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].