Q: Why is Racket’s parsing system so complicated (Scheme)?

A: Circa 2016, Racket’s internal syntax representation system had to be revised because of macros, or “code rewriting” or “code-to-code procedures.” Matthew Flatt can explain in his paper, “Binding as Sets of Scopes” in POPL 2016, and also on his online notes on the new macro expander system for Racket (also formerly PLT Scheme).

Q: But who is Matthew Flatt?

A: Matthew Flatt is the most active contributor to Racket as measured by GitHub’s commit graph, circa 2017. He is also a member of the PLT Racket or Racket contributor group, and a faculty member at the University of Utah.

Q: Okay, so what?

A: Racket’s macro system recently got a rehaul to implement a new syntax system so that it keeps track of scopes. This is effectively so that they can implement hygienic macros more easily (e.g. syntax-case). Essentially, every single identifier can be associated with a scope given that the primitive expressions are indeed the primitive expressions.

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