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Yes: plus, bang, star No: space Yes: slash, question, ampersand
You can't put sharp in a path, so anything goes, really.
Nonetheless, some of these confuse Netscape, so I'll avoid them.
-->
<!--========================================================================-->
<html>
<head>
<meta name="keywords" content="Scheme, programming language, list processing, SRFI" />
<link rev=made href="mailto:cowan@ccil.org" />
<title>SRFI 130: Cursor-based string library</title>
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1" />
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== But my Netscape will completely ignore the tag if I do that.
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/* This stuff is for definition lists of defined procedures.
** A proc-def1 is used when you want a stack of procs to go
** with one 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
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** 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.
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<body>
<!--========================================================================-->
<H1>Title</H1>
<div class=title-text>Cursor-based string library</div>
<!--========================================================================-->
<H1>Author</H1>
John Cowan
<H1>Status</H1>
<p>This SRFI is currently in <em>final</em> status. Here is <a href="https://srfi.schemers.org/srfi-process.html">an explanation</a> of each status that a SRFI can hold. To provide input on this SRFI, please send email to <code><a href="mailto:srfi+minus+130+at+srfi+dotschemers+dot+org">srfi-130@<span class="antispam">nospam</span>srfi.schemers.org</a></code>. To subscribe to the list, follow <a href="http://srfi.schemers.org/srfi-list-subscribe.html">these instructions</a>. You can access previous messages via the mailing list <a href="https://srfi-email.schemers.org/srfi-130">archive</a>.</p>
<ul>
<li>Received: 2015-12-02</li>
<li>60-day deadline: 2016-02-01</li>
<li>Draft #1 published: 2015-12-03</li>
<li>Draft #2 published: 2015-12-13</li>
<li>Draft #3 published: 2016-03-28</li>
<li>Draft #4 published: 2016-04-12</li>
<li>Draft #5 published: 2016-05-14</li>
<li>Draft #6 published: 2016-05-14</li>
<li>Draft #7 published: 2016-05-22</li>
<li>Draft #8 published: 2016-05-23 (code changes only)</li>
<li>Finalized: 2016-05-08</li>
<li>Revised to fix errata:
<ul>
<li>2024-09-02 (Revised to fix missing argument in <a href="#string-fold">examples</a> of <code>string-fold</code> and <code>string-fold-right</code>.)</li></ul></li>
</ul>
<h1>Table of contents</H1>
<!-- A bug in netscape (?) keeps the first link in this UL from being active.
==== So the Abstract link be dead. 99/8/22 -Olin
-->
<ul id=toc-table>
<li><a href="#Abstract">Abstract</a>
<li><a href="#ProcedureIndex">Procedure index</a>
<li><a href="#Rationale">Rationale</a>
<li><a href="#Specification">Specification</a>
<ul>
<li><a href="#StringCursors">String cursors</a>
<li><a href="#CallingPredicates">Calling predicates</a>
<li><a href="#SharedStorage">Shared storage</a>
<li><a href="#NamingConventions">Naming conventions</a>
<li><a href="#Notation">Notation</a>
<li><a href="#Procedures">Procedures</a>
<ul>
<li><a href="#CursorOperations">Cursor operations</a>
<li><a href="#Predicates">Predicates</a>
<li><a href="#Constructors">Constructors</a>
<li><a href="#Conversion">Conversion</a>
<li><a href="#Selection">Selection</a>
<li><a href="#PrefixesSuffixes">Prefixes & suffixes</a>
<li><a href="#Searching">Searching</a>
<li><a href="#TheWholeString">The whole string</a>
</ul>
<li><a href="#SampleImp">Sample implementation</a>
<li><a href="#Acknowledgements">Acknowledgements</a>
<li><a href="#Links">References & Links</a>
<li><a href="#Copyright">Copyright</a>
</ul>
<!--========================================================================-->
<h1><a name="Abstract">Abstract</a></H1>
<p>
<abbr title="Revised<sup>5</sup> Report on Scheme"><a href="#R5RS">R5RS</a></abbr>
Scheme has an impoverished set of string-processing utilities, which is a
problem for authors of portable code. Although
<abbr title="Revised<sup>7</sup> Report on Scheme"><a href="#R7RS">R7RS</a></abbr>
provides some extensions and improvements, it is still very incomplete.
This <abbr title="Scheme Request for
Implementation">SRFI</abbr> proposes a coherent and comprehensive set of
string-processing procedures; it is accompanied by a portable sample implementation
of the spec.
<p>This SRFI is derived from SRFI 13. The biggest difference is that it
allows subsequences of strings to be specified by
<em>cursors</em> as well as the traditional string indexes.
In addition, it omits the comparison, case-mapping, and mutation operations
of SRFI 13, as well as all procedures already present in
<abbr title="Revised<sup>7</sup> Report on Scheme"><a href="#R7RS">R7RS</a></abbr>.
