quote and quasiquote

In LISP, writing (+ 1 2) at the REPL evaluates it as a function call, but sometimes you want the literal list (+ 1 2) itself as data, not its result. The special operator quote, usually written with the shorthand ', tells the evaluator 'don't evaluate this, just return it as-is.' Because LISP represents code and data with the same list structures, '(a b c) and '(+ 1 2) are both just data: lists of symbols, ready to be passed around, inspected, or later evaluated with eval.

Quasiquote (backquote, ) builds a mostly-literal template while allowing specific pieces to be evaluated: inside a backquoted form, a comma switches back to normal evaluation for just that sub-expression. So (a ,b c) produces a list whose first and third elements are the literal symbols a and c, but whose second element is the current value of the variable b. This selective evaluation is exactly what macros need to build replacement code that mixes fixed structure with caller-supplied pieces.

,@ (unquote-splicing) goes further: instead of inserting one value in place, it splices the elements of a list variable directly into the surrounding list, flattening one level. So if items is (1 2 3), then `(a ,@items b) produces (a 1 2 3 b), not (a (1 2 3) b). This is exactly how macros like the unless example insert a caller-supplied &body, which is itself a list of forms, as a sequence of sibling forms inside progn rather than as one nested list.

A common pitfall is forgetting that quoted data is inert: '(1 2 3) will never call any function even if it looks like one, so '(+ 1 2) evaluates to the three-element list (+ 1 2), not 3. The opposite mistake, forgetting to quote, is just as common: writing (list a b) when you meant (list 'a 'b) will try to evaluate a and b as variables instead of treating them as literal symbols, causing an unbound-variable error if no such variables exist.