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Extensions to the standard

[2.1] Identifiers may contain special characters if delimited with | ... |.

[2.3] The brackets [ ... ] are provided as an alternative syntax for ( ... ). A number of reader extensions is provided. See Non-standard read syntax.

[4] Numerous non-standard macros are provided. See Non-standard macros and special forms for more information.

[4.1.4] Extended DSSSL style lambda lists are supported. DSSSL parameter lists are defined by the following grammar:

<parameter-list> ==> <required-parameter>*
                     [(#!optional <optional-parameter>*)]
                     [(#!rest <rest-parameter>)]
                     [(#!key <keyword-parameter>*)]
<required-parameter> ==> <ident>
<optional-parameter> ==> <ident>
                         | (<ident> <initializer>)
<rest-parameter> ==> <ident>
<keyword-parameter> ==> <ident>
                        | (<ident> <initializer>)
<initializer> ==> <expr>

When a procedure is applied to a list of arguments, the parameters and arguments are processed from left to right as follows:

It shall be an error for an <ident> to appear more than once in a parameter-list.

Example:

((lambda x x) 3 4 5 6)       => (3 4 5 6)
((lambda (x y #!rest z) z)
 3 4 5 6)                    => (5 6)
((lambda (x y #!optional z #!rest r #!key i (j 1)) 
    (list x y z i: i j: j))
 3 4 5 i: 6 i: 7)            => (3 4 5 i: 6 j: 1)

[4.1.6] set! for unbound toplevel variables is allowed. set! (PROCEDURE ...) ...) is supported, as CHICKEN implements SRFI-17. [4.2.1] The cond form supports SRFI-61.

[4.2.2] It is allowed for initialization values of bindings in a letrec construct to refer to previous variables in the same set of bindings, so

(letrec ((foo 123)
         (bar foo) )
  bar)

is allowed and returns 123.

[4.2.3] (begin) is allowed in non-toplevel contexts and evaluates to an unspecified value.

[4.2.5] Delayed expressions may return multiple values.

[5.2.2] CHICKEN extends standard semantics by allowing internal definitions everywhere, and not only at the beginning of a body. A set of internal definitions is equivalent to a letrec form enclosing all following expressions in the body:

(let ((foo 123))
  (bar)
  (define foo 456)
  (baz foo) )

expands into

(let ((foo 123))
  (bar)
  (letrec ((foo 456))
    (baz foo) ) )

[5.2] define with a single argument is allowed and initializes the toplevel or local binding to an unspecified value. CHICKEN supports curried definitions, where the variable name may also be a list specifying a name and a nested lambda list. So

(define ((make-adder x) y) (+ x y))

is equivalent to

(define (make-adder x) (lambda (y) (+ x y)))

[6] CHICKEN provides numerous non-standard procedures. See the manual sections on library units for more information.

[6.2.4] The special IEEE floating-point numbers +nan, +inf and -inf are supported, as is negative zero.

[6.3.4] 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.

[6.3.5] 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.

The third argument to substring is optional and defaults to the length of the string.

[6.4] force called with an argument that is not a promise returns that object unchanged. Captured continuations can be safely invoked inside before- and after-thunks of a dynamic-wind form and execute in the outer dynamic context of the dynamic-wind form.

Implicit non-multival continuations accept multiple values by discarding all but the first result. Zero values result in the continuation receiving an unspecified value. Note that this slight relaxation of the behaviour of returning mulitple values to non-multival continuations does not apply to explicit continuations (created with call-with-current-continuation).

[6.5] The second argument to eval is optional and defaults to the value of (interaction-environment). scheme-report-environment and null-environment accept an optional 2nd parameter: if not #f (which is the default), toplevel bindings to standard procedures are mutable and new toplevel bindings may be introduced.

[6.6] The tilde character (~) is automatically expanded in pathnames. Additionally, if a pathname starts with $VARIABLE..., then the prefix is replaced by the value of the given environment variable.

[6.6.1] if the procedures current-input-port and current-output-port are called with an argument (which should be a port), then that argument is selected as the new current input- and output-port, respectively. The procedures open-input-file, open-output-file, with-input-from-file, with-output-to-file, call-with-input-file and call-with-output-file accept an optional second (or third) argument which should be one or more keywords, if supplied. These arguments specify the mode in which the file is opened. Possible values are the keywords #:text, #:binary or #:append.

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