4.11 Quasiquoting: quasiquote and `

Quasiquoting: quasiquote, unquote, and unquote-splicing in Reference: Racket also documents quasiquote.

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 unquote sub-form.

Example:

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

'(1 2 3 4)

This form can be used to write functions that build lists according to certain patterns.

Examples:

> (define (deep n)       (cond         [(zero? n) 0]         [else          (quasiquote ((unquote n) (unquote (deep (- n 1)))))]))

> (deep 8)

'(8 (7 (6 (5 (4 (3 (2 (1 0))))))))

Or even to cheaply construct expressions programmatically. (Of course, 9 times out of 10, you should be using a macro to do this (the 10th time being when you’re working through a textbook like PLAI).)

Examples:

> (define (build-exp n)       (add-lets n (make-sum n)))

> (define (add-lets n body)       (cond         [(zero? n) body]         [else          (quasiquote           (let ([(unquote (n->var n)) (unquote n)])             (unquote (add-lets (- n 1) body))))]))

> (define (make-sum n)       (cond         [(= n 1) (n->var 1)]         [else          (quasiquote (+ (unquote (n->var n))                         (unquote (make-sum (- n 1)))))]))

> (define (n->var n) (string->symbol (format "x~a" n)))

> (build-exp 3)

'(let ((x3 3)) (let ((x2 2)) (let ((x1 1)) (+ x3 (+ x2 x1)))))

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 or a vector. As the name suggests, the resulting list is spliced into the context of its use.

Example:

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

'(1 2 3 4 5)

Using splicing we can revise the construction of our example expressions above to have just a single let expression and a single + expression.

Examples:

> (define (build-exp n)       (add-lets        n        (quasiquote (+ (unquote-splicing                        (build-list                         n                         (λ (x) (n->var (+ x 1)))))))))

> (define (add-lets n body)       (quasiquote        (let (unquote              (build-list               n               (λ (n)                 (quasiquote                  [(unquote (n->var (+ n 1))) (unquote (+ n 1))]))))          (unquote body))))

> (define (n->var n) (string->symbol (format "x~a" n)))

> (build-exp 3)

'(let ((x1 1) (x2 2) (x3 3)) (+ x1 x2 x3))

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.

Example:

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

'(1 2 (quasiquote (unquote (+ 1 2)) (unquote 4)))

The evaluation above will not actually print as shown. Instead, the shorthand form of quasiquote and unquote will be used: ` (i.e., a backquote) and , (i.e., a comma). The same shorthands can be used in expressions:

Example:

> `(1 2 `(,(+ 1 2) ,,(- 5 1)))

'(1 2 `(,(+ 1 2) ,4))

The shorthand for of unquote-splicing is ,@:

Example:

> `(1 2 ,@(list (+ 1 2) (- 5 1)))

'(1 2 3 4)