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Extensions to the R7RS standard

Brackets and braces

The brackets [ ... ] and the braces { ... } are provided as an alternative syntax for ( ... ). A number of reader extensions is provided.

User defined character names

User defined character names are supported. See char-name. Numeric hexadecimal character codes above are supported and can be read using #\uXXXX and #\UXXXXXXXX, in addition to the standard \x...; notation.

Non-standard characters names supported are #\linefeed, #\vtab, #\nul, #\page and #\esc.

Non-standard read syntax

Multiline Block Comment

#|read
#| ... |#

A multiline block comment. May be nested. Implements SRFI-30.

Expression Comment

#;read
#;EXPRESSION

Treats EXPRESSION as a comment. That is, the comment runs through the whole S-expression, regardless of newlines, which saves you from having to comment out every line, or add a newline in the middle of your parens to make the commenting of the last line work, or other things like that. Implements SRFI-62.

External Representation

#,read
#,(CONSTRUCTORNAME DATUM ...)

Allows user-defined extension of external representations. (For more information see the documentation for SRFI-10)

Location Expression

#$EXPRESSIONread

An abbreviation for (location EXPRESSION).

Bytevector strings

#u8"..."read

String syntax for bytevectors, as an alternative to #u8(...). The usual escape sequences for strings are recognized. Note that this syntax is also available for other numeric vectors, like "s8", "u16", "f32", etc. In each case the elements will be the codepoint values of the strings' characters.

#u8{HEX...}read

Hexadecimal bytevector syntax. HEX... should be an even number of hexadecimal characters representing the contents of the vector.

#u8{deadbeef}  ==  #u8(#xde #xad #xbe #xef}

Keyword

#:read
#:SYMBOL
SYMBOL:
:SYMBOL

Syntax for keywords. Keywords are symbol-like objects that evaluate to themselves, and as such don't have to be quoted. Either SYMBOL: or :SYMBOL is accepted, depending on the setting of the keyword-style parameter, but never both. #:SYMBOL is always accepted.

Multiline String Constant

#<<read
#<<TAG

Specifies a multiline string constant. Anything up to a line equal to TAG (or end of file) will be returned as a single string:

(define msg #<<END
 "Hello, world!", she said.
END
)

is equivalent to

(define msg "\"Hello, world!\", she said.")

Multiline String Constant with Embedded Expressions

#<#read
#<#TAG

Similar to #<<, but allows substitution of embedded Scheme expressions prefixed with # and optionally enclosed in curly brackets. Two consecutive #s are translated to a single #:

(define three 3)
(display #<#EOF
This is a simple string with an embedded `##' character
and substituted expressions: (+ three 99) ==> #(+ three 99)
(three is "#{three}")
EOF
)

prints

This is a simple string with an embedded `#' character
and substituted expressions: (+ three 99) ==> 102
(three is "3")

Foreign Declare

#>read
#> ... <#

Abbreviation for (foreign-declare " ... ").

String escape sequences

String-literals may contain the following escape sequences:

Escape sequenceCharacter
\nline feed / newline
\ttab
\rcarriage return
\bbackspace
\abell
\vvertical tab
\fform feed
\xXX;hexadecimal 8-bit character code
\uXXXXhexadecimal 16-bit Unicode character code
\UXXXXXXXXhexadecimal 32-bit Unicode character code
\OOOoctal 8-bit character code
\|   \"    \\    \'the escaped character

Bang

#!read
#!...

Interpretation depends on the directly following characters. Only the following are recognized. Any other case results in a read error.

Line Comment
If followed by whitespace or a slash, then everything up the end of the current line is ignored
Eof Object
If followed by the character sequence eof, then the (self-evaluating) end-of-file object is returned
DSSSL Formal Parameter List Annotation
If followed by any of the character sequences optional, rest or key, then a symbol with the same name (and prefixed with #!) is returned
Read Mark Invocation
If a read mark with the same name as the token is registered, then its procedure is called and the result of the read-mark procedure will be returned (see set-read-syntax!).

Conditional Expansion

#+read
#+FEATURE EXPR

Rewrites to

(cond-expand (FEATURE EXPR) (else))

and performs the feature test at macroexpansion time. Therefore, it may not work as expected when used within a macro form.

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