The Illusyn (Illusionary Syntax) library is an experimental solution for the definition of abstract syntax trees (ASTs) to support program transformations. It is written in Racket, and relies on Racket's macro system for both declaring AST operations and implementing them.

Illusyn is currently only available as part of the Magnolisp language implementation. Magnolisp makes extensive use of the Illusyn library, and can therefore be regarded as a sample application for Illusyn. With Magnolisp installed, it is possible to use the Illusyn library from other applications as well, without having to install a separate copy.