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# Quasiquoting: `quasiquote` and `‘`
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> +\[missing\] in \[missing\] also documents `quasiquote`.
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The `quasiquote` form is similar to `quote`:
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```racket
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(quasiquote datum)
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```
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However, for each `(unquote expr)` that appears within the `datum`, the
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`expr` is evaluated to produce a value that takes the place of the
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`unquote` sub-form.
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Example:
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```racket
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> (quasiquote (1 2 (unquote (+ 1 2)) (unquote (- 5 1))))
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'(1 2 3 4)
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```
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This form can be used to write functions that build lists according to
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certain patterns.
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Examples:
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```racket
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> (define (deep n)
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(cond
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[(zero? n) 0]
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[else
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(quasiquote ((unquote n) (unquote (deep (- n 1)))))]))
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> (deep 8)
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'(8 (7 (6 (5 (4 (3 (2 (1 0))))))))
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```
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Or even to cheaply construct expressions programmatically. \(Of course,
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9 times out of 10, you should be using a macro to do this \(the 10th
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time being when you’re working through a textbook like
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[PLAI](http://www.cs.brown.edu/~sk/Publications/Books/ProgLangs/)\).\)
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Examples:
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```racket
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> (define (build-exp n)
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(add-lets n (make-sum n)))
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> (define (add-lets n body)
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(cond
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[(zero? n) body]
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[else
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(quasiquote
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(let ([(unquote (n->var n)) (unquote n)])
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(unquote (add-lets (- n 1) body))))]))
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> (define (make-sum n)
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(cond
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[(= n 1) (n->var 1)]
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[else
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(quasiquote (+ (unquote (n->var n))
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(unquote (make-sum (- n 1)))))]))
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> (define (n->var n) (string->symbol (format "x~a" n)))
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> (build-exp 3)
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'(let ((x3 3)) (let ((x2 2)) (let ((x1 1)) (+ x3 (+ x2 x1)))))
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```
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The `unquote-splicing` form is similar to `unquote`, but its `expr` must
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produce a list, and the `unquote-splicing` form must appear in a context
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that produces either a list or a vector. As the name suggests, the
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resulting list is spliced into the context of its use.
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Example:
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```racket
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> (quasiquote (1 2 (unquote-splicing (list (+ 1 2) (- 5 1))) 5))
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'(1 2 3 4 5)
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```
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Using splicing we can revise the construction of our example expressions
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above to have just a single `let` expression and a single `+`
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expression.
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Examples:
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```racket
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> (define (build-exp n)
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(add-lets
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n
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(quasiquote (+ (unquote-splicing
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(build-list
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n
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(λ (x) (n->var (+ x 1)))))))))
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> (define (add-lets n body)
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(quasiquote
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(let (unquote
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(build-list
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n
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(λ (n)
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(quasiquote
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[(unquote (n->var (+ n 1))) (unquote (+ n 1))]))))
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(unquote body))))
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> (define (n->var n) (string->symbol (format "x~a" n)))
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> (build-exp 3)
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'(let ((x1 1) (x2 2) (x3 3)) (+ x1 x2 x3))
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```
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If a `quasiquote` form appears within an enclosing `quasiquote` form,
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then the inner `quasiquote` effectively cancels one layer of `unquote`
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and `unquote-splicing` forms, so that a second `unquote` or
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`unquote-splicing` is needed.
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Examples:
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```racket
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> (quasiquote (1 2 (quasiquote (unquote (+ 1 2)))))
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'(1 2 (quasiquote (unquote (+ 1 2))))
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> (quasiquote (1 2 (quasiquote (unquote (unquote (+ 1 2))))))
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'(1 2 (quasiquote (unquote 3)))
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>
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(quasiquote (1 2 (quasiquote ((unquote (+ 1 2)) (unquote (unquote (- 5 1)))))))
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'(1 2 (quasiquote ((unquote (+ 1 2)) (unquote 4))))
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```
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The evaluations above will not actually print as shown. Instead, the
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shorthand form of `quasiquote` and `unquote` will be used: ` \(i.e., a
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backquote\) and `,` \(i.e., a comma\). The same shorthands can be used
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in expressions:
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Example:
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```racket
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> `(1 2 `(,(+ 1 2) ,,(- 5 1)))
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'(1 2 `(,(+ 1 2) ,4))
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```
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The shorthand form of `unquote-splicing` is `,@`:
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Example:
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```racket
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> `(1 2 ,@(list (+ 1 2) (- 5 1)))
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'(1 2 3 4)
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```
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