168 lines
7.1 KiB
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168 lines
7.1 KiB
HTML
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Scoping Example (Guile Reference Manual)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Lexical-Scope.html" rel="up" title="Lexical Scope">
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<body lang="en">
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<div class="subsubsection-level-extent" id="Scoping-Example">
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<div class="nav-panel">
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<p>
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Up: <a href="Lexical-Scope.html" accesskey="u" rel="up">Lexical Scope</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>
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<h4 class="subsubsection" id="An-Example-of-Non_002dLexical-Scoping"><span>3.4.4.1 An Example of Non-Lexical Scoping<a class="copiable-link" href="#An-Example-of-Non_002dLexical-Scoping"> ¶</a></span></h4>
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<p>To demonstrate that non-lexical scoping does exist and can be useful, we
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present the following example from Emacs Lisp, which is a “dynamically
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scoped” language.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(defvar currency-abbreviation "USD")
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(defun currency-string (units hundredths)
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(concat currency-abbreviation
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(number-to-string units)
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"."
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(number-to-string hundredths)))
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(defun french-currency-string (units hundredths)
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(let ((currency-abbreviation "FRF"))
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(currency-string units hundredths)))
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</pre></div>
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<p>The question to focus on here is: what does the identifier
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<code class="code">currency-abbreviation</code> refer to in the <code class="code">currency-string</code>
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function? The answer, in Emacs Lisp, is that all variable bindings go
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onto a single stack, and that <code class="code">currency-abbreviation</code> refers to the
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topmost binding from that stack which has the name
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“currency-abbreviation”. The binding that is created by the
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<code class="code">defvar</code> form, to the value <code class="code">"USD"</code>, is only relevant if none
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of the code that calls <code class="code">currency-string</code> rebinds the name
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“currency-abbreviation” in the meanwhile.
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</p>
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<p>The second function <code class="code">french-currency-string</code> works precisely by
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taking advantage of this behavior. It creates a new binding for the
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name “currency-abbreviation” which overrides the one established by
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the <code class="code">defvar</code> form.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">;; Note! This is Emacs Lisp evaluation, not Scheme!
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(french-currency-string 33 44)
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⇒
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"FRF33.44"
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</pre></div>
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<p>Now let’s look at the corresponding, <em class="emph">lexically scoped</em> Scheme
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code:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define currency-abbreviation "USD")
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(define (currency-string units hundredths)
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(string-append currency-abbreviation
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(number->string units)
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"."
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(number->string hundredths)))
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(define (french-currency-string units hundredths)
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(let ((currency-abbreviation "FRF"))
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(currency-string units hundredths)))
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</pre></div>
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<p>According to the rules of lexical scoping, the
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<code class="code">currency-abbreviation</code> in <code class="code">currency-string</code> refers to the
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variable location in the innermost environment at that point in the code
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which has a binding for <code class="code">currency-abbreviation</code>, which is the
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variable location in the top level environment created by the preceding
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<code class="code">(define currency-abbreviation …)</code> expression.
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</p>
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<p>In Scheme, therefore, the <code class="code">french-currency-string</code> procedure does
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not work as intended. The variable binding that it creates for
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“currency-abbreviation” is purely local to the code that forms the
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body of the <code class="code">let</code> expression. Since this code doesn’t directly use
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the name “currency-abbreviation” at all, the binding is pointless.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(french-currency-string 33 44)
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⇒
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"USD33.44"
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</pre></div>
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<p>This begs the question of how the Emacs Lisp behavior can be
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implemented in Scheme. In general, this is a design question whose
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answer depends upon the problem that is being addressed. In this case,
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the best answer may be that <code class="code">currency-string</code> should be
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redesigned so that it can take an optional third argument. This third
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argument, if supplied, is interpreted as a currency abbreviation that
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overrides the default.
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</p>
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<p>It is possible to change <code class="code">french-currency-string</code> so that it mostly
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works without changing <code class="code">currency-string</code>, but the fix is inelegant,
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and susceptible to interrupts that could leave the
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<code class="code">currency-abbreviation</code> variable in the wrong state:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define (french-currency-string units hundredths)
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(set! currency-abbreviation "FRF")
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(let ((result (currency-string units hundredths)))
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(set! currency-abbreviation "USD")
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result))
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</pre></div>
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<p>The key point here is that the code does not create any local binding
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for the identifier <code class="code">currency-abbreviation</code>, so all occurrences of
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this identifier refer to the top level variable.
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</p>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Up: <a href="Lexical-Scope.html">Lexical Scope</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>
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