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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
[read] #$EXPRESSIONAn abbreviation for (location EXPRESSION).
Bytevector strings
[read] #u8"..."String syntax for bytevectors, as an alternative to #u8(...). The usual escape sequences for strings are recognized.
Keyword
[read] #:#:SYMBOL SYMBOL: :SYMBOL
Syntax for keywords. Keywords are symbols 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 sequence | Character |
|---|---|
| \n | line feed / newline |
| \t | tab |
| \r | carriage return |
| \b | backspace |
| \a | bell |
| \v | vertical tab |
| \f | form feed |
| \xXX; | hexadecimal 8-bit character code |
| \uXXXX | hexadecimal 16-bit Unicode character code |
| \UXXXXXXXX | hexadecimal 32-bit Unicode character code |
| \OOO | octal 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
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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