Pyrefly is not the first Python type-checking tool from Meta. Previously, the company delivered Pyre, written in OCaml. Pyre has since been retired; Pyrefly, written from scratch in Rust, replaces it.

hello.py:7: error: Argument 1 to "hello" has incompatible type ↪"int"; expected "str" hello.py:8: error: Argument 1 to "hello" has incompatible type ↪"List[int]"; expected "str" Sure enough, mypy has ...

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