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    Tips for converting CST to AST

    Once you have copied the generated Boilerplate.ml for your language foo into Parse_foo_tree_sitter.ml, you can start editing it. The goal is replace all the calls like todo env x by the construction of a node of the AST. The destination AST can be a language-specific AST or directly the generic AST. If we're mapping to a language-specific AST, this language-specific AST needs to be created first. The advantage of going through a language-specific AST is more visibility into which constructs are valid for the language, compared to the generic AST which supports many more constructs.

    Besides writing and updating the tree-sitter grammar, this step is where the most time will be spent to integrate a language in semgrep. This is a collection of tips to make this tedious task somewhat easier.

    Use editor/IDE with good OCaml support​

    Make sure to set up your editor with a proper ocaml mode, so that you can see the inferred type of expressions and get the ability to jump to the definitions.

    Popular editors include emacs, vim, vscode. They all have their own OCaml extension or plugin which relies on merlin.

    Editing the boilerplate​

    Study examples​

    Parse_foo_tree_sitter.ml is copied from the generated file Boilerplate.ml. The todo env x calls are typically replaced by the construction of a node of the AST. See how it's done for example in Parse_go_tree_sitter.ml.

    Learn OCaml basics​

    CST and AST type definitions make heavy use of algebraic data types to accommodate nodes of different kinds under the same type. Those are known as variants (e.g. Expr e) and polymorphic variants in OCaml jargon (e.g. `Expr e).

    Parametrized types in OCaml are like generics in languages like Java. The OCaml type for a list of ints is denoted int list, which would be denoted List<Int> in a Java-like language.

    Run utop (opam install utop) and go over this tutorial about OCaml types at ocaml.org.

    Preserve structure, assign useful names​

    Consider this example of typical generated code in Boilerplate.ml file:

    let rec anon_choice_type_id_42c0412 (env : env) (x : CST.anon_choice_type_id_42c0412) =
      match x with
      | `Id tok -> [ identifier env tok ] (* identifier *)
      | `Nested_id x -> nested_identifier env x

    The name anon_choice_type_id_42c0412 was generated from an anonymous node in the grammar and it's not meaningful. However, it's used in multiple spots, which is why it has its own function definition. It occurs for example here:

        | `Choice_id_opt_type_args (v1, v2) ->
            let v1 = anon_choice_type_id_42c0412 env v1 in
            let id = concat_nested_identifier v1 in
            let _v2 =

    Instead, it works better to give it a meaningful name like id_or_nested_id as follows:

    let rec id_or_nested_id (env : env) (x : CST.anon_choice_type_id_42c0412) =
      match x with
      | `Id tok -> [ identifier env tok ] (* identifier *)
      | `Nested_id x -> nested_identifier env x

    and replace it at every point of use. Now, the snippet where it's used makes more sense:

        | `Choice_id_opt_type_args (v1, v2) ->
            let v1 = id_or_nested_id env v1 in
            let id = concat_nested_identifier v1 in
            let _v2 =

    Another helpful transformation is to assign a name to every new value instead of v1, v2, etc. The above snippet becomes something like

        | `Choice_id_opt_type_args (v1, v2) ->
            let ids = id_or_nested_id env v1 in
            let id = concat_nested_identifier ids in
            let type_args =

    Note that we're still keeping the original v1 and v2, because it's not very useful to find names for them.

    Finally, it is very useful to specify the return type of the function so as to figure out type errors a lot more easily.

    let rec id_or_nested_id (env : env) (x : CST.anon_choice_type_id_42c0412) =

    becomes

    let rec id_or_nested_id (env : env) (x : CST.anon_choice_type_id_42c0412) : ident =

    Summary:

    • Replace generated function names by something meaningful.
    • Replace let v1 = by a meaningful name.
    • Specify the return type of functions that map CST to AST.
    • Preserve the general structure of the generated functions.

    This all helps with updating the code when the grammar changes.

    Compile regularly​

    Compile regularly so as to perform type checking. This is general advice for OCaml development. If you're working on a single file, you don't need to recompile the project, though. Merlin will take care of checking types when you save the file.

    The initial template with all the todos should compile successfully, but of course will fail at runtime. Type errors produced by the compiler can be tricky to understand but it's good to learn how to interpret them. Sometimes they're just too long, though.

    Keep the boilerplate structure intact​

    Leave the original structure in place as much as possible. This is important for later when we want to update the grammar and need to compare the new boilerplate with the old/edited one.

    Add type annotations​

    The generated boilerplate looks like this, i.e. the return type is left unspecified.

    and formal_parameter (env : env) (x : CST.formal_parameter) =
      ...

    The return type will be an AST node determined by the programmer. OCaml performs full type inference, so it's not technically required to specify a return type. However, given the peculiar nature of this exercise, we recommend specifying return types. It makes it easier to see expectations and get clear error messages when the time comes to upgrade the grammar. A return type annotation looks like this:

    and formal_parameter (env : env) (x : CST.formal_parameter) : AST.parameter =
      ...

    Consult the original grammar.js​

    The original grammar.js, or sometimes another javascript file, contains the bulk of the original rules for the grammar. This is usually a better reference than the generated code.

    The generated boilerplate Boilerplate.ml is similar to the type definitions CST.ml which is our interpretation of the original grammar.js. So, it is useful to consult CST.ml as well.

    What tends to work well is to keep 4 windows open:

    • Parse_foo_tree_sitter.ml (2 windows)
    • grammar.js
    • AST_generic.ml

    The last file AST_generic.ml contains the type definitions of the AST we're mapping to.

    Extend the generic AST with moderation​

    Each programming language comes with a few features that other languages don't offer, but typically not that many. The generic AST is designed to accommodate all the constructs for all the programming languages. So, we sometimes have to extend the generic AST with new kinds of nodes. We try to do so sparingly, since we try to keep the generic AST as simple as possible and it's already very rich.

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