Other language changes since R⁵RS ¶

This section enumerates the additional differences between this report and the “Revised⁵ report”⁴³.

This list is not authoritative, but is believed to be correct and complete.

• Various minor ambiguities and unclarities in R⁵RS have been cleaned up.
• Libraries have been added as a new program structure to improve encapsulation and sharing of code. Some existing and new identifiers have been factored out into separate libraries. Libraries can be imported into other libraries or main programs, with controlled exposure and renaming of identifiers. The contents of a library can be made conditional on the features of the implementation on which it is to be used. There is an R⁵RS compatibility library.
• The expressions types include, include-ci, and cond-expand have been added to the base library; they have the same semantics as the corresponding library declarations.
• Exceptions can now be signaled explicitly with raise, raise-continuable or error, and can be handled with with-exception-handler and the guard syntax. Any object can specify an error condition; the implementation-defined conditions signaled by error have a predicate to detect them and accessor functions to retrieve the arguments passed to error. Conditions signaled by read and by file-related procedures also have predicates to detect them.
• New disjoint types supporting access to multiple fields can be generated with the define-record-type of SRFI 9⁴⁴.
• Parameter objects can be created with make-parameter, and dynamically rebound with parameterize. The procedures current-input-port and current-output-port are now parameter objects, as is the newly introduced current-error-port.
• Support for promises has been enhanced based on SRFI 45⁴⁵.
• Bytevectors, vectors of exact integers in the range from 0 to 255 inclusive, have been added as a new disjoint type. A subset of the vector procedures is provided. Bytevectors can be converted to and from strings in accordance with the UTF-8 character encoding. Bytevectors have a datum representation and evaluate to themselves.
• Vector constants evaluate to themselves.
• The procedure read-line is provided to make line-oriented textual input simpler.
• The procedure flush-output-port is provided to allow minimal control of output port buffering.
• Ports can now be designated as textual or binary ports, with new procedures for reading and writing binary data. The new predicates input-port-open? and output-port-open? return whether a port is open or closed. The new procedure close-port now closes a port; if the port has both input and output sides, both are closed.
• String ports have been added as a way to read and write characters to and from strings, and bytevector ports to read and write bytes to and from bytevectors.
• There are now I/O procedures specific to strings and bytevectors.
• The write procedure now generates datum labels when applied to circular objects. The new procedure write-simple never generates labels; write-shared generates labels for all shared and circular structure. The display procedure must not loop on circular objects.
• The R⁶RS procedure eof-object has been added. Eof-objects are now required to be a disjoint type.
• Syntax definitions are now allowed wherever variable definitions are.
• The syntax-rules construct now allows the ellipsis symbol to be specified explicitly instead of the default ‘...’, allows template escapes with an ellipsis-prefixed list, and allows tail patterns to follow an ellipsis pattern.
• The syntax-error syntax has been added as a way to signal immediate and more informative errors when a macro is expanded.
• The letrec* binding construct has been added, and internal define is specified in terms of it.
• Support for capturing multiple values has been enhanced with define-values, let-values, and let*-values. Standard expression types which contain a sequence of expressions now permit passing zero or more than one value to the continuations of all non-final expressions of the sequence.
• The case conditional now supports => syntax analogous to cond not only in regular clauses but in the else clause as well.
• To support dispatching on the number of arguments passed to a procedure, case-lambda has been added in its own library.
• The convenience conditionals when and unless have been added.
• The behavior of eqv? on inexact numbers now conforms to the R⁶RS definition.
• When applied to procedures, eq? and eqv? are permitted to return different answers.
• The R⁶RS procedures boolean=? and symbol=? have been added.
• Positive infinity, negative infinity, NaN, and negative inexact zero have been added to the numeric tower as inexact values with the written representations ‘+inf.0’, ‘-inf.0’, ‘+nan.0’, and ‘-0.0’ respectively. Support for them is not required. The representation ‘-nan.0’ is synonymous with ‘+nan.0’.
• The log procedure now accepts a second argument specifying the logarithm base.
• The procedures map and for-each are now required to terminate on the shortest argument list.
• The procedures member and assoc now take an optional third argument specifying the equality predicate to be used.
• The numeric procedures finite?, infinite?, nan?, exact-integer?, square, and exact-integer-sqrt have been added.
• The - and / procedures and the character and string comparison predicates are now required to support more than two arguments.
• The forms ‘#true’ and ‘#false’ are now supported as well as ‘#t’ and ‘#f’.
• The procedures make-list, list-copy, list-set!, string-map, string-for-each, string->vector, vector-append, vector-copy, vector-map, vector-for-each, vector->string, vector-copy!, and string-copy! have been added to round out the sequence operations.
• Some string and vector procedures support processing of part of a string or vector using optional start and end arguments.
• Some list procedures are now defined on circular lists.
• Implementations may provide any subset of the full Unicode repertoire that includes ASCII, but implementations must support any such subset in a way consistent with Unicode. Various character and string procedures have been extended accordingly, and case conversion procedures added for strings. String comparison is no longer required to be consistent with character comparison, which is based solely on Unicode scalar values. The new digit-value procedure has been added to obtain the numerical value of a numeric character.
• There are now two additional comment syntaxes: ‘#;’ to skip the next datum, and ‘#| … |#’ for nestable block comments.
• Data prefixed with datum labels ‘#⟨n⟩=’ can be referenced with ‘#⟨n⟩#’, allowing for reading and writing of data with shared structure.
• Strings and symbols now allow mnemonic and numeric escape sequences, and the list of named characters has been extended.
• The procedures file-exists? and delete-file are available in the (scheme file) library.
• An interface to the system environment, command line, and process exit status is available in the (scheme process-context) library.
• Procedures for accessing time-related values are available in the (scheme time) library.
• A less irregular set of integer division operators is provided with new and clearer names.
• The load procedure now accepts a second argument specifying the environment to load into.
• The call-with-current-continuation procedure now has the synonym call/cc.
• The semantics of read-eval-print loops are now partly prescribed, requiring the redefinition of procedures, but not syntax keywords, to have retroactive effect.
• The formal semantics now handles dynamic-wind.