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<!--
SPDX-FileCopyrightText: 2014 John Cowan <[email protected]>
SPDX-License-Identifier: MIT
-->
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0//EN" "http://www.w3.org/TR/REC-html40/strict.dtd">
<html lang="en-US"><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"/>
<!-- This commented out text is for the brittle SRFI tools -->
<!--
</head>
<body>
<H1>Title</H1>
Immutable List Library
<H1>Author</H1>
John Cowan
<H1>Status</H1>
This SRFI is currently in ``draft'' status.
-->
<title>SRFI 116: Immutable List Library</title>
<!-- Should have a media=all to get, for example, printing to work.
== But my Netscape will completely ignore the tag if I do that.
-->
<style type="text/css">
/* A little general layout hackery for headers and the title. */
body { margin-left: +7%;
font-family: "Helvetica", sans-serif;
}
/* Netscape workaround: */
td, th { font-family: "Helvetica", sans-serif; }
code, pre { font-family: "courier new", "courier"; }
h1 { margin-left: -5%; }
h1, h2 { clear: both; }
h1, h2, h3, h4, h5, h6 { color: blue }
div.title-text { font-size: large; font-weight: bold; }
div.indent { margin-left: 2em; } /* General indentation */
pre.code-example { margin-left: 2em; } /* Indent code examples. */
/* This stuff is for definition lists of defined procedures.
** A proc-def2 is used when you want a stack of procs to go
** with one <dd> ... </dd> body. In this case, make the first
** proc a proc-def1, following ones proc-defi's, and the last one
** a proc-defn.
**
** Unfortunately, Netscape has huge bugs with respect to style
** sheets and dl list rendering. We have to set truly random
** values here to get the rendering to come out. The proper values
** are in the following style sheet, for Internet Explorer.
** In the following settings, the *comments* say what the
** setting *really* causes Netscape to do.
**
** Ugh. Professional coders sacrifice their self-respect,
** that others may live.
*/
/* m-t ignored; m-b sets top margin space. */
dt.proc-def1 { margin-top: 0ex; margin-bottom: 3ex; }
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dt.proc-defn { margin-top: 0ex; margin-bottom: 0ex; }
/* m-t works weird depending on whether or not the last line
** of the previous entry was a pre. Set to zero.
*/
dt.proc-def { margin-top: 0ex; margin-bottom: 3ex; }
/* m-b sets space between dd and dt; m-t ignored. */
dd.proc-def { margin-bottom: 0.5ex; margin-top: 0ex; }
/* Boldface the name of a procedure when it's being defined. */
code.proc-def { font-weight: bold; font-size: 110%}
/* For the index of procedures.
** Same hackery as for dt.proc-def, above.
*/
/* m-b sets space between dd and dt; m-t ignored. */
dd.proc-index { margin-bottom: 0ex; margin-top: 0ex; }
/* What the fuck? */
pre.proc-index { margin-top: -2ex; }
/* Pull the table of contents back flush with the margin.
** Both NS and IE screw this up in different ways.
*/
#toc-table { margin-top: -2ex; margin-left: -5%; }
/* Spread out bibliographic lists. */
/* More Netscape-specific lossage; see the following stylesheet
** for the proper values (used by IE).
*/
dt.biblio { margin-bottom: 1ex; }
/* Links to draft copies (e.g., not at the official SRFI site)
** are colored in red, so people will use them during the
** development process and kill them when the document's done.
*/
a.draft { color: red; }
</style>
<style type="text/css" media="all">
/* Nastiness: Here, I'm using a bug to work around a bug.
** Netscape rendering bugs mean you need bogus dt and dd
** margin settings — settings which screw up IE's proper rendering.
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** media=all style sheet. So I am placing the (proper) IE values
** here. Perhaps, one day, when these rendering bugs are fixed,
** this gross hackery can be removed.
*/
dt.proc-def1 { margin-top: 3ex; margin-bottom: 0ex; }
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dt.proc-defn { margin-top: 0ex; margin-bottom: 0.5ex; }
dt.proc-def { margin-top: 3ex; margin-bottom: 0.5ex; }
pre { margin-top: 1ex; }
dd.proc-def { margin-bottom: 2ex; margin-top: 0.5ex; }
/* For the index of procedures.
** Same hackery as for dt.proc-def, above.
*/
dd.proc-index { margin-top: 0ex; }
pre.proc-index { margin-top: 0ex; }
/* Spread out bibliographic lists. */
dt.biblio { margin-top: 3ex; margin-bottom: 0ex; }
dd.biblio { margin-bottom: 1ex; }
</style>
</head>
<body>
<!--========================================================================-->
<h1>Title</h1><div class="title-text">
Immutable List Library
</div>
<!--========================================================================-->
<h1>Author</h1>
John Cowan
<address>
<a href="http://www.ccil.org/~cowan">http://www.ccil.org/~cowan</a><br/>
<a href="mailto:[email protected]">[email protected]</a>
</address>
<!--========================================================================-->
<h1>Status</h1>
<p>
To see an explanation of
each status that a SRFI can hold, see <a
href="http://srfi.schemers.org/srfi-process.html">here</a>.
To provide input on this SRFI, please
<a href="mailto:srfi minus 116 at srfi dot schemers dot org">mail to
<code><srfi minus 116 at srfi dot schemers dot org></code></a>. See
<a href="../../srfi-list-subscribe.html">instructions here</a> to
subscribe to the list. You can access previous messages via
<a href="mail-archive/maillist.html">the archive of the mailing list</a>.
