200 lines
9.5 KiB
HTML
200 lines
9.5 KiB
HTML
|
<!DOCTYPE html>
|
||
|
|
<html>
|
||
|
|
<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
|
||
|
|
<head>
|
||
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
||
|
|
<!-- This manual documents Guile version 3.0.10.
|
||
|
|
|
||
|
|
Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
|
||
|
|
Inc.
|
||
|
|
|
||
|
|
Copyright (C) 2021 Maxime Devos
|
||
|
|
|
||
|
|
Copyright (C) 2024 Tomas Volf
|
||
|
|
|
||
|
|
|
||
|
|
Permission is granted to copy, distribute and/or modify this document
|
||
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
||
|
|
any later version published by the Free Software Foundation; with no
|
||
|
|
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
|
||
|
|
copy of the license is included in the section entitled "GNU Free
|
||
|
|
Documentation License." -->
|
||
|
|
<title>Parameters (Guile Reference Manual)</title>
|
||
|
|
|
||
|
|
<meta name="description" content="Parameters (Guile Reference Manual)">
|
||
|
|
<meta name="keywords" content="Parameters (Guile Reference Manual)">
|
||
|
|
<meta name="resource-type" content="document">
|
||
|
|
<meta name="distribution" content="global">
|
||
|
|
<meta name="Generator" content=".texi2any-real">
|
||
|
|
<meta name="viewport" content="width=device-width,initial-scale=1">
|
||
|
|
|
||
|
|
<link href="index.html" rel="start" title="Top">
|
||
|
|
<link href="Concept-Index.html" rel="index" title="Concept Index">
|
||
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
||
|
|
<link href="Control-Mechanisms.html" rel="up" title="Control Mechanisms">
|
||
|
|
<link href="Handling-Errors.html" rel="next" title="Handling Errors">
|
||
|
|
<link href="Fluids-and-Dynamic-States.html" rel="prev" title="Fluids and Dynamic States">
|
||
|
|
<style type="text/css">
|
||
|
|
<!--
|
||
|
|
a.copiable-link {visibility: hidden; text-decoration: none; line-height: 0em}
|
||
|
|
div.example {margin-left: 3.2em}
|
||
|
|
span:hover a.copiable-link {visibility: visible}
|
||
|
|
strong.def-name {font-family: monospace; font-weight: bold; font-size: larger}
|
||
|
|
-->
|
||
|
|
</style>
|
||
|
|
<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
|
||
|
|
|
||
|
|
|
||
|
|
</head>
|
||
|
|
|
||
|
|
<body lang="en">
|
||
|
|
<div class="subsection-level-extent" id="Parameters">
|
||
|
|
<div class="nav-panel">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Handling-Errors.html" accesskey="n" rel="next">How to Handle Errors</a>, Previous: <a href="Fluids-and-Dynamic-States.html" accesskey="p" rel="prev">Fluids and Dynamic States</a>, Up: <a href="Control-Mechanisms.html" accesskey="u" rel="up">Controlling the Flow of Program Execution</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<h4 class="subsection" id="Parameters-1"><span>6.11.12 Parameters<a class="copiable-link" href="#Parameters-1"> ¶</a></span></h4>
|
||
|
|
|
||
|
|
<a class="index-entry-id" id="index-SRFI_002d39"></a>
|
||
|
|
<a class="index-entry-id" id="index-parameter-object"></a>
|
||
|
|
<a class="index-entry-id" id="index-Parameter"></a>
|
||
|
|
|
||
|
|
<p>Parameters are Guile’s facility for dynamically bound variables.
|
||
|
|
</p>
|
||
|
|
<p>On the most basic level, a parameter object is a procedure. Calling it
|
||
|
|
with no arguments returns its value. Calling it with one argument sets
|
||
|
|
the value.