<!--========================================================================-->
<h1><a name="ProcedureIndex">Procedure Index</a></h1>
<p>
Here is a list of the procedures provided by this SRFI.
<div class=indent>
<dl>
<dt class=proc-index> Cursor operations
<dd class=proc-index>
<pre class=proc-index>
<a href="#string-cursor-p">string-cursor?</a>
<a href="#string-cursor-start">string-cursor-start</a> <a href="#string-cursor-end">string-cursor-end</a>
<a href="#string-cursor-next">string-cursor-next</a> <a href="#string-cursor-prev">string-cursor-prev</a>
<a href="#string-cursor-forward">string-cursor-forward</a> <a href="#string-cursor-back">string-cursor-back</a>
<a href="#string-cursor-eq">string-cursor=?</a>
<a href="#string-cursor-lt">string-cursor<?</a> <a href="#string-cursor-gt">string-cursor>?</a>
<a href="#string-cursor-le">string-cursor<=?</a> <a href="#string-cursor-ge">string-cursor>=?</a>
<a href="#string-cursor-diff">string-cursor-diff</a>
<a href="#string-cursor2index">string-cursor->index</a> <a href="#string-index2cursor">string-index->cursor</a>
</pre>
<dt class=proc-index> Predicates
<dd class=proc-index>
<pre class=proc-index>
<a href="#string-null-p">string-null?</a>
<a href="#string-every">string-every</a> <a href="#string-any">string-any</a>
</pre>
<dt class=proc-index> Constructors
<dd class=proc-index>
<pre class=proc-index>
<a href="#string-tabulate">string-tabulate</a>
<a href="#string-unfold">string-unfold</a> <a href="#string-unfold-right">string-unfold-right</a>
</pre>
<dt class=proc-index> Conversion
<dd class=proc-index>
<pre class=proc-index>
<a href="#string2list-w-cursors">string->list/cursors</a> <a href="#string2vector-w-cursors">string->vector/cursors</a>
<a href="#reverse-list2string">reverse-list->string</a> <a href="#string-join">string-join</a>
</pre>
<dt class=proc-index> Selection
<dd class=proc-index>
<pre class=proc-index>
<a href="#string-ref-w-cursor">string-ref/cursor</a>
<a href="#substring-w-cursors">substring/cursors</a> <a href="#string-copy-w-cursors">string-copy/cursors</a>
<a href="#string-take">string-take</a> <a href="#string-take-right">string-take-right</a>
<a href="#string-drop">string-drop</a> <a href="#string-drop-right">string-drop-right</a>
<a href="#string-pad">string-pad</a> <a href="#string-pad-right">string-pad-right</a>
<a href="#string-trim">string-trim</a> <a href="#string-trim-right">string-trim-right</a> <a href="#string-trim-both">string-trim-both</a>
</pre>
<dt class=proc-index>Prefixes & suffixes
<dd class=proc-index>
<pre class=proc-index>
<a href="#string-prefix-length">string-prefix-length</a> <a href="#string-suffix-length">string-suffix-length</a>
<a href="#string-prefix-p">string-prefix?</a> <a href="#string-suffix-p">string-suffix?</a>
</pre>
<dt class=proc-index>Searching
<dd class=proc-index>
<pre class=proc-index>
<a href="#string-index">string-index</a> <a href="#string-index-right">string-index-right</a>
<a href="#string-skip">string-skip</a> <a href="#string-skip-right">string-skip-right</a>
<a href="#string-contains">string-contains</a> <a href="#string-contains-right">string-contains-right</a>
</pre>
<dt class=proc-index>The whole string
<dd class=proc-index>
<pre class=proc-index>
<a href="#string-reverse">string-reverse</a>
<a href="#string-concatenate">string-concatenate</a> <a href="#string-concatenate-reverse">string-concatenate-reverse</a>
<a href="#string-fold">string-fold</a> <a href="#string-fold-right">string-fold-right</a>
<a href="#string-for-each-cursor">string-for-each-cursor</a>
<a href="#string-replicate">string-replicate</a> <a href="#string-count">string-count</a>
<a href="#string-replace">string-replace</a> <a href="#string-split">string-split</a>
<a href="#string-filter">string-filter</a> <a href="#string-remove">string-remove</a>
</pre>
</dl>
</div>
<!--========================================================================-->
<h1><a name="Rationale">Rationale</a></h1>
<p>
This SRFI defines a rich set of operations for
manipulating strings. These are frequently useful for scripting and other
text-manipulation applications. The library's design was influenced by the
string libraries found in MIT Scheme, Gambit, RScheme, MzScheme, SLIB, Common
Lisp, Bigloo, Guile, Chez, APL, Java, and the SML standard basis.
All functionality is available in substring and full-string forms.
