Overview

Mogglo is a multi-language AST-based code search and rewriting tool. Mogglo supports embedding Lua code in search patterns and replacements.

Mogglo focuses on the following goals:

• Minimal setup: Mogglo will work right away on any codebase in a supported language.
• Many languages: 12 and counting!
• High-quality grammars: Mogglo is based on tree-sitter grammars.
• Lua: Mogglo exposes a rich API to embedded Lua snippets.

Introduction

The following examples give a taste of Mogglo. Here's how to find pointless assignments of an expression to itself:

mogglo-rust --detail 'let $x = $x;' ./**/*.rs

The --detail flag helps you understand why something matched, it produces fancy output like:

   ╭─[./test/nonlinear.rs:4:1]
   │
 4 │ ╭─▶ let a =
   │ │       ┬
   │ │       ╰── $x
 5 │ ├─▶     a;
   │ │       ┬
   │ │       ╰──── $x
   │ │
   │ ╰──────────── let $x = $x;
   │
   │ Note: Multiple occurrences of $x were structurally equal
───╯

Lua code is wrapped in braces. Lua can recursively match patterns with rec. Here's a pattern to detect out-of-bounds array accesses:

mogglo-rust 'while $i <= $buf.len() { ${{ rec("$buf.get($i)") }} }' ./**/*.rs

Here's how to unroll a simple loop:

mogglo-rust \
  'for $i in 0..$h { $b; }' \
  --where 'h_num = tonumber(h); return h_num ~= nil and h_num % 4 == 0' \
  --replace 'for $i in 0..${{ string.format("%.0f", h / 4) }} { $b; $b; $b; $b; }' \
  ./*/**.rs

This transformation demonstrates the power of using Lua: it can't be done with regular expression substitutions and would be very difficult with other codemod tools.

Lua snippets can match and negate patterns, or even compose new patterns dynamically! See the guide for more detailed explanations, examples, and features.

Supported languages

Mogglo currently ships pre-built executables for the following languages:

• C
• CSS
• Java
• JavaScript
• Rust
• TypeScript
• Swift

Additionally, the following can be built from source or via Cargo/crates.io:

• C++
• Haskell
• HTML
• Python
• Ruby

Languages are very easy to add, so file an issue or a PR if you want a new one!

Comparison to related tools

Mogglo is not as polished as any of the tools mentioned in this section.

Mogglo is most similar to other multi-language code search and codemod tools.

• Mogglo is similar to ast-grep, but supports more languages and allows embedding Lua in patterns.
• Mogglo is similar to Comby. Comby uses lower-fidelity parsers, but is much more battle-tested and better documented. Mogglo also embeds Lua in patterns.
• Mogglo has less semantic understanding of code (e.g., name resolution) than Semgrep or CodeQL, but is much easier to set up.

There are many excellent language-specific code search and codemod tools; these tend to be more polished but less general than Mogglo.

• retrie
• weggli