4.11 Quasiquoting: quasiquote and lsquo

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▼ | 4 | Expressions and Definitions

4.1 | Notation
4.2 | Identifiers and Binding
4.3 | Function Calls
4.4 | Functions: lambda
4.5 | Definitions: define
4.6 | Local Binding
4.7 | Conditionals
4.8 | Sequencing
4.9 | Assignment: set!
4.10 | Quoting: quote and ’
4.11 | Quasiquoting: quasiquote and ‘
4.12 | Simple Dispatch: case
4.13 | Dynamic Binding: parameterize

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#### 4.11`` Quasiquoting: quasiquote and ‘🔗ℹ

> > > Quasiquoting: quasiquote, unquote, and unquote-splicing in The Racket Reference 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.

Examples:

> > (quasiquote (1 2 (quasiquote (unquote (+ 1 2)))))
> '(1 2 (quasiquote (unquote (+ 1 2))))
> > (quasiquote (1 2 (quasiquote (unquote (unquote (+ 1 2))))))
> '(1 2 (quasiquote (unquote 3)))
> > (quasiquote (1 2 (quasiquote ((unquote (+ 1 2)) (unquote (unquote (- 5 1)))))))
> '(1 2 (quasiquote ((unquote (+ 1 2)) (unquote 4))))

The evaluations 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 form of unquote-splicing is ,@:

Example:

> > `(1 2 ,@(list (+ 1 2) (- 5 1)))
> '(1 2 3 4)

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