<p>
When SRFI 13 was defined in 1999, it was intended to provide efficient string operations
on both whole strings and substrings. At that time, it was normal for strings to be
sequences of 8-bit characters, or in a few cases, characters of other fixed lengths.
Consequently, many of the SRFI 13 procedures accept optional exact integer
arguments for each of the string arguments, indexing the beginning and the end
of the substring(s) to be operated on.</p>
<p>
Unfortunately for this design, Unicode has become much more widely used, and it is now
fairly common for implementations to store strings internally as UTF-8 or UTF-16 code
unit sequences, which means that indexing operations are potentially O(n) rather than O(1).
Using opaque cursors makes it possible to iterate much more efficiently through such strings
compared to incrementing or decrementing indexes; however, for backward compatibility,
the procedures defined in this SRFI accept either cursors or indexes.
The results returned are always cursors: the use of indexes is
preserved mainly for the sake of existing code and for implementer
convenience.</p>
<p>
The operations provided here are entirely independent of the character repertoire supported
by the implementation. In particular, this means that the comparison and case conversion
procedures of SRFI 13 are excluded. There is also no provision for
<a class="ext-link" href="http://www.r6rs.org/final/html/r6rs-lib/r6rs-lib-Z-H-2.html#node_idx_54">R6RS normalization procedures</a>
or for a <code>string->integer</code> procedure that was proposed for SRFI 13 but not included. These may appear in future SRFIs.
Furthermore, string mutation can be
extremely expensive if the storage used for the string needs to be expanded, particularly
if the implementation does not use an indirect pointer to it (as in Chicken),
so this SRFI does not provide for it.
The low-level procedures of SRFI 13 are specific to the sample implementation, and have been removed
to make other implementations simpler and easier.</p>
<p>Many SRFI 13 procedures accept either a predicate, a single character, or a
<a href="https://srfi.schemers.org/srfi-14/srfi-14.html">SRFI 14</a> character set.
In this SRFI, only support for predicates is required, though implementations may also support the
other two alternatives. In that case, a single character is interpreted as a predicate which returns true if its
argument is the same (in the sense of <code>eqv?</code>) to that character; a character set is interpreted
as a predicate which returns true if its argument belongs to that character set.
In SRFI 13, character sets are inherently more efficient than predicates
<a class="ext-link" href="https://srfi.schemers.org/srfi-13/mail-archive/msg00052.html">
because testing them is fast and free of side effects</a>,
though how fast character sets actually are if they support full Unicode is very implementation-dependent.
The only procedure that absolutely requires character set support, <code>string-tokenize</code>,
has been replaced here by the more usual <code>string-split</code> procedure provided by Perl,
Python, Java, JavaScript, and other languages.
</p>
<p>The search procedures in SRFI 13 return either an index or <code>#f</code> if the search fails.
Their counterparts in this SRFI return cursors. Left-to-right searches return a cursor representing
the leftmost matching character, or the post-end cursor if there is no match; right-to-left searches
return a cursor representing the <i>successor</i> of the rightmost matching character, or the
start cursor if there is no match. This convention was devised by Alan Watson
and implemented in Chibi Scheme.</p>
<p>
In short, this SRFI is intended to help move the practice of Scheme programming
away from mutable strings, string indexes, and SRFI 13, while largely maintaining
backward compatibility. It does not require any particular run-time efficiencies from its procedures.
<!--========================================================================-->
<h1><a name="Specification">Specification</a></h1>
<!--========================================================================-->
<h2><a name="StringCursors">String cursors</a></h2>
<p>
While indexes are exact integers
ranging from 0 to the length of the string they refer to, cursors are opaque objects
that point into strings.
However, they are not required to belong to a disjoint type, as long as they are either disjoint
from indexes or identical to indexes. For example, they may be negative exact integers representing
indexes into a byte array underlying the string. It is also possible to implement cursors as a record type
or an implementation-specific primitive type.
Additionally, in implementations where no provision has been made for cursors, or there is no benefit in
implementing them separately because strings are in fact arrays of fixed-length characters, it is useful
to allow indexes and cursors to be the same thing.
(Cursors must also be disjoint from <code>#f</code>.)
<p>It is an error to make any use of a cursor referring to a string after
the string, or any string that shares storage with it, has been mutated by a procedure like <code>string-set!</code>,
<code>string-copy!</code>, or <code>string-fill!</code>.
<p>
Given a string of length <em>n</em>, there are <em>n</em> + 1 valid cursors
that refer to it: one for each character in the string,
and one for the position just after the last character, known as the "post-end cursor".
The cursor for the first (or zeroth) position in the string is known as the "start cursor".
The post-end cursor is provided because
when creating a string from cursors the second cursor argument is exclusive.
It is an error if a cursor argument is not one of the valid cursors for the string argument.
