rustlr 0.3.93 - Docs.rs

<html><head><link rel="stylesheet" href="resource://content-accessible/plaintext.css"><style media="screen" id="__markdown-viewer__md_css">/*
 * Copyright (c) Sindre Sorhus <sindresorhus@gmail.com> (https://sindresorhus.com)
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy of
 * this software and associated documentation files (the "Software"), to deal in
 * the Software without restriction, including without limitation the rights to
 * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
 * of the Software, and to permit persons to whom the Software is furnished to do
 * so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

@media (prefers-color-scheme: dark) {
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    --color-prettylights-syntax-constant: #79c0ff;
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    --color-prettylights-syntax-entity-tag: #7ee787;
    --color-prettylights-syntax-keyword: #ff7b72;
    --color-prettylights-syntax-string: #a5d6ff;
    --color-prettylights-syntax-variable: #ffa657;
    --color-prettylights-syntax-brackethighlighter-unmatched: #f85149;
    --color-prettylights-syntax-invalid-illegal-text: #f0f6fc;
    --color-prettylights-syntax-invalid-illegal-bg: #8e1519;
    --color-prettylights-syntax-carriage-return-text: #f0f6fc;
    --color-prettylights-syntax-carriage-return-bg: #b62324;
    --color-prettylights-syntax-string-regexp: #7ee787;
    --color-prettylights-syntax-markup-list: #f2cc60;
    --color-prettylights-syntax-markup-heading: #1f6feb;
    --color-prettylights-syntax-markup-italic: #c9d1d9;
    --color-prettylights-syntax-markup-bold: #c9d1d9;
    --color-prettylights-syntax-markup-deleted-text: #ffdcd7;
    --color-prettylights-syntax-markup-deleted-bg: #67060c;
    --color-prettylights-syntax-markup-inserted-text: #aff5b4;
    --color-prettylights-syntax-markup-inserted-bg: #033a16;
    --color-prettylights-syntax-markup-changed-text: #ffdfb6;
    --color-prettylights-syntax-markup-changed-bg: #5a1e02;
    --color-prettylights-syntax-markup-ignored-text: #c9d1d9;
    --color-prettylights-syntax-markup-ignored-bg: #1158c7;
    --color-prettylights-syntax-meta-diff-range: #d2a8ff;
    --color-prettylights-syntax-brackethighlighter-angle: #8b949e;
    --color-prettylights-syntax-sublimelinter-gutter-mark: #484f58;
    --color-prettylights-syntax-constant-other-reference-link: #a5d6ff;
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    --color-fg-muted: #8b949e;
    --color-fg-subtle: #484f58;
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    --color-canvas-subtle: #161b22;
    --color-border-default: #30363d;
    --color-border-muted: #21262d;
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    --color-accent-fg: #58a6ff;
    --color-accent-emphasis: #1f6feb;
    --color-danger-fg: #f85149;
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}

@media (prefers-color-scheme: light) {
  :root {
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    --color-prettylights-syntax-constant: #0550ae;
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    --color-prettylights-syntax-storage-modifier-import: #24292f;
    --color-prettylights-syntax-entity-tag: #116329;
    --color-prettylights-syntax-keyword: #cf222e;
    --color-prettylights-syntax-string: #0a3069;
    --color-prettylights-syntax-variable: #953800;
    --color-prettylights-syntax-brackethighlighter-unmatched: #82071e;
    --color-prettylights-syntax-invalid-illegal-text: #f6f8fa;
    --color-prettylights-syntax-invalid-illegal-bg: #82071e;
    --color-prettylights-syntax-carriage-return-text: #f6f8fa;
    --color-prettylights-syntax-carriage-return-bg: #cf222e;
    --color-prettylights-syntax-string-regexp: #116329;
    --color-prettylights-syntax-markup-list: #3b2300;
    --color-prettylights-syntax-markup-heading: #0550ae;
    --color-prettylights-syntax-markup-italic: #24292f;
    --color-prettylights-syntax-markup-bold: #24292f;
    --color-prettylights-syntax-markup-deleted-text: #82071e;
    --color-prettylights-syntax-markup-deleted-bg: #FFEBE9;
    --color-prettylights-syntax-markup-inserted-text: #116329;
    --color-prettylights-syntax-markup-inserted-bg: #dafbe1;
    --color-prettylights-syntax-markup-changed-text: #953800;
    --color-prettylights-syntax-markup-changed-bg: #ffd8b5;
    --color-prettylights-syntax-markup-ignored-text: #eaeef2;
    --color-prettylights-syntax-markup-ignored-bg: #0550ae;
    --color-prettylights-syntax-meta-diff-range: #8250df;
    --color-prettylights-syntax-brackethighlighter-angle: #57606a;
    --color-prettylights-syntax-sublimelinter-gutter-mark: #8c959f;
    --color-prettylights-syntax-constant-other-reference-link: #0a3069;
    --color-fg-default: #24292f;
    --color-fg-muted: #57606a;
    --color-fg-subtle: #6e7781;
    --color-canvas-default: #ffffff;
    --color-canvas-subtle: #f6f8fa;
    --color-border-default: #d0d7de;
    --color-border-muted: hsla(210,18%,87%,1);
    --color-neutral-muted: rgba(175,184,193,0.2);
    --color-accent-fg: #0969da;
    --color-accent-emphasis: #0969da;
    --color-danger-fg: #cf222e;
  }
}

