Revised^6 Report on the Algorithmic Language Scheme

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Appendix E
Language changes

This chapter describes most of the changes that have been made to Scheme since the “Revised5 Report” [14] was published:

- Scheme source code now uses the Unicode character set. Specifically, the character set that can be used for identifiers has been greatly expanded.

- Identifiers can now start with the characters `->`.

- Identifiers and symbol literals are now case-sensitive.

- Identifiers and representations of characters, booleans, number objects, and `.` must be explicitly delimited.

- ``#`` is now a delimiter.

- Bytevector literal syntax has been added.

- Matched square brackets can be used synonymously with parentheses.

- The read-syntax abbreviations ``#`’` (for `syntax`), ``#``‘`` (for `quasisyntax`), ``#`,` (for `unsyntax`), and ``#`,@` (for `unsyntax-splicing` have been added; see section 4.3.5.)

- ``#`` can no longer be used in place of digits in number representations.

- The external representation of number objects can now include a mantissa width.

- Literals for NaNs and infinities were added.

- String and character literals can now use a variety of escape sequences.

- Block and datum comments have been added.

- The ``#`!r6rs` comment for marking report-compliant lexical syntax has been added.

- Characters are now specified to correspond to 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; see figure 13.

  identifier | moved to
  `assoc` | `(rnrs lists (6))`
  `assv` | `(rnrs lists (6))`
  `assq` | `(rnrs lists (6))`
  `call-with-input-file` | `(rnrs io simple (6))`
  `call-with-output-file` | `(rnrs io simple (6))`
  `char-upcase` | `(rnrs unicode (6))`
  `char-downcase` | `(rnrs unicode (6))`
  `char-ci=?` | `(rnrs unicode (6))`
  `char-ci<?` | `(rnrs unicode (6))`
  `char-ci>?` | `(rnrs unicode (6))`
  `char-ci<=?` | `(rnrs unicode (6))`
  `char-ci>=?` | `(rnrs unicode (6))`
  `char-alphabetic?` | `(rnrs unicode (6))`
  `char-numeric?` | `(rnrs unicode (6))`
  `char-whitespace?` | `(rnrs unicode (6))`
  `char-upper-case?` | `(rnrs unicode (6))`
  `char-lower-case?` | `(rnrs unicode (6))`
  `close-input-port` | `(rnrs io simple (6))`
  `close-output-port` | `(rnrs io simple (6))`
  `current-input-port` | `(rnrs io simple (6))`
  `current-output-port` | `(rnrs io simple (6))`
  `display` | `(rnrs io simple (6))`
  `do` | `(rnrs control (6))`
  `eof-object?` | `(rnrs io simple (6))`
  `eval` | `(rnrs eval (6))`
  `delay` | `(rnrs r5rs (6))`
  `exact->inexact` | `(rnrs r5rs (6))`
  `force` | `(rnrs r5rs (6))`
  `inexact->exact` | `(rnrs r5rs (6))`
  `member` | `(rnrs lists (6))`
  `memv` | `(rnrs lists (6))`
  `memq` | `(rnrs lists (6))`
  `modulo` | `(rnrs r5rs (6))`
  `newline` | `(rnrs io simple (6))`
  `null-environment` | `(rnrs r5rs (6))`
  `open-input-file` | `(rnrs io simple (6))`
  `open-output-file` | `(rnrs io simple (6))`
  `peek-char` | `(rnrs io simple (6))`
  `quotient` | `(rnrs r5rs (6))`
  `read` | `(rnrs io simple (6))`
  `read-char` | `(rnrs io simple (6))`
  `remainder` | `(rnrs r5rs (6))`
  `scheme-report-environment` | `(rnrs r5rs (6))`
  `set-car!` | `(rnrs mutable-pairs (6))`
  `set-cdr!` | `(rnrs mutable-pairs (6))`
  `string-ci=?` | `(rnrs unicode (6))`
  `string-ci<?` | `(rnrs unicode (6))`
  `string-ci>?` | `(rnrs unicode (6))`
  `string-ci<=?` | `(rnrs unicode (6))`
  `string-ci>=?` | `(rnrs unicode (6))`
  `string-set!` | `(rnrs mutable-strings (6))`
  `string-fill!` | `(rnrs mutable-strings (6))`
  `with-input-from-file` | `(rnrs io simple (6))`
  `with-output-to-file` | `(rnrs io simple (6))`
  `write` | `(rnrs io simple (6))`
  `write-char` | `(rnrs io simple (6))`

  Figure 13: Identifiers moved to libraries

- 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`.

- The following procedures have been removed: `char-ready?`, `transcript-on`, `transcript-off`, `load`.

- 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.

- Libraries have been added to the language.

- A number of standard libraries are described in a separate report [24].

- Many situations that “were an error” now have defined or constrained behavior. In particular, many are now specified in terms of the exception system.

- The full numerical tower is now required.

- The semantics for the transcendental functions has been specified more fully.

- The semantics of `expt` for zero bases has been refined.

- In `syntax-rules` forms, a `_` may be used in place of the keyword.

- The `let-syntax` and `letrec-syntax` no longer introduce a new environment for their bodies.

- For implementations that support NaNs or infinities, many arithmetic operations have been specified on these values consistently with IEEE 754.

- 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.

- Scheme’s real number objects now have an exact zero as their imaginary part.

- The specification of `quasiquote` has been extended. Nested quasiquotations work correctly now, and `unquote` and `unquote-splicing` have been extended to several operands.

- Procedures now may or may not refer to locations. Consequently, `eqv?` is now unspecified in a few cases where it was specified before.

- The mutability of the values of `quasiquote` structures has been specified to some degree.

- The dynamic environment of the before and after procedures of `dynamic-wind` is now specified.

- Various expressions that have only side effects are now allowed to return an arbitrary number of values.

- The order and semantics for macro expansion has been more fully specified.

- Internal definitions are now defined in terms of `letrec*`.

- The old notion of program structure and Scheme’s top-level environment has been replaced by top-level programs and libraries.

- The denotational semantics has been replaced by an operational semantics based on an earlier semantics for the language of the “Revised5 Report” [14, 18].