#### 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.