The index analogue of the post-end cursor is <em>n</em>.
</p>
<h2><a name="CallingPredicates">Calling predicates</a></h2>
<p>
All predicates passed to procedures defined in this SRFI may be called in any order
and any number of times, except as otherwise noted. This is not the case in SRFI 13.
<!--========================================================================-->
<h2><a name="SharedStorage">Shared storage</a></h2>
<p>
Some Scheme implementations, <em>e.g.</em> Guile, provide ways to construct
substrings that share storage with other strings. SRFI 130 provides only
minimal support for such shared substrings. The following SRFI 130 procedures
are allowed to return
a result which shares storage with one or more of their string arguments:
<pre>
substring/cursors
string-take string-take-right
string-drop string-drop-right
string-pad string-pad-right
string-trim string-trim-right string-trim-both
string-split string-filter string-remove
</pre>
<p>In particular, if the result is the same (in the sense of <code>string=?</code>)
as any of the arguments, <i>any</i> implementation of the above procedures may return
the string argument
without copying it. Other procedures such as
<code>string-copy/cursors</code>, as well as all the
<abbr title="Revised<sup>7</sup> Report on Scheme"><a href="#R7RS">R7RS</a></abbr> procedures,
are not permitted to return shared results.
If a shared value is returned, it may be mutable or immutable.
<!--========================================================================-->
<h2><a name="NamingConventions">Naming conventions</a></h2>
<p>
The procedures of this SRFI follow
a consistent naming scheme, and are consistent with the conventions
developed in SRFI 1. The names are composed of smaller lexemes
in a regular way that exposes the structure and relationships between the
procedures. This should help the programmer to recall or reconstitute the name
of the desired procedure. In
particular, the order of common parameters is consistent across the
different procedures.
<p>Procedures that have left/right directional variants
use no suffix to specify left-to-right operation, <code>-right</code> to specify
right-to-left operation, and <code>-both</code> to specify both.
This is a general convention that has been established in other SRFIs;
the value of a convention is proportional to the extent of its use.
</ul>
<!--========================================================================-->
<h2><a name="Notation">Notation</a></h2>
<p>
In the following procedure specifications:
<ul>
<li><p> An <var>s</var> parameter is a string.
<li><p> A <var>char</var> parameter is a character.
<li><p> <var>Start</var> and <var>end</var> parameters are half-open string
cursors or indexes specifying
a substring within a string parameter, and typically restrict a procedure's
action to the indicated substring. When omitted, they default
to 0 and the length of the string, respectively; or from another
point of view, they default to the start cursor
and the post-end cursor, respectively. For indexes, it
must be the case that 0 <= <var>start</var> <= <var>end</var>
<= <code>(string-length <var>s</var>)</code>, for
the corresponding parameter <var>s</var> when <var>start</var>
and <var>end</var> are indexes, and the corresponding relationship
must hold when they are cursors. It is an error unless <var>start</var>
and <var>end</var> are both cursors or both indexes.
<li><p> A <var>pred</var> parameter is a unary character predicate procedure, returning
a true/false value when applied to a character. It is an error if a <var>pred</var>
is not pure and functional.
<li><p> A <var>cursor</var> parameter is either a cursor or an exact non-negative integer
specifying an index into a string.
<li><p> <var>Len</var> and <var>nchars</var> parameters are exact non-negative integers specifying a
length of a string or some number of characters.
<li><p> An <var>obj</var> parameter may be any value at all.
</ul>
<p class=continue>
Passing values to procedures with these parameters that do not satisfy these
types is an error.
<p>
Parameters given in square brackets are optional. Unless otherwise noted in the
text describing the procedure, any prefix of these optional parameters may
be supplied, from zero arguments to the full list. When a procedure returns
multiple values, this is shown by listing the return values in square
brackets, as well. So, for example, the procedure with signature
<pre class=code-example>
halts? <var>f [x init-store]</var> → <var>[boolean integer]</var>
</pre>
would take one (<var>f</var>), two (<var>f</var>, <var>x</var>)
or three (<var>f</var>, <var>x</var>, <var>init-store</var>) input parameters,
and return two values, a boolean and an integer.
<p>
A parameter followed by "<code>...</code>" means zero or more elements.
So the procedure with the signature
<pre class=code-example>
sum-squares <var>x ... </var> → <var>number</var>
</pre>
takes zero or more arguments (<var>x ...</var>),
while the procedure with signature
<pre class=code-example>
spell-check <var>doc dict<sub>1</sub> dict<sub>2</sub> ...</var> → <var>string-list</var>
</pre>
takes two required parameters
(<var>doc</var> and <var>dict<sub>1</sub></var>)
and zero or more optional parameters (<var>dict<sub>2</sub> ...</var>).
