This section enumerates the changes that have been made to Scheme since
the “Revised^4 report” [R4RS] was published.
The report is now a superset of the IEEE standard for Scheme
[IEEEScheme]: implementations that conform to the report will
also conform to the standard. This required the following changes:
The empty list is now required to count as true.
The classification of features as essential or inessential has been
removed. There are now three classes of built-in procedures: primitive,
library, and optional. The optional procedures are load,
with-input-from-file, with-output-to-file,
transcript-on, transcript-off, and
interaction-environment,
and - and / with more than two arguments.
None of these are in the IEEE standard.
Programs are allowed to redefine built-in procedures. Doing so
will not change the behavior of other built-in procedures.
Port has been added to the list of disjoint types.
The macro appendix has been removed. High-level macros are now part
of the main body of the report. The rewrite rules for derived expressions
have been replaced with macro definitions. There are no reserved identifiers.
Syntax-rules now allows vector patterns.
Multiple-value returns, eval, and dynamic-wind have
been added.
The calls that are required to be implemented in a properly tail-recursive
fashion are defined explicitly.
`@' can be used within identifiers. `|' is reserved
for possible future extensions.