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This section enumerates the changes that have been made to Scheme since the
"Revised^4 report" [6] was published.
- The report is now a superset of the IEEE standard for Scheme [13]:
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.
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