</p><ul>
<li>Received: <a
href="http://srfi.schemers.org/srfi-116/srfi-116-1.1.html">2013/05/31</a></li>
<li>Revised: 2014/11/14</li>
<li>Draft: 2014/09/08-2014/11/08</li>
</ul>
<!--========================================================================-->
<h1>Table of contents</h1>
<ul id="toc-table">
<li><a href="#Abstract">Abstract</a>
</li><li><a href="#Rationale">Rationale</a>
</li><li><a href="#ProcedureIndex">Procedure index</a>
</li><li><a href="#GeneralDiscussion">General discussion</a></li>
<ul>
<li><a href="#ImproperIlists">Improper ilists</a>
</li></ul>
<li><a href="#Quotation">Quotation</a></li>
<li><a href="#TheProcedures">The procedures</a>
<ul>
<li><a href="#Constructors">Constructors</a>
</li><li><a href="#Predicates">Predicates</a>
</li><li><a href="#Selectors">Selectors</a>
</li><li><a href="#Miscellaneous">Miscellaneous: length, append, reverse, zip & count</a>
</li><li><a href="#FoldUnfoldMap">Fold, unfold, and map</a>
</li><li><a href="#FilteringPartitioning">Filtering & partitioning</a>
</li><li><a href="#Searching">Searching</a>
</li><li><a href="#Deletion">Deletion</a>
</li><li><a href="#ImmutableassociationLists">Immutable association lists</a>
</li><li><a href="#Replacers">Replacers</a>
</li><li><a href="#Conversion">Conversion</a>
</li><li><a href="#ProcedureApplication">Procedure application</a>
</li><li><a href="#Comparators">Comparators</a>
</li></ul>
</li><li><a href="#SampleImplementation">Sample Implementation</a>
</li><li><a href="#Acknowledgements">Acknowledgements</a>
</li><li><a href="#ReferencesLinks">References & links</a>
</li><li><a href="#Copyright">Copyright</a>
</li></ul>
<!--========================================================================-->
<h1><a name="Abstract">Abstract</a></h1>
<p>
Scheme currently does not provide immutable pairs corresponding to its existing mutable pairs, although most uses of pairs do not exploit their mutability. The <a href="http://www.racket-lang.org">Racket</a> system takes the radical approach of making Scheme's pairs immutable, and providing a minimal library of mutable pairs with procedures named <code>mpair?, mcons, mcar, mcdr, set-mcar!, set-mcdr!</code>. 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 (to systems with SRFI 9) and efficient.</p>
<h1><a name="Rationale">Rationale</a></h1>
<p>The first question about this library is why it should exist at all. Why not simply eliminate mutability from Scheme's ordinary pairs and use a version of SRFI-1 that treats the linear-update procedures (with <code>!</code>) as identical to their functional counterparts, as Racket does? The main answer is that this approach breaks R<sup>5</sup>RS and R<sup>7</sup>RS-small. All the data structures in these versions of Scheme are inherently mutable, and portable code is allowed to depend on that property.</p>
<p>R<sup>6</sup>RS segregates <code>set-car!</code> and <code>set-cdr!</code> into a separate library, thus allowing implementations to provide immutable Scheme pairs if this library is not (transitively) imported into a program, and mutable ones if it is. However, it is not possible to write portable R<sup>6</sup>RS programs that differentiate between mutable and immutable pairs, for example by using immutable pairs most of the time and mutable pairs where necessary.</p>
<p>Because of the Liskov Substitution Principle, it is not possible to treat mutable pairs as either a subtype or a supertype of mutable ones; they must be distinct, and if operations are to apply to both, they can do so only by <em>ad hoc</em> polymorphism of the kind that Scheme traditionally avoids for several reasons, including clarity, efficiency, and flexibility. This proposal, therefore, treats mutable and immutable pairs separately, while allowing easy conversion from one to the other.</p>
<p>
Rather than attempting to design this library from scratch, I have chosen the conservative option of modifying <a href="http://srfi.schemers.org/srfi-1/srfi-1.html">SRFI 1</a>. Consequently, most of the rationale given in that document applies to this one as well. I have made the following changes:</p>
<ul><li>Removed all linear-update procedures ending in <code>!</code></li>
<li>Removed all references to circular lists (there will be a future SRFI for immutable bidirectional cycles).</li>
<li>Removed the O(n<sup>2</sup>) lists-as-sets procedures (there will be a future SRFI supporting O(log n) immutable sets).</li>
<li>Inserted an <code>i</code> 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 <code>ipair</code>, <code>xipair</code> and <code>ipair*</code> instead.</li>
<li>Added procedures for conversion between ordinary and immutable pairs, lists, and trees.</li>
<li>Added an analogue of <code>apply</code> for applying a procedure to an immutable list of arguments.</li>
<li>Added SRFI 114 comparators for immutable pairs, lists, and dotted lists.</li></ul>
<p>
Note: In the prose, immutable pairs and lists are known as ipairs and ilists throughout.
</p>
<!--========================================================================-->
<h1><a name="ProcedureIndex">Procedure Index</a></h1>
<p>
Here is a short list of the procedures provided by this SRFI.
