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diff --git a/devdocs/elisp/deferred-eval.html b/devdocs/elisp/deferred-eval.html deleted file mode 100644 index 18d7dc2f..00000000 --- a/devdocs/elisp/deferred-eval.html +++ /dev/null @@ -1,55 +0,0 @@ - <h3 class="section">Deferred and Lazy Evaluation</h3> <p>Sometimes it is useful to delay the evaluation of an expression, for example if you want to avoid performing a time-consuming calculation if it turns out that the result is not needed in the future of the program. The <samp>thunk</samp> library provides the following functions and macros to support such <em>deferred evaluation</em>: </p> <dl> <dt id="thunk-delay">Macro: <strong>thunk-delay</strong> <em>forms…</em> -</dt> <dd><p>Return a <em>thunk</em> for evaluating the <var>forms</var>. A thunk is a closure (see <a href="closures">Closures</a>) that inherits the lexical environment of the <code>thunk-delay</code> call. Using this macro requires <code>lexical-binding</code>. </p></dd> -</dl> <dl> <dt id="thunk-force">Function: <strong>thunk-force</strong> <em>thunk</em> -</dt> <dd><p>Force <var>thunk</var> to perform the evaluation of the forms specified in the <code>thunk-delay</code> that created the thunk. The result of the evaluation of the last form is returned. The <var>thunk</var> also “remembers” that it has been forced: Any further calls of <code>thunk-force</code> with the same <var>thunk</var> will just return the same result without evaluating the forms again. </p></dd> -</dl> <dl> <dt id="thunk-let">Macro: <strong>thunk-let</strong> <em>(bindings…) forms…</em> -</dt> <dd><p>This macro is analogous to <code>let</code> but creates “lazy” variable bindings. Any binding has the form <code>(<var>symbol</var> <var><span class="nolinebreak">value-form</span></var>)</code>. Unlike <code>let</code>, the evaluation of any <var>value-form</var> is deferred until the binding of the according <var>symbol</var> is used for the first time when evaluating the <var>forms</var>. Any <var>value-form</var> is evaluated at most once. Using this macro requires <code>lexical-binding</code>. </p></dd> -</dl> <p>Example: </p> <div class="example"> <pre class="example">(defun f (number) - (thunk-let ((derived-number - (progn (message "Calculating 1 plus 2 times %d" number) - (1+ (* 2 number))))) - (if (> number 10) - derived-number - number))) -</pre> - -<pre class="example">(f 5) -⇒ 5 -</pre> - -<pre class="example">(f 12) --| Calculating 1 plus 2 times 12 -⇒ 25 -</pre> - -</div> <p>Because of the special nature of lazily bound variables, it is an error to set them (e.g. with <code>setq</code>). </p> <dl> <dt id="thunk-let*">Macro: <strong>thunk-let*</strong> <em>(bindings…) forms…</em> -</dt> <dd><p>This is like <code>thunk-let</code> but any expression in <var>bindings</var> is allowed to refer to preceding bindings in this <code>thunk-let*</code> form. Using this macro requires <code>lexical-binding</code>. </p></dd> -</dl> <div class="example"> <pre class="example">(thunk-let* ((x (prog2 (message "Calculating x...") - (+ 1 1) - (message "Finished calculating x"))) - (y (prog2 (message "Calculating y...") - (+ x 1) - (message "Finished calculating y"))) - (z (prog2 (message "Calculating z...") - (+ y 1) - (message "Finished calculating z"))) - (a (prog2 (message "Calculating a...") - (+ z 1) - (message "Finished calculating a")))) - (* z x)) - --| Calculating z... --| Calculating y... --| Calculating x... --| Finished calculating x --| Finished calculating y --| Finished calculating z -⇒ 8 - -</pre> -</div> <p><code>thunk-let</code> and <code>thunk-let*</code> use thunks implicitly: their expansion creates helper symbols and binds them to thunks wrapping the binding expressions. All references to the original variables in the body <var>forms</var> are then replaced by an expression that calls <code>thunk-force</code> with the according helper variable as the argument. So, any code using <code>thunk-let</code> or <code>thunk-let*</code> could be rewritten to use thunks, but in many cases using these macros results in nicer code than using thunks explicitly. </p><div class="_attribution"> - <p class="_attribution-p"> - Copyright © 1990-1996, 1998-2022 Free Software Foundation, Inc. <br>Licensed under the GNU GPL license.<br> - <a href="https://www.gnu.org/software/emacs/manual/html_node/elisp/Deferred-Eval.html" class="_attribution-link">https://www.gnu.org/software/emacs/manual/html_node/elisp/Deferred-Eval.html</a> - </p> -</div> |