Working with map, filter, fold, and apply

At a glance

(reduce + 0 (map (lambda (x) (* x x)) (filter odd? (list 1 2 3 4))))

Result

10

Working method

• Use map for one result per element, filter for truth-valued selection, and folds for one accumulated result.
• Callbacks execute as guest procedures, so errors, escapes, warnings, and value-context rules remain active.
• Use apply when a final proper list should become positional arguments; it preserves tail application.

Boundaries

• A callback that yields zero or multiple values in a single-value traversal position raises E320.