|
||
|
|
</p>
|
||
|
|
<div class="example">
|
||
|
|
<pre class="example-preformatted">(define my-param (make-parameter 123))
|
||
|
|
(my-param) ⇒ 123
|
||
|
|
(my-param 456)
|
||
|
|
(my-param) ⇒ 456
|
||
|
|
</pre></div>
|
||
|
|
|
||
|
|
<p>The <code class="code">parameterize</code> special form establishes new locations for
|
||
|
|
parameters, those new locations having effect within the dynamic extent
|
||
|
|
of the <code class="code">parameterize</code> body. Leaving restores the previous
|
||
|
|
locations. Re-entering (through a saved continuation) will again use
|
||
|
|
the new locations.
|
||
|
|
</p>
|
||
|
|
<div class="example">
|
||
|
|
<pre class="example-preformatted">(parameterize ((my-param 789))
|
||
|
|
(my-param)) ⇒ 789
|
||
|
|
(my-param) ⇒ 456
|
||
|
|
</pre></div>
|
||
|
|
|
||
|
|
<p>Parameters are like dynamically bound variables in other Lisp dialects.
|
||
|
|
They allow an application to establish parameter settings (as the name
|
||
|
|
suggests) just for the execution of a particular bit of code, restoring
|
||
|
|
when done. Examples of such parameters might be case-sensitivity for a
|
||
|
|
search, or a prompt for user input.
|
||
|
|
</p>
|
||
|
|
<p>Global variables are not as good as parameter objects for this sort of
|
||
|
|
thing. Changes to them are visible to all threads, but in Guile
|
||
|
|
parameter object locations are per-thread, thereby truly limiting the
|
||
|
|
effect of <code class="code">parameterize</code> to just its dynamic execution.
|
||
|
|
</p>
|
||
|
|
<p>Passing arguments to functions is thread-safe, but that soon becomes
|
||
|
|
tedious when there’s more than a few or when they need to pass down
|
||
|
|
through several layers of calls before reaching the point they should
|
||
|
|
affect. Introducing a new setting to existing code is often easier with
|
||
|
|
a parameter object than adding arguments.
|
||
|
|
</p>
|
||
|
|
<dl class="first-deffn">
|
||
|
|
<dt class="deffn" id="index-make_002dparameter"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">make-parameter</strong> <var class="def-var-arguments">init [converter]</var><a class="copiable-link" href="#index-make_002dparameter"> ¶</a></span></dt>
|
||
|
|
<dd><p>Return a new parameter object, with initial value <var class="var">init</var>.
|
||
|
|
</p>
|
||
|
|
<p>If a <var class="var">converter</var> is given, then a call <code class="code">(<var class="var">converter</var>
|
||
|
|
val)</code> is made for each value set, its return is the value stored.
|
||
|
|
Such a call is made for the <var class="var">init</var> initial value too.
|
||
|
|
</p>
|
||
|
|
<p>A <var class="var">converter</var> allows values to be validated, or put into a
|
||
|
|
canonical form. For example,
|
||
|
|
</p>
|
||
|
|
<div class="example">
|
||
|
|
<pre class="example-preformatted">(define my-param (make-parameter 123
|
||
|
|
(lambda (val)
|
||
|
|
(if (not (number? val))
|
||
|
|
(error "must be a number"))
|
||
|
|
(inexact->exact val))))
|
||
|
|
(my-param 0.75)
|
||
|
|
(my-param) ⇒ 3/4
|
||
|
|
</pre></div>
|
||
|
|
</dd></dl>
|
||
|
|
|
||
|
|
<dl class="first-deffn">
|
||
|
|
<dt class="deffn" id="index-parameterize"><span class="category-def">library syntax: </span><span><strong class="def-name">parameterize</strong> <var class="def-var-arguments">((param value) …) body1 body2 …</var><a class="copiable-link" href="#index-parameterize"> ¶</a></span></dt>
|
||
|
|
<dd><p>Establish a new dynamic scope with the given <var class="var">param</var>s bound to new
|
||
|
|
locations and set to the given <var class="var">value</var>s. <var class="var">body1</var> <var class="var">body2</var>
|
||
|
|
… is evaluated in that environment. The value returned is that of
|
||
|
|
last body form.
