Matthias Felleisen, Robert Bruce Findler, Matthew Flatt, Shriram Krishnamurthi, Eli Barzilay, Jay McCarthy, and Sam Tobin-Hochstadt. A Programmable Programming Language. Communications of the ACM 61, pp.62–71. 2018.

Lisp has been described as a “programmable programming language”: the fact that Lisp code is valid Lisp data and can be manipulated as such, coupled with macros, makes it easy to construct new Lisp variants in Lisp, and to combine them with “real” Lisp in interesting ways.

Racket is a Lisp that takes this to its logical conclusion, making domain-specific sub-languages into first-class objects that can be freely interleaved within programs and can be matched to the “natural” expression of the target domain. The languages don’t even have to be Lisp-like.

In the ideal world, software developers would analyze each problem in the language of its domain and then articulate solutions in matching terms.

Racket languages are formed of two parts: a reader that turns strings into properly bracketed S-expressions, and an expander that turns these expressions into “real” Racket (or some other Racket-defined language).

This flexibility adds a whole new level to Lisp programming. For example, it allows languages that don’t readily translate to Lisp style to be implemented and integrated into Lisp. This may simply be for user convenience, or may have some deeper semantic structure. Racket also defines clean interfaces between languages, which is what allows them to be combined. It’s a deeper version of the usual Lisp multi-paradigm approach, reaching out beyond macros working on Lisp syntax.