</p><div class="indent">
<dl>
<dt class="proc-index"> Constructors
</dt><dd class="proc-index">
<pre class="proc-index"><a href="#ipair">ipair</a> <a href="#ilist">ilist</a>
<a href="#xipair">xipair</a> <a href="#ipair*">ipair*</a> <a href="#make-ilist">make-ilist</a> <a href="#ilist-tabulate">ilist-tabulate</a>
<a href="#ilist-copy">ilist-copy</a> <a href="#iiota">iiota</a>
</pre>
</dd><dt class="proc-index"> Predicates
</dt><dd class="proc-index">
<pre class="proc-index"><a href="#ipair-p">ipair?</a>
<a href="#proper-ilist-p">proper-ilist?</a>/<a href="#ilist-p">ilist?</a> <a href="#dotted-ilist-p">dotted-ilist?</a>
<a href="#not-ipair-p">not-ipair?</a> <a href="#null-ilist-p">null-ilist?</a>
<a href="#ilist=">ilist=</a>
</pre>
</dd><dt class="proc-index"> Selectors
</dt><dd class="proc-index">
<pre class="proc-index"><a href="#icar">icar</a> <a href="#icdr">icdr</a> ... <a href="#icddadr">icddadr</a> <a href="#icddddr">icddddr</a> <a href="#ilist-ref">ilist-ref</a>
<a href="#ifirst">ifirst</a> <a href="#isecond">isecond</a> <a href="#ithird">ithird</a> <a href="#ifourth">ifourth</a> <a href="#ififth">ififth</a> <a href="#isixth">isixth</a> <a href="#iseventh">iseventh</a> <a href="#ieighth">ieighth</a> <a href="#ininth">ininth</a> <a href="#itenth">itenth</a>
<a href="#icar+icdr">icar+icdr</a>
<a href="#itake">itake</a> <a href="#idrop">idrop</a>/<a href="#ilist-tail">ilist-tail</a>
<a href="#itake-right">itake-right</a> <a href="#idrop-right">idrop-right</a>
<a href="#isplit-at">isplit-at</a>
<a href="#ilast">ilast</a> <a href="#last-ipair">last-ipair</a>
</pre>
</dd><dt class="proc-index"> Miscellaneous: length, append, concatenate, reverse, zip & count
</dt><dd class="proc-index">
<pre class="proc-index"><a href="#ilength">ilength</a>
<a href="#iappend">iappend</a> <a href="#iconcatenate">iconcatenate</a> <a href="#ireverse">ireverse</a> <a href="#iappend-reverse">iappend-reverse</a>
<a href="#izip">izip</a> <a href="#iunzip1">iunzip1</a> <a href="#iunzip2">iunzip2</a> <a href="#iunzip3">iunzip3</a> <a href="#iunzip4">iunzip4</a> <a href="#iunzip5">iunzip5</a>
<a href="#icount">icount</a>
</pre>
</dd><dt class="proc-index"> Fold, unfold & map
</dt><dd class="proc-index">
<pre class="proc-index"><a href="#imap">imap</a> <a href="#ifor-each">ifor-each</a>
<a href="#ifold">ifold</a> <a href="#iunfold">iunfold</a> <a href="#ipair-fold">ipair-fold</a> <a href="#ireduce">ireduce</a>
<a href="#ifold-right">ifold-right</a> <a href="#iunfold-right">iunfold-right</a> <a href="#ipair-fold-right">ipair-fold-right</a> <a href="#ireduce-right">ireduce-right</a>
<a href="#iappend-map">iappend-map</a> <a href="#ipair-for-each">ipair-for-each</a> <a href="#ifilter-map">ifilter-map</a> <a href="#imap-in-order">imap-in-order</a>
</pre>
</dd><dt class="proc-index"> Filtering & partitioning
</dt><dd class="proc-index">
<pre class="proc-index"><a href="#ifilter">ifilter</a> <a href="#ipartition">ipartition</a> <a href="#iremove">iremove</a>
</pre>
</dd><dt class="proc-index"> Searching
</dt><dd class="proc-index">
<pre class="proc-index"><a href="#imember">imember</a> <a href="#imemq">imemq</a> <a href="#imemv">imemv</a>
<a href="#ifind">ifind</a> <a href="#ifind-tail">ifind-tail</a>
<a href="#iany">iany</a> <a href="#ievery">ievery</a>
<a href="#ilist-index">ilist-index</a>
<a href="#itake-while">itake-while</a> <a href="#idrop-while">idrop-while</a>
<a href="#ispan">ispan</a> <a href="#ibreak">ibreak</a>
</pre>
</dd><dt class="proc-index"> Deleting
</dt><dd class="proc-index">
<pre class="proc-inde x"><a href="#idelete">idelete</a> <a href="#idelete-duplicates">idelete-duplicates</a>
</pre>
</dd><dt class="proc-index"> Immutable association lists
</dt><dd class="proc-index">
<pre class="proc-index"><a href="#iassoc">iassoc</a> <a href="#iassq">iassq</a> <a href="#iassv">iassv</a>
<a href="#ialist-cons">ialist-cons</a> <a href="#ialist-delete">ialist-delete</a>
</pre>
</dd><dt class="proc-index"> Replacement
</dt><dd class="proc-index">
<pre class="proc-index"><a href="#replace-icar">replace-icar</a> <a href="#replace-icdr">replace-icdr</a>
</pre>
</dd><dt class="proc-index"> Conversion
</dt><dd class="proc-index">
<pre class="proc-index"><a href="#pair->ipair">pair->ipair</a> <a href="#ipair->pair">ipair->pair</a>
<a href="#list->ilist">list->ilist</a> <a href="#ilist->list">ilist->list</a>
<a href="#tree->itree">tree->itree</a> <a href="#itree->tree">itree->tree</a>
<a href="#gtree->itree">gtree->itree</a> <a href="#gtree->tree">gtree->tree</a>
</pre>
</dd><dt class="proc-index"> Procedure application
</dt><dd class="proc-index">
<pre class="proc-index"><a href="#iapply">iapply</a>
</pre>
</dd><dt class="proc-index"> Comparators
</dt><dd class="proc-index">
<pre class="proc-index"><a href="#ipair-comparator">ipair-comparator</a> <a href="#ilist-comparator">ilist-comparator</a>
<a href="#make-ilist-comparator">make-ilist-comparator</a> <a href="#make-improper-ilist-comparator">make-improper-ilist-comparator</a>
<a href="#make-icar-comparator">make-icar-comparator</a> <a href="#make-icdr-comparator">make-icdr-comparator</a>
</pre>
</dd></dl>
</div>
<!--========================================================================-->
<h1><a name="GeneralDiscussion">General discussion</a></h1>
<p>
A set of general criteria guided the design of the SRFI-1 library that underlies this library.
