Language changes

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.