Note: The SRFI number is not yet official and is therefore subject to change.

Abstract:

Scheme currently does not provide immutable pairs corresponding to its
existing mutable pairs, although most uses of pairs do not exploit
their mutability. The Racket system takes the radical approach of
making Scheme's pairs immutable, and providing a minimal library of
mutable pairs with procedures named mpair?, mcons, mcar, mcdr, set-mcar!,
set-mcdr!. This SRFI takes the opposite approach of leaving Scheme's pairs
unchanged and providing a full set of routines for creating and dealing
with immutable pairs. The sample implementation is portable and efficient.

Rationale:

Rather than attempting to design this library from scratch, I have chosen
the conservative option of modifying SRFI 1. Consequently, most of the
rationale given in that document applies to this one as well. I have
made the following changes:

* Removed all linear-update procedures ending in !

* Removed all references to circular lists (there will be a future SRFI for
  immutable bidirectional cycles).

* Removed first, second , etc., which are mostly used for poor man's records.

* Removed the O(n2) lists-as-sets procedures (there will be a future SRFI
  supporting O(n log n) immutable sets).

* Inserted an i at a judicious place in each identifier, usually at the
  beginning. However, because "icons" means something else in both ordinary
  English and computer jargon, the basic constructor and its immediate
  relatives are named ipair, xipair and ipair* instead.

* Added procedures for conversion between ordinary and immutable pairs, lists,
  and trees.

* Added an analogue of apply for applying a procedure to an immutable list of
  arguments.

SRFI:  http://www.ccil.org/~cowan/temp/srfi-116.html
Implementation:  http://www.ccil.org/~cowan/temp/ilists.tar.gz

Please send comments to <srfi-116@srfi.schemers.org> when it is created,
or to this list in the meantime.

--
John Cowan          http://www.ccil.org/~cowan        cowan@ccil.org
Schlingt dreifach einen Kreis vom dies!
Schliesst euer Aug vor heiliger Schau,
Denn er genoss vom Honig-Tau,
Und trank die Milch vom Paradies.

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