:root {
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.markdownroot details,
.markdownroot figcaption,
.markdownroot figure {
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}

.markdownroot summary {
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}

.markdownroot a {
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  color: var(--color-accent-fg);
  text-decoration: none;
}

.markdownroot a:active,
.markdownroot a:hover {
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}

.markdownroot abbr[title] {
  border-bottom: none;
  -webkit-text-decoration: underline dotted;
  text-decoration: underline dotted;
}

.markdownroot b,
.markdownroot strong {
  font-weight: 600;
}

.markdownroot dfn {
  font-style: italic;
}

.markdownroot h1 {
  margin: .67em 0;
  font-weight: 600;
  padding-bottom: .3em;
  font-size: 2em;
  border-bottom: 1px solid var(--color-border-muted);
}

.markdownroot mark {
  background-color: #ff0;
  color: var(--color-text-primary);
}

.markdownroot small {
  font-size: 90%;
}

.markdownroot sub,
.markdownroot sup {
  font-size: 75%;
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  position: relative;
  vertical-align: baseline;
}

.markdownroot sub {
  bottom: -0.25em;
}

.markdownroot sup {
  top: -0.5em;
}

.markdownroot img {
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.markdownroot code,
.markdownroot kbd,
.markdownroot pre,
.markdownroot samp {
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.markdownroot figure {
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}

.markdownroot hr {
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  border-bottom: 1px solid var(--color-border-muted);
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  margin: 24px 0;
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.markdownroot html [type=button],
.markdownroot [type=reset],
.markdownroot [type=submit] {
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.markdownroot [type=button]::-moz-focus-inner,
.markdownroot [type=reset]::-moz-focus-inner,
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}

.markdownroot [type=button]:-moz-focusring,
.markdownroot [type=reset]:-moz-focusring,
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.markdownroot [type=checkbox],
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.markdownroot [type=number]::-webkit-inner-spin-button,
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.markdownroot [type=search]::-webkit-search-cancel-button,
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.markdownroot h2 {
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.markdownroot p {
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.markdownroot ol ul ol,
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.markdownroot ::placeholder {
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.markdownroot .pl-c {
  color: var(--color-prettylights-syntax-comment);
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.markdownroot .pl-c1,
.markdownroot .pl-s .pl-v {
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}

.markdownroot .pl-e,
.markdownroot .pl-en {
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}

.markdownroot .pl-smi,
.markdownroot .pl-s .pl-s1 {
  color: var(--color-prettylights-syntax-storage-modifier-import);
}

.markdownroot .pl-ent {
  color: var(--color-prettylights-syntax-entity-tag);
}

.markdownroot .pl-k {
  color: var(--color-prettylights-syntax-keyword);
}

.markdownroot .pl-s,
.markdownroot .pl-pds,
.markdownroot .pl-s .pl-pse .pl-s1,
.markdownroot .pl-sr,
.markdownroot .pl-sr .pl-cce,
.markdownroot .pl-sr .pl-sre,
.markdownroot .pl-sr .pl-sra {
  color: var(--color-prettylights-syntax-string);
}

