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How to add support for a new language

This is about adding support for a new programming language in semgrep using the tree-sitter technology. While new languages should use tree-sitter, semgrep also supports some languages independently if there's a good legacy OCaml parser for them. Check for your language in pfff and if you see it in there, talk to us. Otherwise, let's get started.

Repositories involved directly:

  • semgrep: the semgrep command line program;
  • ocaml-tree-sitter-semgrep: language-specific setup, generates C/OCaml parsers for semgrep;
  • new repo semgrep-X for the new language X: C/OCaml parser generated from ocaml-tree-sitter-semgrep by an admin.

Submodules overview (semgrep repo)โ€‹

There are quite a few GitHub repositories involved in porting a language. Here is the file hierarchy of the semgrep repository:

โ”œโ”€โ”€ ocaml-tree-sitter-core # runtime library for tree-sitter parsers
โ”œโ”€โ”€ pfff # non-tree-sitter parsers
โ””โ”€โ”€ tree-sitter-lang # generated tree-sitter parsers
โ”œโ”€โ”€ semgrep-java
โ””โ”€โ”€ semgrep-ruby

When done with the work in ocaml-tree-sitter-semgrep, you'll need a new repo semgrep-X to host the generated parser code. Ask someone at r2c to create one for you. For this, they should use the template semgrep-lang-template when creating the repo.

The instructions for adding a language start in ocaml-tree-sitter-semgrep (as indicated below). Be careful that you are always in the correct repo!

Setup (ocaml-tree-sitter-semgrep repo)โ€‹

As a model, you can use the existing setup for ruby or javascript. Our most complicated setup is for typescript and tsx.

First install npx following this doc.

Expedited setupโ€‹

If you're lucky, the language you want to add can be added with the script add-simple-lang:

cd lang
./add-simple-lang --help

follow the instructions from --help

This often works with languages that define a single dialect using a grammar.js file at the root of the project. If this simplified approach fails, use the Manual setup instructions below to understand what's going on or to set things up manually.

Manual setupโ€‹

From the ocaml-tree-sitter repo, do the following:

  1. Create a lang/X folder.
  2. Make a test/ok directory. Inside the directory, create a simple hello-world program for the language you are porting. Name the program hello-world.<ext>.
  3. Now make a file called extensions.txt and input all the language extensions (.rb, .kt, etc) for your language in the file.
  4. Create a file called fyi.list with all the information files, such as semgrep-grammars/src/tree-sitter-X/LICENSE, semgrep-grammars/src/tree-sitter-X/grammar.js, semgrep-grammars/src/semgrep-X/grammar.js, etc. to bundle with the final OCaml/C project.
  5. Link the Makefile.common to a Makefile in the directory with: ln -s ../Makefile.common Makefile
  6. Create a test corpus. You can do this by:
    • Running most-starred-for-language in order to gather projects on which to run parsing stats. Run with the following command: ./scripts/most-starred-for-language <lang> <github_username> <api_key>
    • Using github advanced search to find the most starred or most forked repositories.
  7. Copy the generated projects.txt file into the lang/X directory.
  8. Add in extra projects and extra input sets as you see necessary.

Here's the file hierarchy for Ruby:

lang/ruby               # language name of the form [a-z][a-z0-9]*
โ”œโ”€โ”€ extensions.txt # standard name. Required for stats.
โ”œโ”€โ”€ fyi.list # list of informational files to copy. Recommended.
โ”œโ”€โ”€ Makefile -> ../Makefile.common
โ”œโ”€โ”€ projects.txt # standard name. Required for stats.
โ””โ”€โ”€ test # sample input files
โ”œโ”€โ”€ ok # contains input files supported by the current grammar
โ”‚ โ”œโ”€โ”€ comment.rb
โ”‚ โ”œโ”€โ”€ ex1.rb
โ”‚ โ”œโ”€โ”€ ex2.rb
โ”‚ โ”œโ”€โ”€ hello.rb
โ”‚ โ””โ”€โ”€ poly.rb
โ””โ”€โ”€ xfail # contains input files that are expected to fail
โ””โ”€โ”€ rating.rb

To test a language in ocaml-tree-sitter, you must build the ocaml-tree-sitter OCaml code generator, run it to produce a parser, then run some tests for the parser. Full instructions for this are given in updating-a-grammar under "Testing". The short instructions are:

  1. For the first time, build everything with ./scripts/rebuild-everything.
  2. Subsequently, work from the lang/X folder and run make and make test.

The fyi.list fileโ€‹

The fyi.list file was created to specify informational files that should accompany the generated files. These files are typically:

  • the source grammar, most often a single grammar.js file.
  • the licensing conditions usually specified in a LICENSE file.


# Comments are allowed on their own line.
# Blank lines are ok.

# Each path is relative to ocaml-tree-sitter/lang

The files listed in fyi.list end up in a fyi folder in tree-sitter-lang. For example, see ruby/fyi.

Extending the original grammar with semgrep syntaxโ€‹

This is best done after everything else is set up. Some constructs such as semgrep metavariables ($FOO) may already be valid constructs in the language, in which case there's nothing to do. Some support for the semgrep ellipsis ... usually needs to be added as well.

You'll need to learn how to create tree-sitter grammars.

  1. Work from semgrep-grammars/src/semgrep-X and use make and make test to build and test.
  2. Add new test cases to test/corpus/semgrep.text.
  3. Edit grammar.js.
  4. Refer to the original grammar in semgrep-grammars/src/tree-sitter-X to determine which rules to extend.

