The one instance where we deliberately broke compatibility with R5RS in favor of R6RS is by defaulting the reader to case-sensitivity. We also use the R6RS #!fold-case and #!no-fold-case reader extensions for changing the case-sensitivity, and have structured the module system to allow using existing case-insensitive R5RS source unmodified.
The module system does not support phase distinctions, which are unnecessary in the absense of low-level macros (see below), nor versioning, which we feel is an important feature but deserves more experimentation before standardizing.
In addition, the syntax of the module system was deliberately chosen to be syntactically different from R6RS, using define-library instead of library and putting an extra level of indirection around the body, for the following reasons:
Since the system is simple, it is expected that R6RS implementations will be able to support the module syntax in addition to their library syntax.
The modularization of standardized identifiers is different from the R6RS system. In particular, procedures which are optional either expressly or by implication in R5RS have been removed from the base module.
Identifier syntax is not provided. We feel this is a useful feature in some situations, but the existence of such macros means that neither programmers nor other macros can look at an identifier in an evaluated position and know it is a reference -- this in a sense makes all macros slightly weaker. We'd like to see individual implementations continue experimenting with this and other extensions before standardizing.
Internal syntax definitions are allowed, but all references to syntax must follow the definition -- the even/odd example given is R6RS is not allowed.
The R6RS exception system was incorporated as-is, but the condition types have been left unspecified. Specific errors that were required to be signalled in R6RS remain "an error" in R7RS, allowing implementations to provide their own extensions. The condition system and stricter semantics may reappear in the large language or a later SRFI or standard. There is no discussion of "safety."
We do not require full Unicode support, but requiring implementations to be consistent with Unicode in the characters that they do support. Case conversions are described in terms of the Unicode locale-independent mappings, and instead of explicit normalization forms we provide string-ni=? etc. for an implementation-defined normalization form (which may be the identity transformation). Character comparisons are defined by Unicode, but string comparisons are implementation-dependent, and therefore need not be the lexicographic mapping of the corresponding character comparisons (an incompatibility with R5RS). Non-Unicode characters are permitted.
The full numeric tower is optional as in R5RS, but support for IEEE infinities, NaN, and -0.0 were adopted from R6RS. Most clarifications on numeric results were also adopted, but the new procedures real-valued?, rational-valued?, and integer-valued? were not. The R5RS names inexact->exact for exact and exact->inexact for inexact were retained, with a note indicating that their names are historical.
The division operators div, mod, div-and-mod, div0, mod0 and div-and-mod0 have been replaced with a full set of 18 operators describing 6 rounding semantics.
When a result is unspecified, it is still required to be a single value, in the interests of R5RS compatibility. However, non-final expressions in a body may return any number of values.
In the interest of the widespread SRFI 1 support and extensive code using it, the semantics of map and for-each have been changed to use the SRFI 1 early termination behavior instead of an error. Likewise assoc and member take an optional equal? argument as in SRFI 1, instead of the separate assp and memp procedures from R6RS.
We adopted the R6RS quasiquote clarifications, but have not seen convincing enough examples to allow multiple-argument unquote and unquote-splicing.
The case macro has been extended with a => syntax analogous to that in cond.
We've kept R5RS # inexact "placeholder" digits in the interest of backwards compatibility, but not adopted the R6RS mantissa widths.
The low-level macro system and syntax-case were not adopted. There are two general families of macro systems in widespread use -- the syntax-case family and the syntactic-closures family -- and they have neither been shown to be equivalent nor capable of implementing each other. Given this situation we feel we cannot choose one over the other, and so leave low-level macros to the large language.
The new I/O system from R6RS was not adopted. We feel a completely new system deserves a period of usage before being standardized, and were unhappy with the backwards-compatibility "simple I/O" which introduced a redundant API and relegated R5RS code to being a second-class citizen. Instead we added support for binary ports as disjoint from character ports, but in the same style as existing ports which is what most R5RS implementations currently do.
Our string ports are compatible with SRFI 6 rather than R6RS; analogous bytevector ports are also provided.
We felt the R6RS records system was overly complex, and the two layers poorly integrated. We spent a lot of time debating this, but in the end decided to simply use a generative version of SRFI 9, which has near-universal support among implementations. We hope to provide a more powerful records system in the large language.
We have not included enumerations in the small language.
We add R6RS bytevectors, providing only the "u8" procedures in the small language, but keeping the SRFI 4 #u8(...) syntax, acknowledging that with a module system it's easier to change names than reader syntax.
We provide an interface to the current system time in terms of TAI.
The utility macros when and unless are provided, but since it would be meaningless to try to use their result we leave the result unspecified.
We could not agree on many issues with hash tables and have left them for the large language. We've also left sorting and bitwise arithmetic to WG2.
We did not relegate pair and string mutators to separate modules.
We've added support for CL-style #<n>=(... #<n># ...) reader labels. write is required to detect cycles and use these labels in this case. The new write-simple (name pending) is added for when the programmer does not want to output reader labels.
We extend syntax-rules with _ as a general wildcard as in R6RS, but make explicit that both _ and ... can be used in the literals list in which case their usual behavior is overridden. We allow the same pattern language extensions, as well as the SRFI 46 ellipsis specifier syntax.
We provide a new syntax-error form for signalling friendly compile-time errors from syntax-rules macros.
We added parameters compatible with R6RS (and SRFI 39), but in the interest of thread-safety and conflicting behavior among existing implementations left the semantics of mutating a parameter (passing it an argument) unspecified. Parameters in the small language can be altered only with the parameterize form.
equal? is required to always terminate, and to be true if eqv? is true.