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’,
‘transcript-on’, ‘transcript-off’, and
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
- 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.