Wombat - Syntax Highlighting with Rust&#s Bat Called from Crystal

Introduction

This article explores using the Rust-based command-line tool bat as a library within Crystal, a language currently lacking a robust syntax highlighting library. bat, similar to cat, displays file contents but offers enhanced features like line numbering, syntax highlighting, and pagination.

<code>bat hello.rb</code>

Leveraging bat as a library addresses this Crystal limitation.

bat as a Rust Library

bat's PrettyPrinter struct enables its use as a Rust library. However, bat's reliance on the complex Syntect library for syntax highlighting initially presented a challenge. To simplify this, a print_with_writer function was added to PrettyPrinter, allowing direct string syntax highlighting. This addition was contributed via a pull request and is available from bat version 0.25.0 onwards.

use bat::PrettyPrinter;

// ... code utilizing print_with_writer ...

Creating a C Wrapper: bat-c

Because Rust libraries cannot be directly called from Crystal, a lightweight C wrapper library, bat-c, was created. This allows bat to be accessed from Crystal and other languages with C interoperability. The development of bat-c heavily utilized ChatGPT and Copilot due to the author's limited Rust and C expertise. The bat-c repository is available at: https://www.php.cn/link/065982e894fcde21153454b2ea4b2a8a

Key considerations in building bat-c included:

• Memory Management: Careful handling of string memory allocation and deallocation to prevent leaks. Rust-allocated memory requires corresponding Rust-side deallocation functions.
• Cargo.toml Configuration: Configuring Cargo.toml to generate both dynamic (cdylib) and static (staticlib) libraries, enabling flexibility in usage. rpath = true allows relative path library location. Release profile optimization using LTO and codegen-units = 1.
• Versioning: A version function (bat_c_version) was added, storing the version in static memory for easy access.
• Renovate Integration: Renovate was implemented for automated library version updates.
• GitHub Actions: Automated release triggers upon Git tag creation. cargo publish was avoided as bat-c is a purely C library.

Calling bat-c from Crystal: wombat

A Crystal library, wombat, was developed to simplify the interaction with bat-c. The primary challenge was managing the bat-c library download and placement. Since bat-c is not yet widely distributed, a direct download from GitHub Releases was chosen. A static library was preferred for its seamless integration with Crystal. The download process is handled via a shards post_install hook, employing curl (Unix-like) or a batch script (Windows).

Usage Examples

The wombat library provides functions for:

• pretty_print_file: Syntax highlighting a file.
• pretty_print: Syntax highlighting a string.
• pretty_string: Returns a highlighted string.

<code>bat hello.rb</code>

GitHub Actions Workflow

Areas for Improvement

• Library Size: The generated library is relatively large.
• API Design: The API could benefit from a more refined design, ideally considering C compatibility from the initial Rust development.
• Rust/C Expertise: Improved Rust and C knowledge would lead to a more efficient and polished library.

Despite these areas for improvement, the project successfully achieves its primary goal: enabling easy and maintainable use of bat within Crystal. The author welcomes contributions and issue reports.

(Original Japanese post on Qiita: Wombat - RustのBatをCrystalから呼び出しシンタックスハイライティングする. English translation by ChatGPT.)

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