standard-extensions

Extensions to the R5RS standard

Identifiers

Identifiers may contain special characters if delimited with | ... |.

Brackets and braces

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

Non-standard procedures and syntax

CHICKEN provides numerous non-standard procedures. See the manual sections on the included library modules (Included modules) for more information. Here we only document cond-expand because it is always present in a module, even without imports.

cond-expand

(cond-expand FEATURE-CLAUSE ...)syntax

Expands by selecting feature clauses. This form is allowed to appear in non-toplevel expressions.

Predefined feature-identifiers are "situation" specific:

compile: chicken, chicken-5, compiling, library, eval, extras, regex, srfi-0, srfi-2, srfi-4, srfi-6, srfi-8, srfi-9, srfi-10, srfi-11, srfi-12, srfi-15, srfi-16, srfi-17, srfi-23, srfi-26, srfi-28, srfi-30, srfi-31, srfi-39, srfi-55, srfi-61, srfi-62
load: chicken, chicken-5, extras, srfi-0, srfi-2, srfi-6, srfi-8, srfi-9, srfi-10, srfi-12, srfi-17, srfi-23, srfi-28, srfi-30, srfi-39, srfi-55, srfi-61, srfi-62. library is implicit.
eval: csi, chicken, chicken-5, extras, srfi-0, srfi-2, srfi-6, srfi-8, srfi-9, srfi-10, srfi-11, srfi-12, srfi-15, srfi-16, srfi-17, srfi-23, srfi-26, srfi-28, srfi-30, srfi-31, srfi-39, srfi-55, srfi-61, srfi-62. library is implicit.

Also, features of the form chicken-5.X, where X denotes the minor version, are available.

The symbols returned by the following procedures from (chicken platform) are also available as feature-identifiers in all situations: (machine-byte-order), (machine-type), (software-type), (software-version). For example, the machine-type class of feature-identifiers include arm, alpha, mips, etc.

The (chicken platform) module also provides a function (features) that returns a list of all registered features.

In addition the following feature-identifiers may exist: cross-chicken, dload, manyargs, ptables.

For further information, see the documentation for SRFI-0.

User defined character names

User defined character names are supported. See char-name. Characters can be given in hexadecimal notation using the #\xXX syntax where XX specifies the character code. Character codes above 255 are supported and can be read (and are written) using the #\uXXXX and #\UXXXXXXXX notations.

Non-standard characters names supported are #\tab, #\linefeed, #\return, #\alarm, #\vtab, #\nul, #\page, #\esc, #\delete and #\backspace.

Special characters in strings

CHICKEN supports special characters preceded with a backslash \ in quoted string constants. \n denotes the newline-character, \r carriage return, \b backspace, \t TAB, \v vertical TAB, \a alarm, \f formfeed, \xXX a character with the code XX in hex and \uXXXX (and \UXXXXXXXX) a unicode character with the code XXXX. The latter is encoded in UTF-8 format.

Non-standard read syntax

Escapes in symbols

| ... | may be used to escape a sequence of characters when reading a symbol. \X escapes a single character in a symbols name:

 (symbol->string '|abc def|)       =>   "abc def"
 (symbol->string '|abc||def|)      =>   "abcdef"
 (symbol->string '|abc|xyz|def|)   =>   "abcxyzdef"
 (symbol->string '|abc\|def|)      =>   "abc|def"
 (symbol->string 'abc\ def)        =>   "abc def"

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).

Blob literals

#${read

 #${ HEX ... }

Syntax for literal "blobs" (byte-sequences). Expects hexadecimal digits and ignores any whitespace characters:

 #;1> ,d '#${deadbee f}
 blob of size 4:
    0: de ad be ef                                     ....

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 sequence	Character
`\n`	line feed / newline
`\t`	tab
`\r`	carriage return
`\b`	backspace
`\a`	bell
`\v`	vertical tab
`\f`	form feed
`\x`XX	hexadecimal 8-bit character code
`\u`XXXX	hexadecimal 16-bit Unicode character code
`\U`XXXXXXXX	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