README.md

Destructuring AST

🔗 Docs

This section has code examples to help you understand it better. To run the examples:

make demo-destructing_ast

Table of Contents

• Description
• AST Structure Pattern Matching
  • Example: Matching Integer Payload Manually
• Using Ast_pattern High-Level Destructors
  • Example 1: Matching Integer Payload with Ast_pattern
  • Example 2: Simplifying Matching with eint
• Using Metaquot
  • Example 1: Matching Integer Payload with Metaquot
  • Example 2: Matching Complex Expressions with Metaquot and Anti-Quotations
• Conclusion

Description

Destructuring an AST (Abstract Syntax Tree) is essential when creating a PPX (preprocessor extension) in OCaml. To generate or transform code, you must first break down the AST to understand and manipulate its structure.

For example, if you want to transform this code:

let one = [%one]

into:

let one = 1

You’ll need to destructure the AST representing the extension point ([%one]) to replace it with 1. There are several ways to destructure an AST. We’ll explore three methods:

• AST Structure Pattern Matching
• Using Ast_pattern High-Level Destructors
• Using Metaquot

AST Structure Pattern Matching

The most fundamental method for destructuring an AST in PPXLib is by directly matching on the AST’s structure.

Example: Matching Integer Payload Manually

🔗 Sample Code

Let’s say we want to destructure an AST representing the integer 1:

let match_int_payload ~loc payload =
  match payload with
  | PStr
      [
        {
          pstr_desc =
            Pstr_eval
              ({ pexp_desc = Pexp_constant (Pconst_integer (value, None)); _ }, _);
          _;
        };
      ] -> (
      try Ok (value |> int_of_string)
      with Failure _ ->
        Error (Location.Error.createf ~loc "Value is not a valid integer"))
  | _ -> Error (Location.Error.createf ~loc "Wrong pattern")

1. Pattern Matching the Payload:
   • Begins by matching the payload with the expected structure.
   • The pattern expects a structure (PStr) containing a single item.
2. Destructuring the Structure Item:
   • Matches the pstr_desc field, expecting an evaluated expression (Pstr_eval).
   • The expression should be a constant integer (Pexp_constant with Pconst_integer).
   • Captures the integer value as a string in value.
3. Handling the Matched Value:
   • Converts the value to an integer and returns Ok if successful.
   • If conversion fails, returns an error message.
4. Handling Mismatched Patterns:
   • If the payload doesn’t match the expected structure, it returns an error.

While this method is powerful, it can be verbose and difficult to maintain as patterns become more complex.

Using Ast_pattern High-Level Destructors

To make AST destructuring more readable, PPXLib provides the Ast_pattern module, which offers high-level destructors.

Example 1: Matching Integer Payload with Ast_pattern

🔗 Sample Code

Let’s destructure the same integer 1 AST using Ast_pattern:

open Ppxlib

let match_int_payload =
  let open Ast_pattern in
  pstr (pstr_eval (pexp_constant (pconst_integer (string "1") none)) nil ^:: nil)

This code achieves the same result as the previous example but in a more concise and readable way.

• PStr becomes pstr
• Pstr_eval becomes pstr_eval
• Pexp_constant becomes pexp_constant
• Pconst_integer becomes pconst_integer

Example 2: Simplifying Matching with eint

🔗 Sample Code

You can further simplify it:

let match_int_payload =
  let open Ast_pattern in
  pstr (pstr_eval (eint (int 1)) nil ^:: nil)

Using eint instead of pexp_constant and pconst_integer provides better type safety. The int wildcard captures the integer value.

Using Metaquot

Metaquot is a syntax extension that allows you to write and destructure ASTs more intuitively.

Example 1: Matching Integer Payload with Metaquot

🔗 Sample Code

Let’s destructure the same integer 1 AST with Metaquot:

let match_int_payload expr =
  match expr with
  | [%expr 1] -> Ok 1
  | _ -> Error (Location.Error.createf ~loc:expr.pexp_loc "Wrong pattern")

Example 2: Matching Complex Expressions with Metaquot and Anti-Quotations

🔗 Sample Code

For example, to match any expression of the form 1 + <int>:

let match_int_payload expr =
  match expr with
  | [%expr 1 + [%e? e]] -> (
      match e with
      | { pexp_desc = Pexp_constant (Pconst_integer (value, None)); _ } ->
          Ok (1 + int_of_string value)
      | _ -> Error (Location.Error.createf ~loc:e.pexp_loc "Invalid integer"))
  | _ -> Error (Location.Error.createf ~loc:expr.pexp_loc "Wrong pattern")

Metaquot simplifies the process, making the AST patterns more readable, especially for complex structures.

Conclusion

In this section, we explored different methods to destructure an AST using PPXLib:

• AST Structure Pattern Matching: Powerful but verbose.
• Using Ast_pattern High-Level Destructors: More readable and maintainable.
• Using Metaquot: Intuitive and effective for both simple and complex patterns.

There’s no right way to destructure an AST, choose the approach that best fits your use case. Understanding all these methods is valuable for creating robust and maintainable PPXs.

On the next section, we will learn how to write a PPX.