A mutable AST library for Hack with linting and code migrations






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v4.168.3 2023-10-31 17:20 UTC

This package is auto-updated.

Last update: 2024-01-31 00:27:14 UTC


Continuous Integration

HHAST is a toolkit for processing the AST of Hack files.

Abstract syntax trees can be an extremely powerful basis for many kinds of tooling beyond compilers and optimization; HHAST is built on top of Hack's Full Fidelity Parser (FFP), providing a Hack object representation of a mutable AST.

Unlike traditional ASTs, the FFP's AST includes all 'trivia' - such as whitespace and comments - allowing you to fully recreate the file from the AST, or create an updated file after mutating the AST, preserving comments and whitespace.

HHAST has 3 main APIs:

  • a low-level library for inspecting and manipulating the FFP AST
  • a linting framework, with support for auto-fixing linters
  • a migration framework


screenshot of lint errors

Linters are designed for subjective or style changes which do not substantially alter the behavior of the code, and may be rejected on a case-by-case basis. Lint errors can provide a suggested fix, which may be based on an AST mutation, but doesn't have to be.

We've included several linters as a starting point, including:

  • don't use await in a loop
  • methods should be ->lowerCamelCase(), functions should be under_scored()
  • always use braces for control flow
  • always use <<__Override>> where possible

Linters can be used both interactively, or unattended. Autofixing is not supported unattended, however it will exit with non-zero if there are any lint issues, to ease integration with CI systems.

Editor and IDE Support

screenshot of lint errors in VSCode

HHAST is supported by:


screenshot of a migration

Migrations are for sweeping changes you want to apply across your entire codebase, and are often more complex. Taking this into account, the migration framework has built-in support for multi-step migrations (unlike linters). AST-aware migrations can be a powerful tool for:

  • adjusting for changes to the language (for example, the shape changes described below)
  • replacing deprecated APIs with new ones
  • general clean-up of the codebase

Low-level AST library

See the documentation.


HHAST is MIT-licensed.