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1.3.7 Additional Notations

The following list describes additional notations used in Scheme. See Numbers, for a description of the notations used for numbers.

+ - .

The plus sign, minus sign, and period are used in numbers, and may also occur in an identifier. A delimited period (not occurring within a number or identifier) is used in the notation for pairs and to indicate a “rest” parameter in a formal parameter list (see Lambda Expressions).

( )

Parentheses are used for grouping and to notate lists (see Lists).

"

The double quote delimits strings (see Strings).

\

The backslash is used in the syntax for character constants (see Characters) and as an escape character within string constants (see Strings).

;

The semicolon starts a comment.

'

The single quote indicates literal data; it suppresses evaluation (see Quoting).

`

The backquote indicates almost-constant data (see Quoting).

,

The comma is used in conjunction with the backquote (see Quoting).

,@

A comma followed by an at-sign is used in conjunction with the backquote (see Quoting).

#

The sharp (or pound) sign has different uses, depending on the character that immediately follows it:

#t #f

These character sequences denote the boolean constants (see Booleans).

#\

This character sequence introduces a character constant (see Characters).

#(

This character sequence introduces a vector constant (see Vectors). A close parenthesis, ‘)’, terminates a vector constant.

#e #i #b #o #d #l #s #x

These character sequences are used in the notation for numbers (see Numbers).

#|

This character sequence introduces an extended comment. The comment is terminated by the sequence ‘|#’. This notation is an MIT/GNU Scheme extension.

#!

This character sequence is used to denote a small set of named constants. Currently there are only two of these, #!optional and #!rest, both of which are used in the lambda special form to mark certain parameters as being “optional” or “rest” parameters. This notation is an MIT/GNU Scheme extension.

#*

This character sequence introduces a bit string (see Bit Strings). This notation is an MIT/GNU Scheme extension.

#[

This character sequence is used to denote objects that do not have a readable external representation (see Custom Output). A close bracket, ‘]’, terminates the object’s notation. This notation is an MIT/GNU Scheme extension.

#@

This character sequence is a convenient shorthand used to refer to objects by their hash number (see Custom Output). This notation is an MIT/GNU Scheme extension.

#=

##

These character sequences introduce a notation used to show circular structures in printed output, or to denote them in input. The notation works much like that in Common Lisp, and is an MIT/GNU Scheme extension.