chickadee » scheme » quasiquote

"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.