[v3] Implement new Concept Exercise: code-as-data

This issue describes how to implement the code-as-data concept exercise for the Common Lisp track.

Getting started

Please please please read the docs before starting. Posting PRs without reading these docs will be a lot more frustrating for you during the review cycle, and exhaust Exercism's maintainers' time. So, before diving into the implementation, please read up on the following documents:

Please also watch the following video:

The Anatomy of a Concept Exercise.

Learning objectives

Understand, in layman's terms, what "homoiconicity" means for Lisp (lists can be evaluated as code)
Be able to define new, simple macros with defmacro
Know the difference between compile-time / load-time and run-time evaluation in the context of defmacro vs defun
Be aware of the additional differences between defmacro and defun (arguments aren't evaluated, macros can't be mapped, funcalled, or applyd)
Understand quasi-quoting and how it can be used to simplify list construction (`, ,, and ,@)
Be able to explore / debug macros with macroexpand

Out of scope

Compiler macros
Reader macros
Symbol macros
Hygienic macros with gensym
Using macros for optimisation (performing calculations at compile-time)

Concepts

code-as-data

Prerequisites

basic-lists
arithmetic
symbols
function-definition
expressions
enumeration
higher-order-functions

Resources to refer to

Hints

CL Cookbook

After

2 Hours of Macros (YouTube)
CL-PPCRE uses compiler macros to translate many regular expressions into normal lisp code, leading to a very high-performance regex library – it's all just sugar for s-expressions.

Representer

We may need to double check that "variable-like" symbols being defined in macros are properly replaced with a name mapping

Analyzer

Nothing yet, should be able to use macroexpand to help with tests

Implementing

While it's important that the student knows how to debug macros using things like macroexpand, that's not something we need to test for.

This one might be one of the trickier concepts, even if the functions and syntax involved aren't that complex. It's vital that the student understands what macros are good for and what they aren't, so that they don't over-apply them. Convincing examples are important here, even if they aren't super trivial. See "What Macros are For" for an example.

Help

If you have any questions while implementing the exercise, please post the questions as comments in this issue.

exercism / common-lisp