<p>
If a procedure's return value is said to be "unspecified," this means that the procedure
returns a single arbitrary value. Such a procedure is not even
required to be consistent from call to call.
<!--========================================================================-->
<h2><a name="Procedures">Procedures</a></h2>
<!--========================================================================-->
<h3><a name="CursorOperations">Cursor operations</a></h3>
<p>These procedures are mostly taken from Chibi Scheme.
<dl>
<!--
==== string-cursor?
============================================================================-->
<dt class=proc-def1>
<a name="string-cursor-p"></a>
<code class=proc-def>string-cursor?</code><var> obj → boolean</var>
<dd class=proc-def>
Returns <code>#t</code> if <var>obj</var> can be a string cursor,
and <code>#f</code> otherwise. In implementations where cursors
and indexes are the same thing, <code>#t</code> is returned on
any cursor or index; where they are disjoint, <code>#t</code> is
returned on cursors, <code>#f</code> on indexes. If <var>obj</var>
is neither a cursor nor an index, <code>string-cursor?</code> will
always return <code>#f</code>.
</dd>
<dd>
<!--
==== string-cursor-start string-cursor-end
============================================================================-->
<dt class=proc-def1>
<a name="string-cursor-start"></a>
<a name="string-cursor-end"></a>
<code class=proc-def>string-cursor-start</code><var> s → cursor</var>
<dt class=proc-defn><code class=proc-def>string-cursor-end</code><var> s → cursor</var>
<dd class=proc-def>
Returns the start/post-end cursor of <var>s</var> respectively.
</dd>
<!--
==== string-cursor-next string-cursor-prev
============================================================================-->
<dt class=proc-def1>
<a name="string-cursor-next"></a>
<a name="string-cursor-prev"></a>
<code class=proc-def>string-cursor-next</code><var> s cursor → cursor</var>
<dt class=proc-defn><code class=proc-def>string-cursor-prev</code><var> s cursor → cursor</var>
<dd class=proc-def>
Returns the cursor into <var>s</var> following/preceding <var>cursor</var>.
If <var>cursor</var> is an index, returns one more/less than <var>cursor</var>.
It is an error if <var>cursor</var> is the post-end/start cursor of <var>s</var>.
</dd>
<!--
==== string-cursor-forward string-cursor-back
============================================================================-->
<dt class=proc-def1>
<a name="string-cursor-forward"></a>
<a name="string-cursor-back"></a>
<code class=proc-def>string-cursor-forward</code><var> s cursor nchars → cursor</var>
<dt class=proc-defn><code class=proc-def>string-cursor-back</code><var> s cursor nchars → cursor</var>
<dd class=proc-def>
Returns the cursor into <var>s</var> which follows/precedes <var>cursor</var>
by <var>nchars</var> characters.
If <var>cursor</var> is an index, returns <var>nchars</var> more/less than <var>cursor</var>.
It is an error if the result would be an invalid cursor or index.
</dd>
<!--
==== string-cursor comparisons
============================================================================-->
<dt class=proc-def1>
<a name="string-cursor-eq"></a>
<a name="string-cursor-lt"></a>
<a name="string-cursor-gt"></a>
<a name="string-cursor-le"></a>
<a name="string-cursor-ge"></a>
<code class=proc-def>string-cursor=?</code><var> cursor<sub>1</sub> cursor<sub>2</sub> → boolean</var>
<dt class=proc-defi><code class=proc-def>string-cursor<?</code><var> cursor<sub>1</sub> cursor<sub>2</sub> → boolean</var>
<dt class=proc-defi><code class=proc-def>string-cursor>?</code><var> cursor<sub>1</sub> cursor<sub>2</sub> → boolean</var>
<dt class=proc-defi><code class=proc-def>string-cursor<=?</code><var> cursor<sub>1</sub> cursor<sub>2</sub> → boolean</var>
<dt class=proc-defn><code class=proc-def>string-cursor>=?</code><var> cursor<sub>1</sub> cursor<sub>2</sub> → boolean</var>
<dd class=proc-def>
Compares two cursors or two indexes pointing into the same string.
<!--
==== string-cursor-diff
============================================================================-->
<dt class=proc-def>
<a name="string-cursor-diff"></a>
<code class=proc-def>string-cursor-diff</code><var> s start end → nchars</var>
<dd class=proc-def>
Returns the number of characters between <var>start</var> and <var>end</var>
in string <var>s</var>.
Note that the result is always non-negative if <var>start</var>
and <var>end</var> are a valid start-end pair.
</dd>
<!--
==== string-cursor->index string-index->cursor
============================================================================-->
<dt class=proc-def1>
<a name="string-cursor2index"></a>
<a name="string-index2cursor"></a>
<code class=proc-def>string-cursor->index</code><var> s cursor → index</var>
<dt class=proc-defn><code class=proc-def>string-index->cursor</code><var> s index → cursor</var>
<dd class=proc-def>
Converts a cursor/index into <var>s</var> into the corresponding
index/cursor. If the argument is already an index/cursor, it is
returned unchanged.
