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!
> 5
> frov
> 6

Unconfirmed deviations


The procedure char-ready? always returns #t for terminal ports.

Doubtful deviations


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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