They are reproduced here.
</p><p>
List-filtering procedures such as <code>ifilter</code> or <code>idelete</code> do not disorder
lists. Elements appear in the answer list in the same order as they appear in
the argument list. This constrains implementation, but seems like a desirable
feature, since in many uses of lists, order matters. (In particular,
disordering an association list is definitely a bad idea.)
</p><p>
Contrariwise, although the sample implementations of the list-filtering
procedures share longest common tails between argument and answer lists,
it is not part of the spec.
</p><p>
Because ilists are an inherently sequential data structure (unlike, say,
vectors), inspection procedures such as <code>ifind</code>, <code>ifind-tail</code>, <code>ifor-each</code>, <code>iany</code>
and <code>ievery</code> commit to a left-to-right traversal order of their argument list.
</p><p>
However, constructors, such as <code><code>ilist-tabulate</code></code> and the mapping
procedures (<code>iappend-map</code>, <code>ipair-for-each</code>, <code>ifilter-map</code>,
<code>imap-in-order</code>), do <em>not</em> specify the dynamic order in which their procedural
argument is applied to its various values.
</p><p>
Predicates return useful true values wherever possible. Thus <code>iany</code> must return
the true value produced by its predicate, and <code>ievery</code> returns the final true
value produced by applying its predicate argument to the last element of its
argument list.
</p><p>
No special status is accorded Scheme's built-in equality predicate.
Any functionality provided in terms of <code>eq?</code>, <code>eqv?</code>, <code>equal?</code> is also
available using a client-provided equality predicate.
</p><p>
These procedures are <em>not</em> generic as between ordinary pairs/lists and immutable pairs/lists;
they are specific to immutable lists. Like Olin, I prefer to keep the library simple and focused.
However, there are a few conversions between mutable and immutable lists provided.
<!--========================================================================-->
</p>
<!--========================================================================-->
<h2><a name="ImproperIlists">Improper Lists</a></h2>
<p>
Scheme does not properly have a list type, just as C does not have a string
type. Rather, Scheme has a binary-tuple type, from which one can build binary
trees. There is an <em>interpretation</em> of Scheme values that allows one to
treat these trees as lists. The same interpretation is applied to immutable pairs.
</p><p>Because the empty list, written as <code>()</code>, is already immutable, it is shared between mutable and immutable lists as the termination marker. It is the only Scheme object that is both a mutable list and an immutable list.
</p><p>Users should note that dotted lists, whether mutable or immutable, are not commonly
used, and are considered by many Scheme programmers to be an ugly artifact of
Scheme's lack of a true list type.
Dotted ilists are <em>not</em> fully supported by this SRFI. Most procedures are
defined only on proper ilists — that is, <code>()</code>-terminated ilists. The
procedures that will also handle dotted ilists are specifically
marked. While this design decision restricts the domain of possible arguments
one can pass to these procedures, it has the benefit of allowing the
procedures to catch the error cases where programmers inadvertently pass
scalar values to an ilist procedure by accident,
<em>e.g.</em>, by switching the arguments to a procedure call.</p>
<h1><a name="Quotation">Quotation</a></h1>
<p>The various Scheme standards permit, but do not require,
Scheme implementations to treat quoted pairs and lists as immutable.
Thus whereas the expression <code>(set-car! (list 1 2 3) 10)</code>
evaluates to the list <code>(10 2 3)</code>, the expression
<code>(set-car! '(1 2 3) 10)</code> is not portable and in fact an error.</p>
<p>This SRFI recommends that implementations that provide both this SRFI and
immutable quotations should cause quotations to return the same immutable
pairs that this SRFI describes. This means that the standard Scheme pair
and list operations, as well as libraries like SRFI 1 which are built on them,
should accept both mutable and immutable pairs: thus <code>(car (ilist 1 2))</code>
should evaluate to <code>1</code>.</p>
<p>This SRFI further recommends that <code>read</code> should return mutable
pairs and lists when reading list structure. No recommendation is made about the
behavior of <code>write</code>, <code>display</code>, and similar output
procedures on immutable lists.</p>
<p>To make life easier for Scheme programmers, given that many implementations
do not provide immutable quotation, the syntax keyword <code>iq</code> is
provided as part of this SRFI. It is analogous to <code>quote</code>,
taking an arbitrary number of literals and constructing an ilist from them,
with any pairs in the literals converted to ipairs. It is useful
for providing constant ipair-based objects.
Note that pairs within literal vectors or other implementation-dependent literals
will not be converted. Unfortunately, there is no ilist analogue of <code>'</code>,
so we save keystrokes by using <code>iq</code> rather than <code>iquote</code>
and omitting the top-level parentheses.</p>
<!--========================================================================-->
<h1><a name="TheProcedures">The procedures</a></h1>
<p>
The templates given below obey the following conventions for procedure formals:
</p><table>
<tbody><tr valign="baseline"><th align="left"> <var>ilist</var>
</th><td> A proper (<code>()</code>-terminated) ilist
</td></tr><tr valign="baseline"><th align="left"> <var>dilist</var>
</th><td> A proper or dotted ilist
</td></tr><tr valign="baseline"><th align="left"> <var>ipair</var>
</th><td> An immutable pair
</td></tr><tr valign="baseline">
<th align="left"> <var>x</var>, <var>y</var>, <var>d</var>, <var>a</var>
</th><td> Any value
</td></tr><tr valign="baseline"><th align="left"> <var>object</var>, <var>value</var>
</th><td> Any value
</td></tr><tr valign="baseline"><th align="left"> <var>n</var>, <var>i</var>
</th><td> A natural number (an integer >= 0)
</td></tr><tr valign="baseline"><th align="left"> <var>proc</var>
</th><td> A procedure
</td></tr><tr valign="baseline"><th align="left"> <var>pred</var>
</th><td> A procedure whose return value is treated as a boolean
</td></tr><tr valign="baseline"><th align="left"> <var>=</var>
</th><td> A boolean procedure taking two arguments
</td></tr></tbody></table>
<p>To interpret the examples, pretend that they are executed on a Scheme that prints immutable pairs and lists with the syntax of mutable ones.</p>
<p>
It is an error to pass a dotted ilist to a procedure not
defined to accept such an argument.