</dd>
</dl>
<!--========================================================================-->
<h3><a name="Predicates">Predicates</a></h3>
<dl>
<!--
==== string-null?
============================================================================-->
<dt class=proc-def>
<a name="string-null-p"></a>
<code class=proc-def>string-null?</code><var> s → boolean</var>
<dd class=proc-def>
Is <var>s</var> the empty string?
</dd>
<!--
==== string-every string-any
============================================================================-->
<dt class=proc-def1>
<a name="string-every"></a>
<a name="string-any"></a>
<code class=proc-def>string-every</code><var> pred s [start end] → value</var>
<dt class=proc-defn><code class=proc-def>string-any</code><var> pred s [start end] → value</var>
<dd class=proc-def>
Checks to see if every/any character in <var>s</var> satisfies <var>pred</var>
proceeding from left (index <var>start</var>) to right (index <var>end</var>).
The predicate is "witness-generating":
<ul>
<li> If <code>string-any</code> returns true, the returned true value is the one produced
by the application of the predicate.
<li> If <code>string-every</code> returns true, the returned true value is the one
produced by the final application of the predicate to <var>s</var>[<var>end</var>-1].
If <code>string-every</code> is applied to an empty sequence of characters,
it simply returns <code>#t</code>.
</ul>
<p>
The names of these procedures do not end with a question mark — this is to
indicate that they do not return a simple boolean
(<code>#t</code> or <code>#f</code>), but a general value.
</dl>
<!--========================================================================-->
<h3><a name="Constructors">Constructors</a></h3>
<dl>
<!--
==== string-tabulate
============================================================================-->
<dt class=proc-def>
<a name="string-tabulate"></a>
<code class=proc-def>string-tabulate</code><var> proc len → string</var>
<dd class=proc-def>
<var>Proc</var> is an integer → char procedure. Construct a string of size <var>len</var>
by applying <var>proc</var> to each value from 0 (inclusive) to <var>len</var> (exclusive) to produce the corresponding string
element. The order in which <var>proc</var> is applied to the indexes is not
specified.
<p>Note that the order of arguments is not the same as SRFI 1's
<code>list-tabulate</code>, but is the same as tabulation functions
in other SRFIs. When this discrepancy was discovered in SRFI 13,
it was too late to change SRFI 1.
<!--
==== string-unfold
============================================================================-->
<dt class=proc-def>
<a name="string-unfold"></a>
<code class=proc-def>string-unfold</code><var> stop? mapper successor seed [base make-final] → string</var>
<dd class=proc-def>
This is a fundamental constructor for strings.
<ul>
<li> <var>Successor</var> is used to generate a series of "seed" values from the initial seed:
<div class=inset>
<var>seed</var>, (<var>successor</var> <var>seed</var>), (<var>successor<sup>2</sup></var> <var>seed</var>), (<var>successor<sup>3</sup></var> <var>seed</var>), ...
</div>
<li> <var>Stop?</var> tells us when to stop — when it returns true when applied to one
of these seed values.
<li> <var>Mapper</var> maps each seed value to the corresponding character
in the result string. These chars are assembled into the
string in a left-to-right order.
<li> <var>Base</var> is the optional initial/leftmost portion of the constructed string;
it defaults to the empty string "".
<li> <var>Make-final</var> is applied to the terminal seed value (on which <var>stop?</var> returns
true) to produce the final/rightmost portion of the constructed string.
It defaults to <code>(lambda (x) "")</code>.
</ul>
<p>
<code>string-unfold</code> is a fairly powerful string constructor — you can use it to
convert a list to a string, read a port into a string, reverse a string,
copy a string, and so forth. Examples:
<pre class=code-example>
(port->string p) = (string-unfold eof-object? values
(lambda (x) (read-char p))
(read-char p))
(list->string lis) = (string-unfold null? car cdr lis)
(string-tabulate f size) = (string-unfold (lambda (i) (= i size)) f add1 0)
</pre>
<p>
To map <var>f</var> over a list <var>lis</var>, producing a string:
<pre class=code-example>
(string-unfold null? (compose f car) cdr lis)
</pre>
<p>
Interested functional programmers may enjoy noting that
<code>string-fold-right</code>
and <code>string-unfold</code> are in some sense inverses. That is, given operations
<var>knull?</var>, <var>kar</var><var>, kdr</var>, <var>kons</var>, and <var>knil</var> satisfying
<pre class=code-example>
(<var>kons</var> (<var>kar</var> x) (<var>kdr</var> x)) = x and (<var>knull?</var> <var>knil</var>) = #t
</pre>
then
<pre class=code-example>
(string-fold-right <var>kons</var> <var>knil</var> (string-unfold <var>knull?</var> <var>kar</var> <var>kdr</var> <var>x</var>)) = <var>x</var>
</pre>
and
<pre class=code-example>
(string-unfold <var>knull?</var> <var>kar</var> <var>kdr</var> (string-fold-right <var>kons</var> <var>knil</var> <var>s</var>)) = <var>s</var>.
