What are some examples of open source Lisp projects and codebases whose features and elegance could have only been executed as well as they are in the language, or are just great codebases in general to study?
It also demonstrates that Scheme is flexible enough to easily implement features that the language designers did not, such as monads and laziness. While both can be done in almost any language, I think that Scheme's macros allow for exceptionally seamless implementations.
I'm trying to look up what this means. Do you have a link?
That's not the only instance of "staged" execution. Guix introduces G-expressions, a quoted expression in a build context where unquoted package values are replaced with absolute directory names that are not known until execution time.
Quoting and unquoting code and dealing with different strata of evaluation come natural in Scheme.
Another simpler example of staged execution might be the remote code execution feature of Guile-SSH.
You may also like to read about multitier programming in Hop: http://queue.acm.org/detail.cfm?id=2330089
Axiom is a FOSS computer algebra system implemented in about 1.2 million lines of Common Lisp.
Axiom relies heavily on the ability to dynamically define and re-define functions. It relies heavily on the macro facility. It presents a domain specific language (called SPAD) which implements mathematically friendly syntax and semantics. (https://github.com/daly/axiom)