Language changes

This section enumerates the changes that have been made to Scheme since the ``Revised³ report'' [#!R3RS!#] was published.

• Although implementations may extend Scheme, they must offer a syntactic mode that adds no reserved words and preempts no lexical conventions of Scheme.

• Implementations may report implementation errors caused by implementation restrictions.

• The sets defined by boolean?, pair?, symbol?, number?, char?, string?, vector?, and procedure? are required to be disjoint.

• The variables bound by a lambda, let, letrec, and do must not contain duplicates.

• Nested begin expressions containing definitions are treated as a sequence of definitions.

• The eqv? procedure is no longer required to be true of any two empty strings or two empty vectors.

• The syntax of numeric literals has been changed, and the exactness implied by each syntax has been specified.

• The semantics of many numeric procedures have been clarified.

• Rationalize has been restricted to two arguments and its specification clarified.

• The number->string and string->number procedures have been changed.

• Integer->char now requires an exact integer argument.

• Variables removed: t, nil.

• Procedures removed: approximate, last-pair.

• Procedures added: peek-char.

• Syntaxes made essential: case, and, or, quasiquote.

• Procedures made essential: reverse, max, min, modulo, gcd, lcm, floor, ceiling, truncate, round, char-ci=?, char-ci<?, char-ci>?, char-ci<=?, char-ci>=?, char-alphabetic char-numeric?, char-whitespace?, char-lower-case? char-upper-case?, char-upcase, char-downcase, make-string, string-set!, string-ci=?, string-ci<?, string-ci>?, string-ci<=?, string-ci>=?, string-append, open-input-file, open-output-file, close-input-port, close-output-port, number->string, string->number.

• Procedures required to accept more general numbers of arguments: append, +, *, - (one argument), / (one argument), =, <, >, <=, >=, map, for-each.

  Call-with-input-file and call-with-output-file renamed to call-with-input-port and call-with-output-port