</pre>
The final string constructed does not share storage with either <var>base</var>
or the value produced by <var>make-final</var>.
<p>
This combinator sometimes is called an "anamorphism."
<p>
Note: implementations should take care that runtime stack limits do not
cause overflow when constructing large (<em>e.g.</em>, megabyte) strings with
<code>string-unfold</code>.
<!--
==== string-unfold-right
============================================================================-->
<dt class=proc-def>
<a name="string-unfold-right"></a>
<code class=proc-def>string-unfold-right</code><var> stop? mapper successor seed [base make-final] → string</var>
<dd class=proc-def>
This is a fundamental constructor for strings. It is equivalent to <code>string-unfold</code>,
except that the results of <var>mapper</var> are assembled into the
string in a right-to-left order, <var>base</var> is the optional rightmost portion
of the constructed string, and <var>make-final</var>
produces the leftmost portion of the constructed string.
</dl>
<h3><a name="Conversion">Conversion</a></h3>
<dl>
<!--
==== string->list/cursors string->vector/cursors
============================================================================-->
<dt class=proc-def1>
<a name="string2list-w-cursors"></a>
<a name="string2vector-w-cursors"></a>
<code class=proc-def>string->list/cursors</code><var> s [start end] → char-list</var>
<dt class=proc-defn><code class=proc-def>string->vector/cursors</code><var> s [start end] → char-vector</var>
<dd class=proc-def>
<code>string->list/cursors</code> and <code>string->vector/cursors</code> return
a newly allocated list or vector of the characters
that make up the given string. They differ from the
<abbr title="Revised<sup>7</sup> Report on Scheme"><a href="#R7RS">R7RS</a></abbr> procedures
<code>string->list</code> and <code>string->vector</code>
by accepting either cursors or indexes.
<!--
==== reverse-list->string
============================================================================-->
<dt class=proc-def>
<a name="reverse-list2string"></a>
<code class=proc-def>reverse-list->string</code><var> char-list → string</var>
<dd class=proc-def>
An efficient implementation of <code>(compose list->string reverse)</code>:
<pre class=code-example>
(reverse-list->string '(#\a #\B #\c)) → "cBa"
</pre>
This is a common idiom in the epilog of string-processing loops
that accumulate an answer in a reverse-order list. (See also
<code>string-concatenate-reverse</code> for the "chunked" variant.)
<!--
==== string-join
============================================================================-->
<dt class=proc-def>
<a name="string-join"></a>
<code class=proc-def>string-join</code><var> string-list [delimiter grammar] → string</var>
<dd class=proc-def>
This procedure is a simple unparser —- it pastes strings together using
the delimiter string.
<p>
The <var>grammar</var> argument is a symbol that determines how the delimiter is
used, and defaults to <code>'infix</code>.
<ul>
<li> <code>'infix</code> means an infix or separator grammar:
insert the delimiter
between list elements. An empty list will produce an empty string —
note, however, that parsing an empty string with an infix or separator
grammar is ambiguous. Is it an empty list, or a list of one element,
the empty string?
<li> <code>'strict-infix</code> means the same as <code>'infix</code>,
but will signal an error if given an empty list.
<li> <code>'suffix</code> means a suffix or terminator grammar:
insert the delimiter
after every list element. This grammar has no ambiguities.
<li> <code>'prefix</code> means a prefix grammar: insert the delimiter
before every list element. This grammar has no ambiguities.
</ul>
The delimiter is the string used to delimit elements; it defaults to
a single space " ".
<pre class=code-example>
(string-join '("foo" "bar" "baz") ":") => "foo:bar:baz"
(string-join '("foo" "bar" "baz") ":" 'suffix) => "foo:bar:baz:"
;; Infix grammar is ambiguous wrt empty list vs. empty string,
(string-join '() ":") => ""
(string-join '("") ":") => ""
;; but suffix & prefix grammars are not.
(string-join '() ":" 'suffix) => ""
(string-join '("") ":" 'suffix) => ":"
</pre>
</dl>
<!--========================================================================-->
<h3><a name="Selection">Selection</a></h3>
<dl>
<!--
==== string-ref/cursor
============================================================================-->
<dt class=proc-def>
<a name="string-ref-w-cursor"></a>
<code class=proc-def>string-ref/cursor</code><var> s cursor → char</var>
<dd class=proc-def>
Returns character <var>s[i]</var> using a valid cursor or index of <var>s</var>.