<!--========================================================================-->
</p><h2><a name="Constructors">Constructors</a></h2>
<p>
</p><dl>
<!--
==== ipair
============================================================================-->
<dt class="proc-def">
<a name="ipair"></a>
<code class="proc-def">ipair</code> <var>a d -> ipair</var>
</dt><dd class="proc-def">
The primitive constructor. Returns a newly allocated ipair whose icar is
<var>a</var> and whose icdr is <var>d</var>.
The ipair is guaranteed to be different (in the sense of <code>eqv?</code>)
from every existing object.
<pre class="code-example">(ipair 'a '()) => (a)
(ipair (iq a) (iq b c d)) => ((a) b c d)
(ipair "a" (iq b c)) => ("a" b c)
(ipair 'a 3) => (a . 3)
(ipair (iq a b) 'c) => ((a b ) . c)
</pre>
<!--
==== ilist
============================================================================-->
</dd><dt class="proc-def">
<a name="ilist"></a>
<code class="proc-def">ilist</code> <var>object ... -> ilist</var>
</dt><dd class="proc-def">
Returns a newly allocated ilist of its arguments.
<pre class="code-example">(ilist 'a (+ 3 4) 'c) => (a 7 c)
(ilist) => ()
</pre>
<!--
==== xipair
============================================================================-->
</dd><dt class="proc-def">
<a name="xipair"></a>
<code class="proc-def">xipair</code> <var>d a -> ipair</var>
</dt><dd class="proc-def">
<pre>(lambda (d a) (ipair a d))
</pre>
Of utility only as a value to be conveniently passed to higher-order
procedures.
<pre class="code-example">(xipair (iq b c) 'a) => (a b c)
</pre>
The name stands for "eXchanged Immutable PAIR."
<!--
==== ipair*
============================================================================-->
<a name="ipair*"></a>
</dd><dt class="proc-def"><code class="proc-def">ipair*</code><var> elt<sub>1</sub> elt<sub>2</sub> ... -> object</var>
</dt><dd class="proc-def">
Like <code>ilist</code>,
but the last argument provides the tail of the constructed ilist,
returning
<div class="indent"><code>
(ipair <var>elt<sub>1</sub></var> (ipair <var>elt<sub>2</sub></var> (ipair ... <var>elt<sub>n</sub></var>)))
</code></div>
<pre class="code-example">(ipair* 1 2 3 4) => (1 2 3 . 4)
(ipair* 1) => 1
</pre>
<!--
==== make-ilist
============================================================================-->
<a name="make-ilist"></a>
</dd><dt class="proc-def"> <code class="proc-def">make-ilist</code> <var>n [fill] -> ilist</var>
</dt><dd class="proc-def">
Returns an <var>n</var>-element ilist,
whose elements are all the value <var>fill</var>.
If the <var>fill</var> argument is not given, the elements of the ilist may
be arbitrary values.
<pre class="code-example">(make-ilist 4 'c) => (c c c c)
</pre>
<!--
==== ilist-tabulate
============================================================================-->
<a name="ilist-tabulate"></a>
</dd><dt class="proc-def"><code class="proc-def">ilist-tabulate</code><var> n init-proc -> ilist</var>
</dt><dd class="proc-def">
Returns an <var>n</var>-element ilist. Element <var>i</var> of the ilist, where 0 <= <var>i</var> < <var>n</var>,
is produced by <code>(<var>init-proc</var> <var>i</var>)</code>. No guarantee is made about the dynamic
order in which <var>init-proc</var> is applied to these indices.
<pre class="code-example">(ilist-tabulate 4 values) => (0 1 2 3)
</pre>
<!--
==== ilist-copy
============================================================================-->
<a name="ilist-copy"></a>
</dd><dt class="proc-def"><code class="proc-def">ilist-copy</code><var> dilist -> dilist</var>
</dt><dd class="proc-def">
Copies the spine of the argument, including the ilist tail.
<!--
==== iiota
============================================================================-->
<a name="iiota"></a>
</dd><dt class="proc-def"><code class="proc-def">iiota</code><var> count [start step] -> ilist</var>
</dt><dd class="proc-def">
Returns an ilist containing the elements
<pre class="code-example">(<var>start</var> <var>start</var>+<var>step</var> ... <var>start</var>+(<var>count</var>-1)*<var>step</var>)
</pre>
The <var>start</var> and <var>step</var> parameters default to 0 and 1, respectively.
This procedure takes its name from the APL primitive.
<pre class="code-example">(iiota 5) => (0 1 2 3 4)
(iiota 5 0 -0.1) => (0 -0.1 -0.2 -0.3 -0.4)
</pre>
</dd></dl>
<!--========================================================================-->
<h2><a name="Predicates">Predicates</a></h2>
<dl>
<!--
==== proper-ilist?
==== ilist?
