(r5rs.info.gz) Quasiquotation
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4.2.6 Quasiquotation
--------------------
-- syntax: quasiquote <qq template>
-- syntax: `<qq template>
"Backquote" or "quasiquote" expressions are useful for
constructing a list or vector structure when most but not all of
the desired structure is known in advance. If no commas appear
within the <qq template>, the result of evaluating `<qq template>
is equivalent to the result of evaluating '<qq template>. If a
comma appears within the <qq template>, however, the expression
following the comma is evaluated ("unquoted") and its result is
inserted into the structure instead of the comma and the
expression. If a comma appears followed immediately by an at-sign
(@), then the following expression must evaluate to a list; the
opening and closing parentheses of the list are then "stripped
away" and the elements of the list are inserted in place of the
comma at-sign expression sequence. A comma at-sign should only
appear within a list or vector <qq template>.
`(list ,(+ 1 2) 4) ==> (list 3 4)
(let ((name 'a)) `(list ,name ',name))
==> (list a (quote a))
`(a ,(+ 1 2) ,@(map abs '(4 -5 6)) b)
==> (a 3 4 5 6 b)
`((`foo' ,(- 10 3)) ,@(cdr '(c)) . ,(car '(cons)))
==> ((foo 7) . cons)
`#(10 5 ,(sqrt 4) ,@(map sqrt '(16 9)) 8)
==> #(10 5 2 4 3 8)
Quasiquote forms may be nested. Substitutions are made only for
unquoted components appearing at the same nesting level as the
outermost backquote. The nesting level increases by one inside
each successive quasiquotation, and decreases by one inside each
unquotation.
`(a `(b ,(+ 1 2) ,(foo ,(+ 1 3) d) e) f)
==> (a `(b ,(+ 1 2) ,(foo 4 d) e) f)
(let ((name1 'x)
(name2 'y))
`(a `(b ,,name1 ,',name2 d) e))
==> (a `(b ,x ,'y d) e)
The two notations `<qq template> and (quasiquote <qq template>)
are identical in all respects. `,<expression>' is identical to
`(unquote <expression>)', and `,@<expression>' is identical to
`(unquote-splicing <expression>)'. The external syntax generated
by `write' for two-element lists whose car is one of these symbols
may vary between implementations.
(quasiquote (list (unquote (+ 1 2)) 4))
==> (list 3 4)
'(quasiquote (list (unquote (+ 1 2)) 4))
==> `(list ,(+ 1 2) 4)
__i.e., (quasiquote (list (unquote (+ 1 2)) 4))
Unpredictable behavior can result if any of the symbols
`quasiquote', `unquote', or `unquote-splicing' appear in positions
within a <qq template> otherwise than as described above.
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