For an example of how to extend a language, you can:

Avoiding parsing conflicts is the trickiest part. Asking for help is encouraged.

๐Ÿ’ก A note on the JavaScript syntax that's heavily used to define and extend grammars:

When possible, we prefer the shorthand notation for anonymous functions made of a single expression:

(x) => x

which is the same as

(x) => { return x; }

which is itself the same as

function(x) { return x; }

When extending any rule with an alternate choice such as $.ellipsis, the simpler way is this one:

expression: ($, previous) => choice(previous, $.ellipsis),

However, if the previous rule is known to be a choice(), we can avoid one level of nesting and append to the original list of choices, which is done as follows:

expression: ($, previous) => choice(...previous.members, $.ellipsis),

Whether to use one or the other is a matter of taste.

Finally, on rare occasions where the rule body is more than a single expression, you'll have to use the curly-brace/return syntax:

expression: ($, previous) => {
if (semgrep_ext)
return choice(...previous.members, $.ellipsis);
return previous;

Parsing statisticsโ€‹

From a language's folder such as lang/csharp, two targets are available to exercise the generated parser:

  • make test: runs on test/ok and test/xfail
  • make stat: downloads the code specified in projects.txt and parses the files whose extension matches those in extensions.txt, reporting parsing success in the form of a CSV file.

For gathering a good test corpus, you can use GitHub Search or the script provided in scripts/ For github searches, filter by programming language and use a constraint to select large projects, such as "> 100 forks". Collect the repository URLs and put them into projects.txt.

Publishing generated parsersโ€‹

After you have pushed your ocaml-tree-sitter changes to the main branch, do the following:

  1. Check that the original grammar.js, src/scanner.c/.cc (if applicable) look clean and have minimal external dependencies.
  2. In ocaml-tree-sitter/lang/Makefile, add language under 'SUPPORTED_LANGUAGES' and 'STAT_LANGUAGES'.
  3. In ocaml-tree-sitter/lang directory, run ./release X --dry-run. If this looks good, please ask someone at r2c to publish the code using ./release X.


Various errors can occur along the way.

Compilation errors in C or C++ are usually due to a missing source file scanner.c or, or a grammar with a name that doesn't match the name inside the scanner file. Javascript files may also be missing, in particular in the case of grammars that extend existing grammars such as C++ for C or TypeScript for JavaScript. Check for require() calls in grammar.js and learn how this NodeJS primitive resolves paths.

There may also be errors when generating or compiling OCaml code. These are likely bugs in ocaml-tree-sitter and they should be reported or fixed right away.

Here are some known types of parsing errors:

  • A syntax error. The input program is in the wrong syntax or uses a recent feature that's not supported yet: make test or directly the parse_X program will show the tree produced by tree-sitter with one or more ERROR nodes.
  • A "reparsing" error. It's an error generated after the first successful parsing pass by the tree-sitter parser, during the reparsing pass by the OCaml code performed by the generated file. The error message should tell you something like "cannot interpret tree-sitter's output", with details on what code failed to match what pattern. This is most likely a bug in ocaml-tree-sitter.
  • A segmentation fault. This could be due to a bug in the OCaml/tree-sitter C bindings and should be fixed. A simple test case that reproduces the problem would be nice. See

Parsing errors that are due to an incomplete or incorrect grammar should be recorded, and eventually reported and/or fixed in the upstream project. We keep failing test cases in a fail/ folder, preferably in the form of the minimal program suitable for a bug report, with a comment describing what was expected and what's going on.


Pfff defines a list programming languages, some of which have parsers in pfff itself. Others are tree-sitter parsers which are otherwise independent from pfff. You need to add the new language to the list of languages in pfff.

Look under Adding a Language in pfff for step-by-step instructions.


Now that you have added your new language 'X' to pfff, do the following:

  1. Add the new pfff submodule to semgrep-core.

  2. In, add 'X' to lang_has_no_dollar_ids/ If the grammar has no dollar identifiers, add it above 'true'. Otherwise, add it above 'false'.

  3. In synthesizing/, add 'X' to the appropriate functions:

    • print_bool
    • if_stmt
    • while_stmt
    • do_while
    • for_stmt
    • def_stmt
    • return
    • break
    • continue
    • literal
  4. In parsing/, add in 'X' to dump_tree_sitter_cst_lang. You can look to the other languages as reference to what code to add.

  5. Create a file parsing/ Add basic functionality to define the function parse and import module Parse_tree_sitter_helpers. You can look at csharp and kotlin files in order to get a better idea of how to define the parse file function, but this file should contain something similar to:

    module H = Parse_tree_sitter_helpers

    let parse file =
    (fun () ->
    Parallel.backtrace_when_exn := false
    Parallel.invoke Tree_sitter_X.Parse.file file ()
  6. In parsing/tree_sitter/dune, add tree-sitter-lang.X.

  7. Write a basic test case for your language in tests/X/hello-world.X. This can just be a hello-world function.

  8. Test that the command semgrep-core/bin/semgrep-core -dump_tree_sitter_cst test/X/hello-world prints out a CST for your language.

Be thankful for the authors of the original code, keep clearly visible license notices, and make it easy to get back to the original projects:

  • Make sure to preserve the LICENSE files. This should be listed in the fyi.list file.
  • For sample input in test/, consider Public Domain ("The Unlicense") files or write your own, for simplicity. GitHub Search allows you to filter projects by license and by programming language.

See alsoโ€‹

How to upgrade the grammar for a language