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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 120 (1000 on some common hardware platforms like x86). Likewise, the maximum number of values that can be passed to continuations captured using call-with-current-continuation is 120. This is an implementation restriction that is unlikely to be lifted.
numerator, denominator and rationalize
The numerator and denominator procedures cannot be applied to inexact numbers, and the procedure rationalize is not implemented at all.
Numeric string-conversion considerations
The runtime system uses the numerical string-conversion routines of the underlying C library and so does only understand standard (C-library) syntax for floating-point constants. Consequently, the procedures string->number, read, write, and display do not obey read/write invariance to inexact numbers.
Environments and non-standard syntax
Code evaluated in scheme-report-environment or null-environment still sees non-standard syntax.
The procedure char-ready? always returns #t for terminal ports.
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
No built-in support for bignums
There is no built-in support for exact rationals, complex numbers or extended-precision integers (bignums). The routines complex?, real? and rational? are identical to the standard procedure number?. The procedures make-rectangular and make-polar are not implemented. Fixnums are limited to 2^30 (or 2^62 on 64-bit hardware). Support for the full numeric tower is available as a separate package (=> numbers), provided the GNU multiprecision library is installed.
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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