.markdownroot .pl-v,
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  color: var(--color-prettylights-syntax-variable);
}

.markdownroot .pl-bu {
  color: var(--color-prettylights-syntax-brackethighlighter-unmatched);
}

.markdownroot .pl-ii {
  color: var(--color-prettylights-syntax-invalid-illegal-text);
  background-color: var(--color-prettylights-syntax-invalid-illegal-bg);
}

.markdownroot .pl-c2 {
  color: var(--color-prettylights-syntax-carriage-return-text);
  background-color: var(--color-prettylights-syntax-carriage-return-bg);
}

.markdownroot .pl-sr .pl-cce {
  font-weight: bold;
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}

.markdownroot .pl-ml {
  color: var(--color-prettylights-syntax-markup-list);
}

.markdownroot .pl-mh,
.markdownroot .pl-mh .pl-en,
.markdownroot .pl-ms {
  font-weight: bold;
  color: var(--color-prettylights-syntax-markup-heading);
}

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}

.markdownroot .pl-mb {
  font-weight: bold;
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}

.markdownroot .pl-md {
  color: var(--color-prettylights-syntax-markup-deleted-text);
  background-color: var(--color-prettylights-syntax-markup-deleted-bg);
}

.markdownroot .pl-mi1 {
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  background-color: var(--color-prettylights-syntax-markup-inserted-bg);
}

.markdownroot .pl-mc {
  color: var(--color-prettylights-syntax-markup-changed-text);
  background-color: var(--color-prettylights-syntax-markup-changed-bg);
}

.markdownroot .pl-mi2 {
  color: var(--color-prettylights-syntax-markup-ignored-text);
  background-color: var(--color-prettylights-syntax-markup-ignored-bg);
}

.markdownroot .pl-mdr {
  font-weight: bold;
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}

.markdownroot .pl-ba {
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}

.markdownroot .pl-sg {
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}

.markdownroot .pl-corl {
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  color: var(--color-prettylights-syntax-constant-other-reference-link);
}

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  font-size: 1em;
  font-style: normal !important;
  font-weight: 400;
  line-height: 1;
  vertical-align: -0.075em;
}

.markdownroot g-emoji img {
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.markdownroot::before {
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.markdownroot::after {
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.markdownroot>*:last-child {
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}

.markdownroot a:not([href]) {
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}

.markdownroot .absent {
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}

.markdownroot .anchor {
  float: left;
  padding-right: 4px;
  margin-left: -20px;
  line-height: 1;
}

.markdownroot .anchor:focus {
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.markdownroot p,
.markdownroot blockquote,
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.markdownroot table,
.markdownroot pre,
.markdownroot details {
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.markdownroot blockquote>:last-child {
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.markdownroot h2 .octicon-link,
.markdownroot h3 .octicon-link,
.markdownroot h4 .octicon-link,
.markdownroot h5 .octicon-link,
.markdownroot h6 .octicon-link {
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}

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.markdownroot h3:hover .anchor,
.markdownroot h4:hover .anchor,
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.markdownroot h5:hover .anchor .octicon-link,
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.markdownroot ol[type="1"] {
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.markdownroot ol[type=a] {
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.markdownroot li>p {
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.markdownroot li+li {
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.markdownroot dl dt {
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  font-style: italic;
  font-weight: 600;
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.markdownroot dl dd {
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.markdownroot table th {
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.markdownroot table th,
.markdownroot table td {
  padding: 6px 13px;
  border: 1px solid var(--color-border-default);
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.markdownroot table tr {
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  border-top: 1px solid var(--color-border-muted);
}