============================================================================-->
<dt class="proc-def1">
<code class="proc-def">proper-ilist?</code><var> x -> boolean</var>
<a name="proper-ilist-p"></a>
</dt><dt class="proc-defn">
<code class="proc-def">ilist?</code><var> x -> boolean</var>
<a name="ilist-p"></a>
</dt><dd class="proc-def">
These identifiers are bound either to the same procedure, or to procedures of equivalent behavior. In either case, true is returned iff <var>x</var> is a proper ilist — a <code>()</code>-terminated ilist.
<p>
More carefully: The empty list is a proper ilist. An ipair whose icdr is a
proper ilist is also a proper ilist. Everything else is a dotted ilist. This includes
non-ipair, non-() values (<em>e.g.</em> symbols, numbers, mutable pairs),
which are considered to be dotted ilists of length 0.
</p>
<!--
==== dotted-ilist?
============================================================================-->
<a name="dotted-ilist-p"></a>
</dd><dt class="proc-def"><code class="proc-def">dotted-ilist?</code><var> x -> boolean</var>
</dt><dd class="proc-def">
True if <var>x</var> is a finite, non-nil-terminated ilist. That is, there exists
an <var>n</var> >= 0 such that icdr<sup><var>n</var></sup>(<var>x</var>) is neither an ipair nor ().
This includes
non-ipair, non-() values (<em>e.g.</em> symbols, numbers),
which are considered to be dotted ilists of length 0.
<pre>(dotted-ilist? <var>x</var>) = (not (proper-ilist? <var>x</var>))
</pre>
<!--
==== ipair?
============================================================================-->
<a name="ipair-p"></a>
</dd><dt class="proc-def"><code class="proc-def">ipair?</code><var> object -> boolean</var>
</dt><dd class="proc-def">
Returns #t if <var>object</var> is an ipair; otherwise, #f.
<pre class="code-example">(ipair? (ipair 'a 'b)) => #t
(ipair? (iq a b c)) => #t
(ipair? (cons 1 2)) => #f
(ipair? '()) => #f
(ipair? '#(a b)) => #f
(ipair? 7) => #f
(ipair? 'a) => #f
</pre>
<!--
==== null-ilist?
============================================================================-->
<a name="null-ilist-p"></a>
</dd><dt class="proc-def"><code class="proc-def">null-ilist?</code><var> ilist -> boolean</var>
</dt><dd class="proc-def">
<var>Ilist</var> is a proper ilist. This procedure returns true if
the argument is the empty list (), and false otherwise. It is an
error to pass this procedure a value which is not a proper ilist.
This procedure is recommended as the termination condition for
ilist-processing procedures that are not defined on dotted ilists.
<!--
==== not-ipair?
============================================================================-->
</dd><dt class="proc-def">
<a name="not-ipair-p"></a>
<code class="proc-def">not-ipair?</code><var> x -> boolean</var>
</dt><dd class="proc-def">
<pre>(lambda (x) (not (ipair? x)))</pre>
Provided as a procedure as it can be useful as the termination condition
for ilist-processing procedures that wish to handle all ilists,
both proper and dotted.
<!--
==== ilist=
============================================================================-->
</dd><dt class="proc-def">
<a name="list="></a>
<code class="proc-def">ilist=</code><var> elt= ilist<sub>1</sub> ... -> boolean</var>
</dt><dd class="proc-def">
Determines ilist equality, given an element-equality procedure.
Proper ilist <var>A</var> equals proper ilist <var>B</var>
if they are of the same length,
and their corresponding elements are equal,
as determined by <var>elt=</var>.
If the element-comparison procedure's first argument is
from <var>ilist<sub>i</sub></var>,
then its second argument is from <var>ilist<sub>i+1</sub></var>,
<em>i.e.</em> it is always called as
<code>(<var>elt=</var> <var>a</var> <var>b</var>)</code>
for <var>a</var> an element of ilist <var>A</var>,
and <var>b</var> an element of ilist <var>B</var>.
<p>
In the <var>n</var>-ary case,
every <var>ilist<sub>i</sub></var> is compared to
<var>ilist<sub>i+1</sub></var>
(as opposed, for example, to comparing
<var>ilist<sub>1</sub></var> to <var>ilist<sub>i</sub></var>,
for <var>i</var>>1).
If there are no ilist arguments at all,
<code>ilist=</code> simply returns true.
</p><p>
It is an error to apply <code>ilist=</code> to anything except proper ilists.
It
cannot reasonably be extended to dotted ilists, as it provides no way to
specify an equality procedure for comparing the ilist terminators.
</p><p>
Note that the dynamic order in which the <var>elt=</var> procedure is
applied to pairs of elements is not specified.
For example, if <code>ilist=</code> is applied
to three ilists, <var>A</var>, <var>B</var>, and <var>C</var>,
it may first completely compare <var>A</var> to <var>B</var>,
then compare <var>B</var> to <var>C</var>,
or it may compare the first elements of <var>A</var> and <var>B</var>,
then the first elements of <var>B</var> and <var>C</var>,
then the second elements of <var>A</var> and <var>B</var>, and so forth.
</p><p>
The equality procedure must be consistent with <code>eq?</code>.
That is, it must be the case that
</p><div class="indent">
<code>(eq? <var>x</var> <var>y</var>)</code> => <code>(<var>elt=</var> <var>x</var> <var>y</var>)</code>.
</div>
Note that this implies that two ilists which are <code>eq?</code>
are always <code>ilist=</code>, as well; implementations may exploit this
fact to "short-cut" the element-by-element comparisons.
