Use case: I want to compile a single ReScript file and run it immediately.
With this PR I could dorescript compile-file MyFile.res | bun - and bun automatically runs it.

(Would not be a stretch to create aBun plugin and just be able to runbun run MyFile.res if plugin is configured)

Going to keep this as draft until after v12.

Proof of concept: one shot file compilation

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commit:102f806

Extract -bs-package-output into multiple arguments

c3cc9a8

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Split bsc module system flags for stdout compilation

Problem

Whenbsc compiles to stdout (no-bs-package-output specified), it hardcoded the Commonjs module system:

(* compiler/core/lam_compile_main.ml:295 - old code*)ifJs_packages_info.is_empty package_info&&!Js_config.js_stdoutthenJs_dump_program.dump_deps_program~output_prefixCommonjs(* <- hardcoded!*)

This meant stdout output was always Commonjs, regardless of project configuration. Since the newcompile-file command outputs to stdout, it could only generate Commonjs, not ES6 modules.

Solution

Split-bs-package-output into three independent flags:

Before (coupled):

-bs-package-output esmodule:lib/es6:.mjs# Format: module_system:path:suffix (all three or none)

After (decoupled):

-bs-module-system esmodule# Controls import/export syntax-bs-suffix .mjs# Controls import path extensions-bs-package-output lib/es6# Output directory (optional)

Implementation

New compiler state (compiler/common/js_config.ml):

let default_module_system=refExt_module_system.Commonjslet default_suffix=refLiterals.suffix_js

New flags (compiler/bsc/rescript_compiler_main.ml):

("-bs-module-system", string_call set_module_system,"Set module system: commonjs, esmodule, es6-global");("-bs-suffix", string_call set_suffix,"Set import file suffix: .js, .mjs, .cjs");

Stdout output now uses configured values (compiler/core/lam_compile_main.ml):

ifJs_packages_info.is_empty package_info&&!Js_config.js_stdoutthen(* Use configured module system instead of hardcoded Commonjs*)Js_dump_program.dump_deps_program~output_prefix!Js_config.default_module_system

Import path generation uses configured suffix (compiler/core/js_name_of_module_id.ml):

(* For Package_script mode (stdout), use configured suffix*)let js_file=Ext_namespace.js_name_of_modulename dep_module_id.id.name case!Js_config.default_suffix

Critical exception: Runtime package imports (@rescript/runtime) always use.js because the runtime is pre-compiled and distributed with.js files:

(* Runtime package is pre-compiled and always uses .js suffix*)let js_file=Ext_namespace.js_name_of_modulename dep_module_id.id.nameUpperLiterals.suffix_js

Build System Updates

Rewatch (rewatch/src/build/compile.rs):

// Old format:"-bs-package-output","esmodule:lib/es6:.mjs"// New format:"-bs-module-system","esmodule","-bs-suffix",".mjs","-bs-package-output","lib/es6"

Legacy bsb (compiler/bsb/bsb_package_specs.ml):

(* Old: single compound flag*)"-bs-package-output esmodule:lib/es6:.mjs"(* New: three separate flags*)"-bs-module-system esmodule -bs-suffix .mjs -bs-package-output lib/es6"

Backward Compatibility

The-bs-package-output parser still accepts the old format:

module:path:suffix - all three explicit (old format)
module:path - uses configured suffix
path - uses configured module system and suffix

This allows gradual migration, though since these are internal flags (users don't callbsc directly), backward compatibility isn't critical.

Result

bsc can now output ES6 modules to stdout:

$ bsc -bs-module-system esmodule -bs-suffix .mjs file.ast// Generated by ReScript, PLEASE EDIT WITH CAREimport* as MyDep from"./MyDep.mjs";import* as Belt_Array from"@rescript/runtime/lib/es6/Belt_Array.js";export { ... }

Notice:

User module imports:.mjs (from-bs-suffix)
Runtime imports:.js (always, because runtime is pre-compiled)

This enablescompile-file to respect project configuration when outputting to stdout.

//cc@cristianoc ,@zth

nojaf added8 commits

November 4, 2025 19:50

Fix legacy bsb to use source subdirectory from output_prefix

4b9142e

The previous implementation used a ninja variable $in_d which doesn't exist,causing files to be written to the wrong location. Now bsc extracts thesource subdirectory directly from the output_prefix parameter.For in-source builds with output_prefix='src/Node', files are written to:  project_root / '.' / 'src' / 'Node.mjs'For out-of-source builds with output_prefix='src/Node', files go to:  project_root / 'lib/es6' / 'src' / 'Node.mjs'This ensures source directory structure is preserved in the output.

Fix path construction to handle both bsb and rewatch

6d4c90f

bsb passes path='.' and expects source_subdir to be extracted from output_prefix.rewatch passes path=file_path.parent() which already contains the full directory.Now check if path is a base directory (., lib/es6, etc.) and only thenextract source_subdir from output_prefix. Otherwise use path as-is.

Fix JavaScript import paths for files in subdirectories after splitti…

957acbf

…ng -bs-package-output

Fix JavaScript import paths for in-source builds after splitting -bs-…

e22f5bb

…package-outputFor in-source builds, extract source subdirectories from output_prefix and .cmj file locations to generate correct relative import paths.

Maybe this fixed something

6c6f2cf

Windows thingy

aebccbd

Another windows thing

22e4f13

This is getting out of hand

102f806

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Hi@cristianoc , the splitting of the bsc arg turned out quite ugly.
To satisfy bsb, a lot of code was generated while it seems a lot more straightforward in Rewatch.
Any pointers on what could be improved?

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nojaf commentedNov 4, 2025

Split bsc module system flags for stdout compilation

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Build System Updates

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Proof of concept: one shot file compilation#8002

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Proof of concept: one shot file compilation#8002

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nojaf commentedNov 4, 2025

Split bsc module system flags for stdout compilation

Problem

Solution

Implementation

Build System Updates

Backward Compatibility

Result

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nojaf commentedNov 5, 2025

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