.markdownroot table tr:nth-child(2n) {
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.markdownroot table img {
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.markdownroot img[align=right] {
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.markdownroot img[align=left] {
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</style><style id="__markdown-viewer__custom_css"></style><meta name="viewport" content="width=device-width, initial-scale=1"><title>Chapter 3: Multiple Value Types with LBox&lt;dyn Any&gt;</title></head><body><div id="__markdown-viewer__tools" class="floating"><input type="checkbox" id="__markdown-viewer__show-tools"><label for="__markdown-viewer__show-tools"></label><div id="__markdown-viewer__toc" class="toggleable"><ul><li><a href="#chapter-3-multiple-value-types-with-lboxdyn-any">Chapter 3: Multiple Value Types with LBox&lt;dyn Any&gt;</a><ul><li><a href="#tradeoff">TRADEOFF</a></li></ul></li></ul></div><p class="toggleable">Pick a markdown and code style:<br><select id="__markdown-viewer__mdselect"><option value="sss">default</option><option value="github">github</option></select><select id="__markdown-viewer__hlselect"><option value="a11y-dark">a11y-dark</option><option value="a11y-light">a11y-light</option><option value="a11y-auto">a11y-auto</option><option 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<p>Rustlr requires that the value returned by the smantics actions of all grammar rules be of
the same type, declared using the <code>valuetype</code> or <code>absyntype</code>
directive.  It only allows one other type, the <code>externtype</code> which
means that these actions can be stateful.  However, sometimes it may
still be more convenient to allow different rules to return values of
different types.  Theoretically, this can be accomplished by generating
a enum-type internally that would encompass all possible types
returned by the rules.  In other words, instead of leaving the
definition of an enum with a large number of variants to the user, it
is created internally.  The problem with this approach is
that it is not quite compatible with the goal of <em>decoupling</em> the
parser from the lexical analyzer.  The tokens returned by a lexical
scanner must also carry values, such as numerical constants and string
literals, and these values must also be of the same "absyntype" as any
that appear on the parse stack.  Integrating such values into a
generated enum will tie the rustlr runtime parsers to one specific type of
lexical token.  Rustlr does include a
<a href="https://docs.rs/rustlr/latest/rustlr/lexer_interface/enum.RawToken.html">RawToken</a> type, but this type was not created to cover all possible
scenarios when a parser might be needed.  It only exists for the
purpose of including a usable tokenizer, <a href="https://docs.rs/rustlr/latest/rustlr/lexer_interface/struct.StrTokenizer.html">StrTokenizer</a>, with the
rustlr crate.  We wish to allow rustlr to parse any type of input,
including binary formated input, as long as tokenizers can be provided
for them and adopted to the <a href="https://docs.rs/rustlr/latest/rustlr/lexer_interface/trait.Tokenizer.html">Tokenizer</a> trait.</p>
<p>The currently available approach to allowing different grammar rules
to return differently typed values borrows a page from object-oriented
programming and relies on the Any trait, specifically <strong><a href="https://docs.rs/rustlr/latest/rustlr/generic_absyn/struct.LBox.html">LBox</a>&lt;dyn
Any&gt;</strong>.  When a terminal or nonterminal symbol of the grammar is
declared, one can optionally specify a type associated with it that's
distinct from the overall absyntype (but which also must impl
Default).  <strong>When the absyntype of the grammar is declared to
be LBox&lt;dyn Any&gt;, rustlr will automatically generate code to
downcast the values attached to grammar symbols and upcast the values
returned by the semantic actions to the supertype.</strong> The following is
another grammar for a calculator program, but which demonstrates this
option.</p>
<pre><code>!<span class="hljs-keyword">use</span> rustlr::{unbox};
!<span class="hljs-keyword">pub</span> <span class="hljs-class"><span class="hljs-keyword">enum</span> <span class="hljs-title">Expr</span></span>
!{
!   Val(<span class="hljs-built_in">i64</span>),
!   Plus(LBox&lt;Expr&gt;,LBox&lt;Expr&gt;), 
!   Times(LBox&lt;Expr&gt;,LBox&lt;Expr&gt;),
!   Divide(LBox&lt;Expr&gt;,LBox&lt;Expr&gt;),
!   Minus(LBox&lt;Expr&gt;,LBox&lt;Expr&gt;),
!   Negative(LBox&lt;Expr&gt;),
!   Nothing,                   
!}
!<span class="hljs-keyword">impl</span> <span class="hljs-built_in">Default</span> <span class="hljs-keyword">for</span> Expr { <span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">default</span></span>()-&gt;<span class="hljs-keyword">Self</span> {Nothing} }

absyntype LBox&lt;<span class="hljs-keyword">dyn</span> Any&gt;
nonterminal E Expr
nonterminal ES <span class="hljs-built_in">Vec</span>&lt;LBox&lt;Expr&gt;&gt;
terminal + - * / ( ) ;
typedterminal int Expr
topsym ES
resync ;

left * <span class="hljs-number">500</span>
left / <span class="hljs-number">500</span>
left + <span class="hljs-number">400</span>
left - <span class="hljs-number">400</span>

lexvalue int Num(n) Val(n)

