Confirmed deviations from R5RS
Identifiers are by default case-sensitive (see Using the compiler).
Number of arguments to procedures and macros
The maximal number of arguments that may be passed to a compiled procedure or macro is limited to around 1000. Likewise, the maximum number of values that can be passed to continuations captured using call-with-current-continuation is 1000. This is an implementation restriction that is unlikely to be lifted.
Numeric string-conversion considerations
In some cases the runtime system uses the numerical string-conversion routines of the underlying C library. Consequently, the procedures string->number, read, write, and display do not obey read/write invariance for inexact numbers.
Environments and non-standard syntax
In addition to the standard bindings, scheme-report-environment and null-environment contain additional non-standard bindings for the following syntactic forms: import, require-extension, require-library, begin-for-syntax, export, module, cond-expand, syntax, reexport, import-for-syntax.
Assignment to unbound variables
set! may assign values to unbound variables; this creates a new top-level binding for the variable, as if define had been used instead. This extension must be used with care, as typos might cause unexpected results:
> (let ((frob 5))
(set! frov (+ frob 1)) ; oops!
frob)
> 5
> frov
> 6
Unconfirmed deviations
char-ready?
The procedure char-ready? always returns #t for terminal ports.
Doubtful deviations
letrec
letrec does evaluate the initial values for the bound variables sequentially and not in parallel, that is:
(letrec ((x 1) (y 2)) (cons x y))
is equivalent to
(let ((x (void)) (y (void)))
(set! x 1)
(set! y 2)
(cons x y) )
where R5RS requires
(let ((x (void)) (y (void)))
(let ((tmp1 1) (tmp2 2))
(set! x tmp1)
(set! y tmp2)
(cons x y) ) )
It is unclear whether R5RS permits this behavior or not; in any case, this only affects letrecs where the bound values are not lambda-expressions.
Non-deviations that might surprise you
let-syntax and letrec-syntax
let-syntax and letrec-syntax introduce a new scope.
equal? compares all structured data recursively
equal? compares all structured data with the exception of procedures recursively, while R5RS specifies that eqv? is used for data other than pairs, strings and vectors. However, R5RS does not dictate the treatment of data types that are not specified by R5RS
transcript-on and transcript-off are not implemented
The transcript-on and transcript-off procedures are not implemented. R5RS does not require them.
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