140 lines
5.1 KiB
HTML
140 lines
5.1 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>OO Closure (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="About-Closure.html" rel="up" title="About Closure">
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<link href="Callback-Closure.html" rel="prev" title="Callback Closure">
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<body lang="en">
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<div class="subsection-level-extent" id="OO-Closure">
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<div class="nav-panel">
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<p>
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Previous: <a href="Callback-Closure.html" accesskey="p" rel="prev">Example 3: The Callback Closure Problem</a>, Up: <a href="About-Closure.html" accesskey="u" rel="up">The Concept of Closure</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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</div>
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<hr>
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<h4 class="subsection" id="Example-4_003a-Object-Orientation"><span>3.4.9 Example 4: Object Orientation<a class="copiable-link" href="#Example-4_003a-Object-Orientation"> ¶</a></span></h4>
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<p>Closure is the capture of an environment, containing persistent variable
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bindings, within the definition of a procedure or a set of related
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procedures. This is rather similar to the idea in some object oriented
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languages of encapsulating a set of related data variables inside an
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“object”, together with a set of “methods” that operate on the
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encapsulated data. The following example shows how closure can be used
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to emulate the ideas of objects, methods and encapsulation in Scheme.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define (make-account)
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(let ((balance 0))
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(define (get-balance)
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balance)
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(define (deposit amount)
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(set! balance (+ balance amount))
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balance)
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(define (withdraw amount)
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(deposit (- amount)))
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(lambda args
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(apply
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(case (car args)
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((get-balance) get-balance)
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((deposit) deposit)
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((withdraw) withdraw)
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(else (error "Invalid method!")))
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(cdr args)))))
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</pre></div>
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<p>Each call to <code class="code">make-account</code> creates and returns a new procedure,
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created by the expression in the example code that begins “(lambda
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args”.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define my-account (make-account))
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my-account
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⇒
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#<procedure args>
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</pre></div>
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<p>This procedure acts as an account object with methods
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<code class="code">get-balance</code>, <code class="code">deposit</code> and <code class="code">withdraw</code>. To apply one of
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the methods to the account, you call the procedure with a symbol
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indicating the required method as the first parameter, followed by any
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other parameters that are required by that method.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(my-account 'get-balance)
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⇒
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0
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(my-account 'withdraw 5)
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⇒
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-5
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(my-account 'deposit 396)
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⇒
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391
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(my-account 'get-balance)
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⇒
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391
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</pre></div>
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<p>Note how, in this example, both the current balance and the helper
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procedures <code class="code">get-balance</code>, <code class="code">deposit</code> and <code class="code">withdraw</code>, used
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to implement the guts of the account object’s methods, are all stored in
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variable bindings within the private local environment captured by the
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<code class="code">lambda</code> expression that creates the account object procedure.
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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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Previous: <a href="Callback-Closure.html">Example 3: The Callback Closure Problem</a>, Up: <a href="About-Closure.html">The Concept of Closure</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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