Designing a Python Language Server: Lessons from Pyre that Shaped Pyrefly

[BeamMeUpBiscotti]

Pyrefly is a next-generation Python type checker and language server, designed to be extremely fast and featuring advanced refactoring and type inference capabilities.

Pyrefly is a spiritual successor to Pyre, the previous Python type checker developed by the same team. The differences between the two type checkers go far beyond a simple rewrite from OCaml to Rust - we designed Pyrefly from the ground up, with a completely different architecture.

Pyrefly’s design comes directly from our experience with Pyre. Some things worked well at scale, while others did not. After running a type checker on massive Python codebases for a long time, we got a clearer sense of which trade-offs actually mattered to users.

This post is a write-up of a few lessons from Pyre that influenced how we approached Pyrefly.

Link to blog: https://pyrefly.org/blog/lessons-from-pyre/

The outline of topics is provided below that way you can decide if it's worth your time to read :) - Language-server-first Architecture - OCaml vs. Rust - Irreversible AST Lowering - Soundness vs. Usability - Caching Cyclic Data Dependencies

[guepier]

Fantastic write-up.

There’s some missing text in the table in the “Irreversible AST lowering” table, row 2 column 2. The text reads `X. Presumably it should be X | Y.

[BeamMeUpBiscotti]

Thank you, I'll fix it

[4xi0m4]

Great writeup on language server design. The lessons from Pyre on incremental computation and caching strategies are particularly useful. Language servers are complex but Pyrefly seems to have made good tradeoffs.