|
||
|
|
</p>
|
||
|
|
<p>Each <var class="var">param</var> is an expression which is evaluated to get the
|
||
|
|
parameter object. Often this will just be the name of a variable
|
||
|
|
holding the object, but it can be anything that evaluates to a
|
||
|
|
parameter.
|
||
|
|
</p>
|
||
|
|
<p>The <var class="var">param</var> expressions and <var class="var">value</var> expressions are all
|
||
|
|
evaluated before establishing the new dynamic bindings, and they’re
|
||
|
|
evaluated in an unspecified order.
|
||
|
|
</p>
|
||
|
|
<p>For example,
|
||
|
|
</p>
|
||
|
|
<div class="example">
|
||
|
|
<pre class="example-preformatted">(define prompt (make-parameter "Type something: "))
|
||
|
|
(define (get-input)
|
||
|
|
(display (prompt))
|
||
|
|
...)
|
||
|
|
|
||
|
|
(parameterize ((prompt "Type a number: "))
|
||
|
|
(get-input)
|
||
|
|
...)
|
||
|
|
</pre></div>
|
||
|
|
</dd></dl>
|
||
|
|
|
||
|
|
<p>Parameter objects are implemented using fluids (see <a class="pxref" href="Fluids-and-Dynamic-States.html">Fluids and Dynamic States</a>), so each dynamic state has its own parameter
|
||
|
|
locations. That includes the separate locations when outside any
|
||
|
|
<code class="code">parameterize</code> form. When a parameter is created it gets a
|
||
|
|
separate initial location in each dynamic state, all initialized to the
|
||
|
|
given <var class="var">init</var> value.
|
||
|
|
</p>
|
||
|
|
<p>New code should probably just use parameters instead of fluids, because
|
||
|
|
the interface is better. But for migrating old code or otherwise
|
||
|
|
providing interoperability, Guile provides the <code class="code">fluid->parameter</code>
|
||
|
|
procedure:
|
||
|
|
</p>
|
||
|
|
<dl class="first-deffn">
|
||
|
|
<dt class="deffn" id="index-fluid_002d_003eparameter"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">fluid->parameter</strong> <var class="def-var-arguments">fluid [conv]</var><a class="copiable-link" href="#index-fluid_002d_003eparameter"> ¶</a></span></dt>
|
||
|
|
<dd><p>Make a parameter that wraps a fluid.
|
||
|
|
</p>
|
||
|
|
<p>The value of the parameter will be the same as the value of the fluid.
|
||
|
|
If the parameter is rebound in some dynamic extent, perhaps via
|
||
|
|
<code class="code">parameterize</code>, the new value will be run through the optional
|
||
|
|
<var class="var">conv</var> procedure, as with any parameter. Note that unlike
|
||
|
|
<code class="code">make-parameter</code>, <var class="var">conv</var> is not applied to the initial value.
|
||
|
|
</p></dd></dl>
|
||
|
|
|
||
|
|
<p>As alluded to above, because each thread usually has a separate dynamic
|
||
|
|
state, each thread has its own locations behind parameter objects, and
|
||
|
|
changes in one thread are not visible to any other. When a new dynamic
|
||
|
|
state or thread is created, the values of parameters in the originating
|
||
|
|
context are copied, into new locations.
|
||
|
|
</p>
|
||
|
|
<a class="index-entry-id" id="index-SRFI_002d39-1"></a>
|
||
|
|
<p>Guile’s parameters conform to SRFI-39 (see <a class="pxref" href="SRFI_002d39.html">SRFI-39 - Parameters</a>).
|
||
|
|
</p>
|
||
|
|
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<div class="nav-panel">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Handling-Errors.html">How to Handle Errors</a>, Previous: <a href="Fluids-and-Dynamic-States.html">Fluids and Dynamic States</a>, Up: <a href="Control-Mechanisms.html">Controlling the Flow of Program Execution</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
</body>
|
||
|
|
</html>
|