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Hopefully final changes to SRFI-13



I have incorporated the results of the last several months of discussion
on the SRFI-13 mailing list into a new draft of the SRFI. The new draft will
appear at
    http://srfi.schemers.org/srfi-13/srfi-13.html
I assume Mike will email the mailing list when he has it installed there.

Here are the principle changes to the previous draft:

- SRFI-13 is now completely backwards-compatible with R5RS.

- STRING-UNFOLD & STRING-UNFOLD-RIGHT have been slightly generalised.

- I have added STRING-HASH and STRING-HASH-CI.

- I have carefully pinned down what these string operators do in a super-ASCII
  world, especially in Latin-1 & Unicode. A short summary of the high-level
  design decisions:

  + All routines are locale- and context-insensitive.
    Hence hash-tables, symbol tables, file-name directories, etc. won't
    have their invariants corrupted by a shift in locale.

    Locale- and context-sensitive "text" operations are important, and should
    be treated in another SRFI.

  + Case-mapping is restricted to 1-1 maps, and must be consistent with
    the Unicode 1-1 case maps. Same rationale as above.

  + Case-insensitive operations are carried out by case-folding strings with
        (char-downcase (char-upcase c))
    where these two operations are assumed to be consistent with the
    case-mapping definitions above.

  + String comparison is done code-point by code-point. 
    Same rationale as above.

I have cleaned up the text and expanded the rationale section.

Note that I have also brought the companion SRFI-14 (character sets) into
similar alignment with modern, super-ASCII considerations. You can check
out the latest draft at
    http://srfi.schemers.org/srfi-14/srfi-14.txt
A description of the super-ASCII issues in the SRFI-14 context and a summary
of the changes can be seen at
    http://srfi.schemers.org/srfi-14/mail-archive/msg00002.html

Dealing with super-ASCII issues has been a bear; I hope that the new design is
acceptable. I am grateful for the help and advice I got on these issues. If
there are no glaring problems, we can move forward and declare this SRFI
final.

Comments welcome.
    -Olin