E --&gt; int:m { unbox!(m) } 
E --&gt; E:e1 + E:e2 { Plus(e1,e2) }
E --&gt; E:e1 - E:e2 { Minus(e1,e2) }
E --&gt; E:e1 / E:e2 { Divide(e1,e2) }
E --&gt; E:e1 * E:e2 { Times(e1,e2) }
E --&gt; - E:e { Negative(e) }
E --&gt; ( E:e )  { *e.exp }
ES --&gt; E:n ; { <span class="hljs-built_in">vec!</span>[n] }
ES ==&gt; ES:v E:e ;  {
   v.push(e);
   unbox!(v)
   } &lt;==

EOF

</code></pre>
<p>Both the <code>nonterminal</code> and the <code>typedterminal</code> directives allow a type
to be associated with grammar symbol.  The default type is
the same as the absyntype (LBox&lt;dyn Any&gt;) unless so defined.
In this grammar, the type associated with the non-terminal E is
Expr, but that for ES is Vec&lt;LBox&lt;Expr&gt;&gt;.  This is an alternative
to including a <code>Seq(Vec&lt;LBox&lt;Expr&gt;&gt;)</code> variant of Expr, which was
used in the chapter 2 example.</p>
<p>In the  LBox&lt;dyn Any&gt; special setting, the labels attached to grammar symbols on
the right-hand side of productions no longer represent value of type
<a href="https://docs.rs/rustlr/latest/rustlr/zc_parser/struct.StackedItem.html">StackedItem</a>, but are of type <a href="https://docs.rs/rustlr/latest/rustlr/generic_absyn/struct.LBox.html">LBox</a>&lt;Ty&gt;, where Ty is the type
associated with that grammar symbol.  That is, in writing the semantic
action for a rule such as</p>
<pre><code><span class="hljs-attribute">E</span> --&gt; E:e<span class="hljs-number">1</span> + E:e<span class="hljs-number">2</span> { Plus(e<span class="hljs-number">1</span>,e<span class="hljs-number">2</span>) }
</code></pre>
<p>the labels e1 and e2 are automatically downcast to <strong>LBox&lt;Expr&gt;</strong>.  The
semantic action should return a value of type <strong>Expr</strong> because that is
the type associated with the nonterminal <strong>E</strong>.  The returned value will
be placed in a LBox and
upcast to LBox&lt;dyn Any&gt; by rustlr before being pushed onto the parse stack.
Pattern labels can still be used to describe the values.  A pattern
inside @..@ will match the down-casted value inside the LBox.
As a much large example, <a href="https://cs.hofstra.edu/~cscccl/rustlr_project/minijava/lbamj.grammar">this grammar</a>,
which defines a scaled-down version of Java, demonstrates how the LBox&lt;dyn Any&gt;
type can still be used alongside patterns.</p>
<p>Macros including <a href="https://docs.rs/rustlr/latest/rustlr/macro.unbox.html">unbox!</a>
allow the semantic values to be extracted from the LBox.</p>
<p>The final value returned by the parser will also be of type
<code>LBox&lt;dyn Any&gt;</code> and must be downcast to a usable value using the provided
<a href="https://docs.rs/rustlr/0.2.1/rustlr/generic_absyn/struct.LBox.html#method.downcast">LBox::downcast</a> function.</p>
<h4 id="tradeoff"><strong>TRADEOFF</strong></h4>
<p>The major downside of using this object-oriented approach is that the
abtract syntax types cannot contain non-static references, because the Any type
does not cover such references.  Even
though rustlr allows lexical scanners to be zero-copy, the <code>lifetime</code>
directive is meaningless in this mode.  Basically, this means that
instead of <code>&amp;'t str</code> one may have to use owned strings in the abstract
syntax representation.  Thus there is a non-zero runtime overhead to
this approach.  In addition to being slower, using the Any trait with
downcasting also sacrafices a degree of static type safety. Thus, the
tradeoff here reflects the fundamental tradeoff between Rust and
object-oriented languages in general.  Those choosing the LBox&lt;dyn Any&gt;
option for its convenience should understand and accept this tradeoff.</p>
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