: Why do char switches now use int32 bounds instead of 0..max_int? A char can now hold any int, including negative ones, so a switch can no longer assume `x >= 0`. Two committed snapshots change for that reason. Review: Do integer range patterns such as `1 .. 5` work now? No, they still report `Invalid_interval`. Char ranges expand to one case per value, which does not scale to arbitrary ints; that needs interval patterns of their own. Area: typechecker lambda analysis tests Output: changed tests/tests/src/adt_optimize_test.mjs and string_unicode_test.mjs, char switches whose range starts at '\000' now send negative values to the default case Formatting: unchanged Formats: none PPX: unchanged Editor: pipe completion on a `char` value offers `Int` functions; it offered none before (tests/analysis_tests/tests/src/CompletionChar.res) Tests: +tests/tests/src/char_int_alias_test.res +tests/analysis_tests/tests/src/CompletionChar.res +tests/build_tests/super_errors/fixtures/warning_8_char_int_patterns.res +tests/build_tests/super_errors/fixtures/warning_11_equivalent_char_int_patterns.res ~tests/tests/src/adt_optimize_test.mjs ~tests/tests/src/string_unicode_test.mjs Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01JALXUyHhkDqzcB3WuW9Svo Signed-off-by: Christoph Knittel <ck@cca.io>

`Primitive_string_extern.getChar` returned `option<char>`; it returns `option<int>` now that `char` is an alias of `int`, which leaves no use of `char` in the runtime. The generated JavaScript is unchanged. Area: runtime Output: unchanged Tests: none, because the type is an alias of the old one and the output is unchanged Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01JALXUyHhkDqzcB3WuW9Svo Signed-off-by: Christoph Knittel <ck@cca.io>

github-actions · 2026-10-19T06:38:59Z

Semantic diff: https://cdn.rescript-lang.org/semantic-diff/pr-8892/index.html?v=eaf8b90be

cknitt marked this pull request as ready for review October 18, 2026 07:59

cknitt requested a review from cristianoc October 18, 2026 08:37

This was referenced Oct 18, 2026

Represent char range patterns with a typed-tree node #8893

Merged

Support int range patterns #8894

Merged

cknitt added this pull request to stack #8895 October 18, 2026 09:55

cknitt force-pushed the char-int-alias branch 2 times, most recently from 8b63ce5 to 001621d Compare October 18, 2026 17:49

cristianoc approved these changes Oct 18, 2026

View reviewed changes

cknitt and others added 2 commits October 19, 2026 06:32

cknitt force-pushed the char-int-alias branch from 001621d to eaf8b90 Compare October 19, 2026 06:38

cknitt merged commit 3354d90 into master Oct 19, 2026
31 checks passed

cknitt deleted the char-int-alias branch October 19, 2026 07:15

cknitt mentioned this pull request Oct 19, 2026

RFC: Revise char primitive #7028

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Make `char` an alias of `int` - #8892

Make `char` an alias of `int`#8892
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