Five to six to seven

These are the language changes listed in R6RS Appendix E, with notes on the extent to which R7RS-small has taken them up.

• Scheme source code now uses the Unicode character set. Specifically, the character set that can be used for identifiers has been greatly expanded.
  • Permitted but not required.
• Identifiers can now start with the characters ->.
  • Adopted.
• Identifiers and symbol literals are now case-sensitive.
  • Adopted.
• Identifiers and representations of characters, booleans, number objects, and . must be explicitly delimited.
  • Adopted.
• # is now a delimiter.
  • Rejected for compatibility with existing Schemes.
• Bytevector literal syntax has been added.
  • Adopted in principle, but SRFI 4 lexical syntax is used.
• Matched square brackets can be used synonymously with parentheses.
  • Rejected on the grounds that implementations may wish to use brackets for better things.
• The read-syntax abbreviations #' (for syntax), #-backquote (for quasisyntax), #, (for unsyntax), and #,@ (for unsyntax-splicing) have been added.
  • Irrelevant, because syntax-case is not part of R7RS-small.
• # can no longer be used in place of digits in number representations.
  • No longer required, but still permitted.
• The external representation of number objects can now include a mantissa width.
  • Rejected.
• Literals for NaNs and infinities were added.
  • Adopted.
• String and character literals can now use a variety of escape sequences.
  • Adopted.
• Block and datum comments have been added.
  • Adopted.
• The #!r6rs comment for marking report-compliant lexical syntax has been added.
  • Rejected; there is no R7RS analogue of it.
• Characters are now specified to correspond to Unicode scalar values.
  • Adopted in part: implementations are not required to handle all Unicode scalar values.
• Many of the procedures and syntactic forms of the language are now part of the (rnrs base (6)) library. Some procedures and syntactic forms have been moved to other libraries.
  • Adopted in principle, but the details differ.
• The base language has the following new procedures and syntactic forms: letrec*, let-values, let*-values, real-valued?, rational-valued?, integer-valued?, exact, inexact, finite?, infinite?, nan?, div, mod, div-and-mod, div0, mod0, div0-and-mod0, exact-integer-sqrt, boolean=?, symbol=?, string-for-each, vector-map, vector-for-each, error, assertion-violation, assert, call/cc, identifier-syntax.
  • All adopted except the -valued procedures, the division procedures (R7RS-small has different ones), assertions, and identifier syntax.
• The following procedures have been removed: char-ready?, transcript-on, transcript-off, load.
  • Only transcript-on and transcript-off are removed.
• The case-insensitive string comparisons (string-ci=?, string-ci<?, string-ci>?, string-ci<=?, string-ci>=?) operate on the case-folded versions of the strings rather than as the simple lexicographic ordering induced by the corresponding character comparison procedures.
  • Adopted. However, the non-ci versions are implementation-dependent in R7RS-small.
• Libraries have been added to the language.
  • Adopted, but the details differ slightly.
• A number of standard libraries are described.
  • Adopted, but the details differ.
• Many situations that "were an error" now have defined or constrained behavior. In particular, many are now specified in terms of the exception system.
  • Rejected for backward compatibility with existing Schemes.
• The full numerical tower is now required.
  • Rejected.
• The semantics for the transcendental functions has been specified more fully.
  • Adopted.
• The semantics of expt for zero bases has been refined.
  • Adopted.
• In syntax-rules forms, a _ may be used in place of the keyword.
  • Adopted.
• The let-syntax and letrec-syntax no longer introduce a new environment for their bodies.
  • Rejected for backward compatibility with existing Schemes.
• For implementations that support NaNs or infinities, many arithmetic operations have been specified on these values consistently with IEEE 754.
  • Adopted.
• For implementations that support a distinct -0.0, the semantics of many arithmetic operations with regard to -0.0 has been specified consistently with IEEE 754.
  • Adopted.
• Scheme's real number objects now have an exact zero as their imaginary part.
  • Rejected for backward compatibility with existing Schemes.
• The specification of quasiquote has been extended. Nested quasiquotations work correctly now, and unquote and unquote-splicing have been extended to several operands.
  • Adopted in part.
• Procedures now may or may not refer to locations. Consequently, eqv? is now unspecified in a few cases where it was specified before.
  • Mostly rejected, although eq? may now distinguish between procedures where eqv? does not.
• The mutability of the values of quasiquote structures has been specified to some degree.
  • Rejected.
• The dynamic environment of the before and after procedures of dynamic-wind is now specified.
  • Adopted.
• Various expressions that have only side effects are now allowed to return an arbitrary number of values.
  • Rejected for backward compatibility with existing Schemes.
• The order and semantics for macro expansion has been more fully specified.
  • Irrelevant.
• Internal definitions are now defined in terms of letrec*.
  • Adopted.
• The old notion of program structure and Scheme's top-level environment has been replaced by top-level programs and libraries.
  • Adopted in parellel with the old semantics.
• The denotational semantics has been replaced by an operational semantics.
  • The denotational semantics was updated rather than replaced.

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