It differs from the
<abbr title="Revised<sup>7</sup> Report on Scheme"><a href="#R7RS">R7RS</a></abbr> procedure <code>string-ref</code>
by accepting either a cursor or an index.
<!--
==== string-copy substring/cursors
============================================================================-->
<dt class=proc-def1>
<a name="substring-w-cursors"></a>
<a name="string-copy-w-cursors"></a>
<code class=proc-def>substring/cursors</code><var> s start end → string</var>
<dt class=proc-defn><code class="proc-def">string-copy/cursors</code><var> s [start end] → string</var>
<dd class=proc-def>
These procedures return a string whose contents are the characters of <var>s</var>
beginning with index <var>start</var> (inclusive) and ending with index <var>end</var>
(exclusive).
If <code>substring/cursors</code> produces the entire string, it may return either
<var>s</var> or a copy of <var>s</var>; in some implementations, proper substrings may share
memory with <var>s</var>.
However, <code>string-copy/cursors</code> always returns a newly allocated string.
They differ from the
<abbr title="Revised<sup>7</sup> Report on Scheme"><a href="#R7RS">R7RS</a></abbr> procedures
<code>substring</code> and <code>string-copy</code>
by accepting either cursors or indexes.
<!--
==== string-take string-drop string-take-right string-drop-right
============================================================================-->
<dt class=proc-def1>
<a name="string-take"></a>
<a name="string-drop"></a>
<a name="string-take-right"></a>
<a name="string-drop-right"></a>
<code class=proc-def>string-take</code><var> s nchars → string</var>
<dt class=proc-defi><code class=proc-def>string-drop</code><var> s nchars → string</var>
<dt class=proc-defi><code class=proc-def>string-take-right</code><var> s nchars → string</var>
<dt class=proc-defn><code class=proc-def>string-drop-right</code><var> s nchars → string</var>
<dd class=proc-def>
<code>string-take</code> returns the first <var>nchars</var> of <var>s</var>;
<code>string-drop</code> returns all but the first <var>nchars</var> of <var>s</var>.
<code>string-take-right</code> returns the last <var>nchars</var> of <var>s</var>;
<code>string-drop-right</code> returns all but the last <var>nchars</var> of <var>s</var>.
If these procedures produce the entire string, they may return either
<var>s</var> or a copy of <var>s</var>; in some implementations, proper substrings may share
memory with <var>s</var>.
<pre class=code-example>
(string-take "Pete Szilagyi" 6) => "Pete S"
(string-drop "Pete Szilagyi" 6) => "zilagyi"
(string-take-right "Beta rules" 5) => "rules"
(string-drop-right "Beta rules" 5) => "Beta "
</pre>
It is an error to take or drop more characters than are in the string:
<pre class=code-example>
(string-take "foo" 37) => <em>error</em>
</pre>
<!--
==== string-pad string-pad-right
============================================================================-->
<dt class=proc-def1>
<a name="string-pad"></a>
<a name="string-pad-right"></a>
<code class=proc-def>string-pad</code><var> s len [char start end] → string</var>
<dt class=proc-defn><code class=proc-def>string-pad-right</code><var> s len [char start end] → string</var>
<dd class=proc-def>
Build a string of length <var>len</var> comprised of <var>s</var> padded on the left (right)
by as many occurrences of the character <var>char</var> as needed. If <var>s</var> has more
than <var>len</var> chars, it is truncated on the left (right) to length <var>len</var>. <var>Char</var>
defaults to <code>#\space</code>.
<p>
If <var>len</var> <= <var>end</var>-<var>start</var>, the returned value is allowed to share storage
with <var>s</var>, or be exactly <var>s</var> (if <var>len</var> = <var>end</var>-<var>start</var>).
<pre class=code-example>
(string-pad "325" 5) => " 325"
(string-pad "71325" 5) => "71325"
(string-pad "8871325" 5) => "71325"
</pre>
<!--
==== string-trim string-trim-right string-trim-both
============================================================================-->
<dt class=proc-def1>
<a name="string-trim"></a>
<a name="string-trim-right"></a>
<a name="string-trim-both"></a>
<code class=proc-def>string-trim </code><var> s [pred start end] → string</var>
<dt class=proc-defi><code class=proc-def>string-trim-right</code><var> s [pred start end] → string</var>
<dt class=proc-defi><code class=proc-def>string-trim-both </code><var> s [pred start end] → string</var>
<dd class=proc-defn>
Trim <var>s</var> by skipping over all characters on the left / on the right /
on both sides that satisfy the second parameter <var>pred</var>:
<var>pred</var> defaults to <code>char-whitespace?</code>.
<p>
If no trimming occurs, these functions may return either <var>s</var> or a copy of <var>s</var>;
in some implementations, proper substrings may share memory with <var>s</var>.