<pre class="code-example">(ilist= eq?) => #t ; Trivial cases
(ilist= eq? (iq a)) => #t
</pre>
</dd></dl>
<!--========================================================================-->
<h2><a name="Selectors">Selectors</a></h2>
<dl>
<!--
==== icar icdr
============================================================================-->
<a name="icar"></a>
<a name="icdr"></a>
<dt class="proc-def1"><code class="proc-def">icar</code><var> ipair -> value</var>
</dt><dt class="proc-defn"><code class="proc-def">icdr</code><var> ipair -> value</var>
</dt><dd class="proc-def">
These procedures return the contents of the icar and icdr field of their
argument, respectively.
Note that it is an error to apply them to the empty ilist.
<pre class="code-example">(icar (iq a b c)) => a (icdr (iq a b c)) => (b c)
(icar (iq (a) b c d)) => (a) (icdr (iq (a) b c d)) => (b c d)
(icar (ipair 1 2)) => 1 (icdr (ipair 1 2)) => 2
(icar '()) => *error* (icdr '()) => *error*
</pre>
<!--
==== icaar icadr ... icdddar icddddr
============================================================================-->
<a name="icaar"></a>
<a name="icadr"></a>
<a name="icdddar"></a>
<a name="icddddr"></a>
</dd><dt class="proc-def1"><code class="proc-def">icaar</code><var> ipair -> value</var>
</dt><dt class="proc-defi"><code class="proc-def">icadr</code><var> ipair -> value</var>
</dt><dt class="proc-defi"><code class="proc-def">:</code>
</dt><dt class="proc-defi"><code class="proc-def">icdddar</code><var> ipair -> value</var>
</dt><dt class="proc-defn"><code class="proc-def">icddddr</code><var> ipair -> value</var>
</dt><dd class="proc-def">
These procedures are compositions of <code>icar</code> and <code>icdr</code>,
where for example <code>icaddr</code> could be defined by
<pre class="code-example">
(define icaddr (lambda (x) (icar (icdr (icdr x))))).
</pre>
Arbitrary compositions, up to four deep, are provided. There are
twenty-eight of these procedures in all.
<!--
==== ilist-ref
============================================================================-->
<a name="ilist-ref"></a>
</dd><dt class="proc-def"><code class="proc-def">ilist-ref</code><var> ilist i -> value</var>
</dt><dd class="proc-def">
Returns the <var>i</var><sup>th</sup> element of <var>ilist</var>.
(This is the same as the icar of
<code>(idrop <var>ilist</var> <var>i</var>)</code>.)
It is an error if <var>i</var> >= <var>n</var>,
where <var>n</var> is the length of <var>ilist</var>.
<pre class="code-example">
(ilist-ref (iq a b c d) 2) => c
</pre>
<!--
==== itenth
==== ininth
==== ieighth
==== iseventh
==== isixth
==== ififth
==== ifourth
==== ithird
==== isecond
==== ifirst
============================================================================-->
</dd><dt class="proc-def1">
<a name="ifirst"></a>
<code class="proc-def">ifirst </code><var>ipair -> object </var>
</dt><dt class="proc-defi">
<a name="isecond"></a>
<code class="proc-def">isecond </code><var>ipair -> object </var>
</dt><dt class="proc-defi">
<a name="ithird"></a>
<code class="proc-def">ithird </code><var>ipair -> object </var>
</dt><dt class="proc-defi">
<a name="ifourth"></a>
<code class="proc-def">ifourth </code><var>ipair -> object </var>
</dt><dt class="proc-defi">
<a name="ififth"></a>
<code class="proc-def">ififth </code><var>ipair -> object </var>
</dt><dt class="proc-defi">
<a name="isixth"></a>
<code class="proc-def">isixth </code><var>ipair -> object </var>
</dt><dt class="proc-defi">
<a name="iseventh"></a>
<code class="proc-def">iseventh </code><var>ipair -> object </var>
</dt><dt class="proc-defi">
<a name="ieighth"></a>
<code class="proc-def">ieighth </code><var>ipair -> object </var>
</dt><dt class="proc-defi">
<a name="ininth"></a>
<code class="proc-def">ininth </code><var>ipair -> object </var>
</dt><dt class="proc-defn">
<a name="itenth"></a>
<code class="proc-def">itenth </code><var>ipair -> object </var>
</dt><dd class="proc-def">
Synonyms for <code>car</code>, <code>cadr</code>, <code>caddr</code>, ...
<pre class="code-example">(ithird '(a b c d e)) => c
</pre>
<!--
==== icar+icdr
============================================================================-->
</dd><dt class="proc-def">
<a name="icar+icdr"></a>
<code class="proc-def">icar+icdr</code><var> ipair -> [x y]</var>
</dt><dd class="proc-def">
The fundamental ipair deconstructor:
<pre class="code-example">(lambda (p) (values (icar p) (icdr p)))
</pre>
This can, of course, be implemented more efficiently by a compiler.
<!--
==== idrop
==== itake
============================================================================-->
</dd><dt class="proc-def1">
<a name="itake"></a>
<code class="proc-def">itake</code><var> x i -> ilist</var>
</dt><dt class="proc-defi">
<a name="idrop"></a>
<code class="proc-def">idrop</code><var> x i -> object</var>
</dt><dt class="proc-defn">
<a name="idrop"></a>
<code class="proc-def">ilist-tail</code><var> x i -> object</var>
</dt><dd class="proc-def">
<code>itake</code> returns the first <var>i</var> elements of ilist <var>x</var>.<br/>
<code>idrop</code> returns all but the first <var>i</var> elements of ilist <var>x</var>.<br/>
<code>ilist-tail</code> is either the same procedure as <code>idrop</code> or else a procedure with the same behavior.
