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Incompatibilities with R⁶RS

This section enumerates the incompatibilities between R⁷RS and the “Revised⁶ report” [R6RS] and its accompanying Standard Libraries document.

This list is not authoritative, and is possibly incomplete.

• R⁷RS libraries begin with the keyword define-library rather than library in order to make them syntactically distinguishable from R⁶RS libraries. In R⁷RS terms, the body of an R⁶RS library consists of a single export declaration followed by a single import declaration, followed by commands and definitions. In R⁷RS, commands and definitions are not permitted directly within the body: they have to be be wrapped in a begin library declaration.
• There is no direct R⁶RS equivalent of the include, include-ci, include-library-declarations, or cond-expand library declarations. On the other hand, the R⁷RS library syntax does not support phase or version specifications.
• The grouping of standardized identifiers into libraries is different from the R⁶RS approach. In particular, procedures which are optional in R⁵RS, either expressly or by implication, have been removed from the base library. Only the base library itself is an absolute requirement.
• No form of identifier syntax is provided.
• Internal syntax definitions are allowed, but uses of a syntax form cannot appear before its definition; the even/odd example given in R⁶RS is not allowed.
• The R⁶RS exception system was incorporated as-is, but the condition types have been left unspecified. In particular, where R⁶RS requires a condition of a specified type to be signaled, R⁷RS says only “it is an error”, leaving the question of signaling open.
• Full Unicode support is not required. Normalization is not provided. Character comparisons are defined by Unicode, but string comparisons are implementation-dependent. Non-Unicode characters are permitted.
• The full numeric tower is optional as in R⁵RS, but optional support for IEEE infinities, NaN, and -0.0 was adopted from R⁶RS. Most clarifications on numeric results were also adopted, but the R⁶RS procedures real-valued?, rational-valued?, and integer-valued? were not. The R⁶RS division operators div, mod, div-and-mod, div0, mod0 and div0-and-mod0 are not provided.
• When a result is unspecified, it is still required to be a single value. However, non-final expressions in a body can return any number of values.
• The semantics of map and for-each have been changed to use the SRFI 1 [srfi1] early termination behavior. Likewise, assoc and member take an optional equal? argument as in SRFI 1, instead of the separate assp and memp procedures of R⁶RS.
• The R⁶RS quasiquote clarifications have been adopted, with the exception of multiple-argument unquote and unquote-splicing.
• The R⁶RS method of specifying mantissa widths was not adopted.
• String ports are compatible with SRFI 6 [srfi6] rather than R⁶RS.
• R⁶RS-style bytevectors are included, but only the unsigned byte (u8) procedures have been provided. The lexical syntax uses #u8 for compatibility with SRFI 4 [srfi4], rather than the R⁶RS #vu8 style.
• The utility macros when and unless are provided, but their result is left unspecified.
• The remaining features of the Standard Libraries document were left to future standardization efforts.