14 Quasiquoting

[Explain why...]

14.1 Escapes: quasiquote, unquote, and unquote-splicing

The quasiquote form is similar to quote:

(quasiquote datum)

However, for each (unquote expr) that appears within the datum, the expr is evaluated to produce a value that takes the place of the unsyntax sub-form.

Examples:

> (quasiquote (1 2 (unquote (+ 1 2)) (unquote (- 5 1))))

(1 2 3 4)

The unquote-splicing form is similar to unquote, but its expr must produce a list, and the unquote-splicing form must appear in a context that produces either a list of vector. As the name suggests, the resulting list spliced into the context of its use.

Examples:

> (quasiquote (1 2 (unquote-splicing (list (+ 1 2) (- 5 1))) 5))

(1 2 3 4 5)

If a quasiquote form appears within an enclosing quasiquote form, then the inner quasiquote effectively cancels one layer of unquote and unquote-splicing forms, so that a second unquote or unquote-splicing is needed.

14.2 Abbreviating with `, ,, and ,@