SRFI 52: Permitting and Supporting Extended Character Sets

by Thomas Lord ( aka

status: withdrawn (2004/06/17)


This SRFI describes how to modify the Revised Report (R5RS) in order to enable conforming implementations to use an extended character set such as (but not limited to) Unicode.

Changes to some requirements of the report are recommended. Currently, the Revised Report contains requirements which are difficult or impossible to satisfy with some extended character sets.

New required procedures are proposed, specified, and included in the reference implementation. These procedures enable portable Scheme programs to manipulate Scheme source texts and source data accurately, even in implementations using extended character sets.

This SRFI concludes with some suggestions for implementors interested in providing good Unicode support, using these suggestions to illustrate how the proposed changes to the Revised Report can "play out" in Unicode-based Scheme.

This SRFI does not attempt to provide a comprehensive library for global text processing. For example, one issue in global text processing is the need for linguistically-sensitive, locale-sensitive procedures for sorting strings. Such procedures are beyond the scope of this SRFI. On the other hand, by making Scheme compatible with extended character sets, this SRFI is a step in the direction of permitting global text processing standard libraries to be developed in a form portable across all conforming implementations.

This SRFI does not propose that implementations be required to support Unicode or any other extended character set. It does not specify a representation for Unicode characters or strings. It does revise the specifications of the report so that char? values may be Unicode (or other) characters.

The reference implementation included should prove to be easily ported to and effective for all ASCII-only implementations and for many implementations using an 8-bit character set which is an extension of ASCII (it will require very minor modifications for each particular implementation). Other implementations may need to use a different implementation.