Conditionals and Derived Forms

When this matters

Choose a conditional by the shape of the question: one yes/no test, several predicates, one value compared with fixed datums, or a side effect that should run only in one case. Remember that only #f is false.

See it run

LISPEX

(case 2 ((1) 'one) ((2) 'two) (else 'other))

Observed result

OUTPUT

two

Read the example

case evaluates 2 once, compares it with each listed datum using eqv?, and selects the second clause. The quoted symbol two is the result; the else clause is not evaluated.

How to reason about it

cond, case, and, or, when, unless, let*, named let, do, and quasiquote preserve pinned evaluation and tail positions.
Generated temporaries and hidden intrinsics cannot be captured or shadowed by user names.
case compares datums with eqv?; when and unless false paths produce zero values.

Choose quickly

Form	Use it when	Result behavior
`if`	there are exactly two branches	returns the selected branch
`cond`	different predicates name several cases	returns the first matching clause
`case`	one value is compared with fixed datums	evaluates the key once
`and` / `or`	tests form a short-circuit chain	returns an operand value, not a coerced boolean
`when` / `unless`	a one-sided effect is clearer	false path returns zero values

A common mistake

There is no define-syntax or user macro expansion.

Current boundaries

Bare unquote and unquote-splicing outside quasiquote are static errors.

Keep going

The data-and-decisions lesson teaches the everyday forms. Derived Forms gives their exact normalization shapes.

Data and decisions · Derived Forms