6.18.1.1 Expression Syntax

An expression to be evaluated takes one of the following forms.

symbol

A symbol is evaluated by dereferencing. A binding of that symbol is sought and the value there used. For example,

(define x 123)
x ⇒ 123

(proc args…)

A parenthesised expression is a function call. proc and each argument are evaluated, then the function (which proc evaluated to) is called with those arguments.

The order in which proc and the arguments are evaluated is unspecified, so be careful when using expressions with side effects.

(max 1 2 3) ⇒ 3

(define (get-some-proc)  min)
((get-some-proc) 1 2 3) ⇒ 1

The same sort of parenthesised form is used for a macro invocation, but in that case the arguments are not evaluated. See the descriptions of macros for more on this (see Macros, and see Syntax Rules).

constant

Number, string, character and boolean constants evaluate “to themselves”, so can appear as literals.

123     ⇒ 123
99.9    ⇒ 99.9
"hello" ⇒ "hello"
#\z     ⇒ #\z
#t      ⇒ #t

Note that an application must not attempt to modify literal strings, since they may be in read-only memory.

(quote data)

'data

Quoting is used to obtain a literal symbol (instead of a variable reference), a literal list (instead of a function call), or a literal vector. ' is simply a shorthand for a quote form. For example,

'x                   ⇒ x
'(1 2 3)             ⇒ (1 2 3)
'#(1 (2 3) 4)        ⇒ #(1 (2 3) 4)
(quote x)            ⇒ x
(quote (1 2 3))      ⇒ (1 2 3)
(quote #(1 (2 3) 4)) ⇒ #(1 (2 3) 4)

Note that an application must not attempt to modify literal lists or vectors obtained from a quote form, since they may be in read-only memory.

(quasiquote data)

`data

Backquote quasi-quotation is like quote, but selected sub-expressions are evaluated. This is a convenient way to construct a list or vector structure most of which is constant, but at certain points should have expressions substituted.

The same effect can always be had with suitable list, cons or vector calls, but quasi-quoting is often easier.

(unquote expr)

,expr

Within the quasiquote data, unquote or , indicates an expression to be evaluated and inserted. The comma syntax , is simply a shorthand for an unquote form. For example,

`(1 2 (* 9 9) 3 4)       ⇒ (1 2 (* 9 9) 3 4)
`(1 2 ,(* 9 9) 3 4)      ⇒ (1 2 81 3 4)
`(1 (unquote (+ 1 1)) 3) ⇒ (1 2 3)
`#(1 ,(/ 12 2))          ⇒ #(1 6)

(unquote-splicing expr)

,@expr

Within the quasiquote data, unquote-splicing or ,@ indicates an expression to be evaluated and the elements of the returned list inserted. expr must evaluate to a list. The “comma-at” syntax ,@ is simply a shorthand for an unquote-splicing form.

(define x '(2 3))
`(1 ,x 4)                           ⇒ (1 (2 3) 4)
`(1 ,@x 4)                         ⇒ (1 2 3 4)
`(1 (unquote-splicing (map 1+ x)))  ⇒ (1 3 4)
`#(9 ,@x 9)                        ⇒ #(9 2 3 9)

Notice ,@ differs from plain , in the way one level of nesting is stripped. For ,@ the elements of a returned list are inserted, whereas with , it would be the list itself inserted.