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How to upgrade the grammar for a language

Like for adding a language, most of these instructions happen in ocaml-tree-sitter.

Let's call our language "X".

Summary (ocaml-tree-sitter)โ€‹

In ocaml-tree-sitter:

  1. Update submodule tree-sitter-X.
  2. From lang/, run ./test-lang X.
  3. From lang/, ask an r2c developer to run ./release X.

In semgrep:

  1. In the semgrep repo, update submodule semgrep-X.
  2. In the semgrep repo, update the OCaml code that maps the CST to the generic AST.

In the end, make sure the generated code used by the main branch of semgrep can be regenerated from the main branch of ocaml-tree-sitter:

  1. Merge your semgrep branch.
  2. Merge your ocaml-tree-sitter branch.


Here are the main components:

  • the OCaml code generator ocaml-tree-sitter: generates OCaml parsing code from tree-sitter grammars extended with ... and such. Publishes code into the git repos of the form semgrep-X.
  • the original tree-sitter grammar tree-sitter-X e.g., tree-sitter-ruby: the original tree-sitter grammar for the language. This is the git submodule lang/semgrep-grammars/src/tree-sitter-X in ocaml-tree-sitter. It is installed at the project's root in node_modules by invoking npm install.
  • syntax extensions to support semgrep patterns, such as ellipses (...) and metavariables ($FOO). This is lang/semgrep-grammars/src/semgrep-X. It can be tested from that folder with make && make test.
  • an automatically-modified grammar for language X in lang/X. It is modified so as to accommodate various requirements of the ocaml-tree-sitter code generator. lang/X/src and lang/X/ocaml-src contain the C/C++/OCaml code that will published into semgrep-X e.g. semgrep-ruby and used by semgrep.
  • semgrep-X: provides generated OCaml/C parsers as a dune project. Is a submodule of semgrep.
  • semgrep: uses the parsers provided by semgrep-X, which produce a CST. The program's CST or pattern's CST is further transformed into an AST suitable for pattern matching.

Make sure the above is clear in your mind before proceeding further. If you have questions, the best way is reach out on our community Slack channel.

Before upgradingโ€‹

Make sure the grammar.js file or equivalent source files defining the grammar are included in the fyi.list file in ocaml-tree-sitter/lang/X.

Why: It is important for tracking and understanding the changes made at the source.

How: See How to add support for a new language.

Upgrade the tree-sitter-X submoduleโ€‹

Say you want to upgrade (or downgrade) tree-sitter-X from some old commit to commit 602f12b. This uses the git submodule way, without anything weird. The commands might be something like this:

git submodule update --init --recursive --depth 1
git checkout -b upgrade-X
cd lang/semgrep-grammars/src/tree-sitter-X
git fetch origin --unshallow
git checkout 602f12b
cd ..


First, build and install ocaml-tree-sitter normally, based on the instructions found in the main README.

make setup
make install

Then, build support for your language in lang/. The following commands will build and test the language:

cd lang
./test-lang X

โš ๏ธ Check the generated code for the presence of Blank nodes. Those correspond to missing tokens. Check with:

grep Blank lang/X/ocaml-src/lib/

If anything comes up, you must modify the grammar so as to create a named rule for the node of the Blank kind. Eventually, the generated should not have Blank nodes anymore but a token type instead. Where a Blank node exists, we won't be able to get a token or its location at parsing time.

If this works, we're all set. Commit the new commit for the tree-sitter-X submodule:

git status
git commit semgrep-languages/semgrep-X
git push origin upgrade-X

Then make a pull request to merge this into ocaml-tree-sitter's main branch. It's ok to merge at this point, even if the generated code hasn't been exported (Publishing section below) or if you haven't done the necessary changes in semgrep (Semgrep integration below).

We can now consider publishing the code to semgrep-X.


Please ask someone at r2c to run this step.

From the lang folder of ocaml-tree-sitter, we'll perform the release. This step redoes some of the work that was done earlier and checks that everything is clean before committing and pushing the changes to semgrep-X.

cd lang
./release --dry-run X # dry-run release
... # 'git status' will show changes for language X
./release X # commits and pushes to semgrep-X

This step is safe. Semgrep at this point is unaffected by those changes. There is now a new commit at e.g. The fyi/ folder contains original files from which the code was generated. fyi/versions shows the last change for each file, allowing you to check that you got the correct version of grammar.js or some other source file.

Semgrep integrationโ€‹

From the semgrep repository, point the submodule for semgrep-X to the latest commit from the "Publishing" step. Then rebuild semgrep-core, which will normally fail if the grammar changed. If the source grammar.js was included in the fyi folder for semgrep-X (as it should), git diff HEAD^ should help figure out the changes since the last version.


The main difficulty is to understand how the different git projects interact and to not make mistakes when dealing with git submodules, which takes a bit of practice.

See alsoโ€‹

How to add support for a new language