eval

Runs at runtime.

Takes a data structure (usually a list) and evaluates it as Lisp code.

Example: (eval '(+ 1 2)) ; => 3

Use case: Dynamically construct and run code while the program is running.

Macros

Runs at compile time (or macro-expansion time).

Transform Lisp code before it's evaluated. They return new code (a Lisp form) to be compiled/evaluated later.

Example: (defmacro my-unless (condition body) `(if (not ,condition) ,body))

(my-unless (= 1 2) (print "Not equal")) ; expands to (if (not (= 1 2)) (print ...))

Use case: Extend the language by defining new syntactic constructs. Enables powerful DSLs and optimizations.

https://www.lispworks.com/documentation/HyperSpec/Body/f_eva...

Macros are really only instructions for the compiler, how to compile things faster.

The syntax improvement aspect is minuscule, because Lisp has no actual syntax perse.

Without macros you could implement eval still but your internal lisp implementation could only work on quoted s-expressions, there would be no way to get back to the base unquoted level of lisp code (assuming you can't use the primitive eval, since you're trying to implement lisp in lisp).

Another use case is implementing a shortcutting boolean and function. You can't do the shortcutting without macros because all of the arguments get eval'd before passing to your and function.

When a macro is used for any given purpose, what happens is a bit more general because it first "does something with" its argument (rather than necessarily straight-up evaluating it). Then the result of that is evaluated.