Need help with syntax?
Click the “chat” button below for chat support from the developer who created it, or find similar developers for support.

About the developer

142 Stars 10 Forks MIT License 186 Commits 90 Opened issues


ReScript's newest syntax as a standalone repo.

Services available


Need anything else?

Contributors list

ReScript Syntax Tests

Blog post:


This repo is the source of truth for the ReScript parser & printer. Issues go here.

You don't need this repo to use the ReScript syntax. This comes with BuckleScript 8.1. This repo is for syntax contributors.



A detailed discussion by Jonathan Blow and Casey Muratori on why you would hand-roll a parser for a production quality programming language Discussion: Making Programming Language Parsers, etc

"One reason why I switched off these parser tools is that the promises didn't really materialize. The amount of work that I had to do change a yacc script from one language to a variant of that language was more than if I hand wrote the code myself. " J. Blow.

Setup & Usage

Currently for devs only.

Required: - NodeJS - Ocaml 4.06.1 - OS: Mac

opam switch create 4.06.1 && eval $(opam env)
git clone
cd syntax
npm install
make # or: make -j9 for faster build

This will produce the final binary

used for testing

First build is super slow because we're also building our vendored

(only used for the conversion tool). Subsequent builds should be <2s. If not, please file an issue (build speed is a priority).

We only build production binary, even in dev mode. No need for a separate dev binary when the build is fast enough. Plus, this encourages proper benchmarking of the (production) binary each diff.

After you make a change:


Run the core tests:

make test

Run the extended tests:

make roundtrip-test

Update jest snapshots:

./node_modules/.bin/jest -u

Debug a file: ```sh

write code in test.res

./lib/rescript.exe test.res # test printer ./lib/rescript.exe -print ast test.res # print ast ./lib/rescript.exe -print ml test.res # show ocaml code ./lib/rescript.exe -print res -width 80 test.res # test printer and change default print width ```


make bench

Development Docs

contains all the source code. Don't change folder structure without notice; BuckleScript uses this repo as a submodule and assumes

Example File Conversion

In a random project of yours:

node_modules/.bin/bsrefmt --print=binary | your/path/to/rescript.exe -parse reasonBinary -print ns > myFile.res
node_modules/.bin/bsrefmt --print=binary --interface=true myFile.rei | your/path/to/rescript.exe -parse reasonBinary -print ns -interface > myFile.resi
mv # random backup name. Could be anything

Example API usage

module Parser = ResCore.Parser
module Diagnostics = ResCore.Diagnostics

let filename = "foo.res" let src = FS.readFile filename

let p = (* intended for ocaml compiler ) let mode = Parser.ParseForTypeChecker in ( if you want to target the printer use: let mode = Parser.Default in*) Parser.make ~mode src filename

let structure = ResParser.parseImplementation p let signature = ResParser.parseInterface p

let () = match p.Parser.diagnostics with | [] -> () (* no problems ) | diagnostics -> ( parser contains problems *) prerr_string ( Diagnostics.stringOfReport ~style:Diagnostics.Pretty diagnostics src )

We use cookies. If you continue to browse the site, you agree to the use of cookies. For more information on our use of cookies please see our Privacy Policy.