LISP Syntax and S-Expressions

LISP syntax is built entirely from S-expressions: parenthesized lists where the operator or function name comes first, followed by its arguments, a style called prefix notation. So instead of writing 2 + 3, LISP is written (+ 2 3). This may look unfamiliar at first, but the consistency pays off: every operation, from arithmetic to function calls to control flow, follows the exact same structural rule, which makes LISP's grammar unusually small and regular compared to languages with many different syntactic forms.

S-expressions nest freely, and LISP always evaluates from the innermost parentheses outward. In (* (+ 1 2) (- 5 2)), the evaluator first resolves (+ 1 2) to 3 and (- 5 2) to 3, and only then multiplies those two results to get 9. There is no separate table of operator precedence to memorize, the way you need for a + b * c in infix languages, because the nesting of parentheses itself dictates the exact order of evaluation.

Every S-expression is either an atom or a list. An atom is an indivisible value: a number like 42, a symbol like hello, a string like a piece of text in double quotes, or a character. A list is a parenthesized sequence that groups together atoms and/or other lists, built internally from linked cons cells. This two-way split, atom versus list, is the entire grammar of LISP; there is no separate syntax for arrays, statements, or expressions the way most languages have.

By default, evaluating a list treats its first element as a function name and applies it to the evaluated rest, which means typing (a b c) at the REPL errors unless A is actually a defined function. To stop that evaluation and treat a list as literal data, you quote it, either with the special form (quote (a b c)) or its shorthand '(a b c). Special forms such as quote, if, defun, and let are not ordinary functions at all: they follow custom evaluation rules, deciding for themselves which of their arguments get evaluated and when.