Hacktoberfest 2026: the issues maintainers tagged for October, open and beginner-friendly. Browse Hacktoberfest issues

Make filename-to-module-name derivation consistent across compiler and Rewatch

Closed
  • Is something wrong?
#8,726 0 comments 0 reactions 0 assignees View on GitHub

Maintainers usually reply within 1 day

A pull request for this has already been merged.

  • #8797 by @cknitt — merged

Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
35/100
Issue type
Refactor
Clarity
Mostly clear
Activity status
Active
Tech stack
javascript, ocaml
Domain
build-system, compilers

Research direction

Start by tracing Ext_filename.module_name, Js_implementation.module_of_filename, Rewatch’s filename conversion, Ext_char.is_lower_case, and the generated JavaScript dependency filename logic. Compare their handling of ASCII, non-ASCII names, extensions, namespaces, paired .res/.resi files, and -o overrides. Done means the supported contract is explicit, all paths agree, and unsupported filenames receive consistent diagnostics with regression coverage.

Written by the indexing model from the issue text.

Description

Problem

Filename-to-module-name conversion currently follows different rules across the compiler, build system, and generated import paths.

  • Implementation compilation uses Ext_filename.module_name, which applies legacy Latin-1 byte capitalization through Ext_string.capitalize_ascii / capitalize_sub.
  • Interface compilation uses Js_implementation.module_of_filename, which applies String.capitalize_ascii.
  • Rewatch uses Unicode uppercasing of the first character.
  • .cmj filename-case metadata uses Ext_char.is_lower_case, with the same legacy Latin-1 byte ranges.
  • JavaScript dependency filenames are reconstructed with ASCII-only uncapitalization.

For ordinary ASCII filenames these rules agree. For non-ASCII input they can disagree or corrupt the module name.

Confirmed examples

Filename stem Implementation conversion Interface conversion Rewatch conversion
foo Foo Foo Foo
éclair éclair éclair Éclair
ßeta ßeta ßeta SSeta
中文 Invalid UTF-8 Unchanged Unchanged
😀foo Invalid UTF-8 Unchanged Unchanged

The invalid UTF-8 results occur because the legacy implementation subtracts 32 from the first byte, treating a UTF-8 leading byte as a Latin-1 lowercase character.

The compiler accepts these non-ASCII filenames without an early naming diagnostic. However, Éclair is not an ordinary ReScript module identifier, and quoting it does not make it valid in a module alias.

Ext_filename.as_module has ASCII-oriented validation, but the compilation paths above do not use it. Rewatch’s is_non_exotic_module_name filters generated namespace aliases rather than rejecting source filenames during discovery.

Why this matters

Inconsistent derivation can make the build graph disagree with compiler module identities. Inconsistent case metadata and filename reconstruction can also produce dependency imports that do not match emitted files.

Unicode uppercasing is not generally reversible: for example, ß becomes SS. A binary upper/lower case flag cannot recover the original filename from such a transformation.

Proposed direction

Define the supported filename-to-module-name contract explicitly and apply it consistently:

  • Use ASCII-only capitalization for conventional module filenames.
  • Decide how unsupported or exotic filenames should be handled, and report a clear diagnostic where appropriate.
  • Share compiler derivation between implementations and interfaces, returning the module name and filename-case metadata together.
  • Make Rewatch follow the same contract.
  • Remove Ext_char and the legacy Latin-1 capitalization once their responsibilities have been replaced.

Also reconcile extension stripping: interface derivation stops at the first dot, implementation derivation at the last dot, and Rewatch removes the final source extension. Any change must preserve intended handling of generated filenames.

Regression coverage

Cover lowercase/uppercase ASCII names, non-ASCII first and subsequent characters, Unicode case expansion, multiple dots, namespaces and namespace entries, paired .res/.resi files, and compiler output overrides (-o).

For supported names, verify that compiler identities, Rewatch dependency keys, emitted filenames, and generated imports agree. For unsupported names, verify consistent diagnostics before compilation artifacts are produced.

Dominant language
OCaml
Stars
7.5k
Forks
484
Avg merge
19h 20m
Merged PRs (30d)
96

Getting set up

Open in Codespaces

Starts the project's dev container in your browser, under your own GitHub account.

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

More from rescript-lang/rescript

All issues in rescript-lang/rescript

Similar issues

More Build System issues

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.