<pre class="code-example">(itake (iq a b c d e) 2) => (a b)
(idrop (iq a b c d e) 2) => (c d e)
</pre>
<var>x</var> may be any value — a proper or dotted ilist:
<pre class="code-example">(itake (ipair 1 (ipair 2 (ipair 3 'd))) => (1 2)
(idrop (ipair 1 (ipair 2 (ipair 3 'd))) 2) => (3 . d)
(itake (ipair 1 (ipair 2 (ipair 3 'd))) 3) => (1 2 3)
(idrop (ipair 1 (ipair 2 (ipair 3 'd))) 3) => d
</pre>
For a legal <var>i</var>, <code>itake</code> and <code>idrop</code> partition the ilist in a manner which
can be inverted with <code>iappend</code>:
<pre class="code-example">(iappend (itake <var>x</var> <var>i</var>) (idrop <var>x</var> <var>i</var>)) = <var>x</var>
</pre>
<code>idrop</code> is exactly equivalent to performing <var>i</var> icdr operations on <var>x</var>;
the returned value shares a common tail with <var>x</var>.
<!--
==== idrop-right
==== itake-right
============================================================================-->
</dd><dt class="proc-def1">
<a name="itake-right"></a>
<code class="proc-def">itake-right</code><var> dilist i -> object</var>
</dt><dt class="proc-defn">
<a name="idrop-right"></a>
<code class="proc-def">idrop-right</code><var> dilist i -> ilist</var>
</dt><dd class="proc-def">
<code>itake-right</code> returns the last <var>i</var> elements of <var>dilist</var>.<br/>
<code>idrop-right</code> returns all but the last <var>i</var> elements of <var>dilist</var>.
<pre class="code-example">(itake-right (iq a b c d e) 2) => (d e)
(idrop-right (iq a b c d e) 2) => (a b c)
</pre>
The returned ilist may share a common tail with the argument ilist.
<p>
<var>dilist</var> may be any ilist, either proper or dotted:
</p><pre class="code-example">(itake-right (iq ipair 1 (ipair 2 (ipair 3 'd))) 2) => (2 3 . d)
(idrop-right (ipair 1 (ipair 2 (ipair 3 'd))) 2) => (1)
(itake-right (ipair 1 (ipair 2 (ipair 3 'd))) 0) => d
(idrop-right (ipair 1 (ipair 2 (ipair 3 'd))) 0) => (1 2 3)
</pre>
For a legal <var>i</var>, <code>itake-right</code> and <code>idrop-right</code> partition the ilist in a manner
which can be inverted with <code>iappend</code>:
<pre class="code-example">(iappend (itake <var>dilist</var> <var>i</var>) (idrop <var>dilist</var> <var>i</var>)) = <var>dilist</var>
</pre>
<code>itake-right</code>'s return value is guaranteed to share a common tail with <var>dilist</var>.
<!--
==== isplit-at
============================================================================-->
</dd><dt class="proc-def">
<a name="isplit-at"></a>
<code class="proc-def">isplit-at </code><var> x i -> [ilist object]</var>
</dt><dd class="proc-def">
<code>isplit-at</code> splits the ilist <var>x</var>
at index <var>i</var>, returning an ilist of the
first <var>i</var> elements, and the remaining tail. It is equivalent
to
<pre class="code-example">(values (itake x i) (idrop x i))
</pre>
<!--
==== last-ipair
==== ilast
============================================================================-->
</dd><dt class="proc-def1">
<a name="ilast"></a>
<code class="proc-def">ilast</code><var> ipair -> object</var>
</dt><dt class="proc-defn">
<a name="last-ipair"></a>
<code class="proc-def">last-ipair</code><var> ipair -> ipair</var>
</dt><dd class="proc-def">
Returns the last element of the non-empty, possibly dotted, ilist <var>ipair</var>.
<code>last-ipair</code> returns the last ipair in the non-empty
ilist <var>pair</var>.
<pre class="code-example">(ilast (iq a b c)) => c
(last-ipair (iq a b c)) => (c)
</pre>
</dd></dl>
<!--========================================================================-->
<h2><a name="Miscellaneous">Miscellaneous: length, append, concatenate, reverse, zip & count</a></h2>
<dl>
<!--
==== ilength
============================================================================-->
<dt class="proc-def">
<a name="ilength"></a>
<code class="proc-def">ilength </code><var>ilist -> integer</var>
</dt><dd class="proc-def">
Returns the length of its argument.
It is an error to pass a value to <code>ilength</code> which is not a proper
ilist (<code>()</code>-terminated).<p>
The length of a proper ilist is a non-negative integer <var>n</var> such that <code>icdr</code>
applied <var>n</var> times to the ilist produces the empty list.
<!--
==== iappend
============================================================================-->
</p></dd><dt class="proc-def">
<a name="iappend"></a>
<code class="proc-def">iappend </code><var> ilist<sub>1</sub> ... -> ilist</var>
</dt><dd class="proc-def">
Returns an ilist consisting of the elements
of <var>ilist<sub>1</sub></var>
followed by the elements of the other ilist parameters.
<pre class="code-example">(iappend (iq x) (iq y)) => (x y)
(iappend (iq a) (iq b c d)) => (a b c d)
(iappend (iq a (b)) (iq (c))) => (a (b) (c))
</pre>
The resulting ilist is always newly allocated, except that it
shares structure with the final <var>ilist<sub>i</sub></var> argument.
This last argument may be any value at all;
an improper ilist results if it is not
a proper ilist. All other arguments must be proper ilists.
<pre class="code-example">(iappend (iq a b) (ipair 'c 'd)) => (a b c . d)
(iappend '() 'a) => a
(iappend (iq x y)) => (x y)
(iappend) => ()
</pre>
<!--
==== iconcatenate
============================================================================-->