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<ol class="chapter"><li class="chapter-item expanded "><a href="../about_salsa.html"><strong aria-hidden="true">1.</strong> About salsa</a></li><li class="chapter-item expanded affix "><li class="part-title">How to use Salsa</li><li class="chapter-item expanded "><a href="../overview.html"><strong aria-hidden="true">2.</strong> Overview</a></li><li class="chapter-item expanded "><a href="../tutorial.html"><strong aria-hidden="true">3.</strong> Tutorial: calc language</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../tutorial/structure.html"><strong aria-hidden="true">3.1.</strong> Basic structure</a></li><li class="chapter-item expanded "><a href="../tutorial/jar.html"><strong aria-hidden="true">3.2.</strong> Jars and databases</a></li><li class="chapter-item expanded "><a href="../tutorial/db.html"><strong aria-hidden="true">3.3.</strong> Defining the database struct</a></li><li class="chapter-item expanded "><a href="../tutorial/ir.html"><strong aria-hidden="true">3.4.</strong> Defining the IR: the various "salsa structs"</a></li><li class="chapter-item expanded "><a href="../tutorial/parser.html"><strong aria-hidden="true">3.5.</strong> Defining the parser: memoized functions and inputs</a></li><li class="chapter-item expanded "><a href="../tutorial/accumulators.html" class="active"><strong aria-hidden="true">3.6.</strong> Defining the parser: reporting errors</a></li><li class="chapter-item expanded "><a href="../tutorial/debug.html"><strong aria-hidden="true">3.7.</strong> Defining the parser: debug impls and testing</a></li><li class="chapter-item expanded "><a href="../tutorial/checker.html"><strong aria-hidden="true">3.8.</strong> Defining the checker</a></li><li class="chapter-item expanded "><a href="../tutorial/interpreter.html"><strong aria-hidden="true">3.9.</strong> Defining the interpreter</a></li></ol></li><li class="chapter-item expanded "><a href="../reference.html"><strong aria-hidden="true">4.</strong> Reference</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../reference/algorithm.html"><strong aria-hidden="true">4.1.</strong> Algorithm</a></li></ol></li><li class="chapter-item expanded "><a href="../common_patterns.html"><strong aria-hidden="true">5.</strong> Common patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../common_patterns/selection.html"><strong aria-hidden="true">5.1.</strong> Selection</a></li><li class="chapter-item expanded "><a href="../common_patterns/on_demand_inputs.html"><strong aria-hidden="true">5.2.</strong> On-demand (Lazy) inputs</a></li></ol></li><li class="chapter-item expanded "><a href="../tuning.html"><strong aria-hidden="true">6.</strong> Tuning</a></li><li class="chapter-item expanded "><a href="../cycles.html"><strong aria-hidden="true">7.</strong> Cycle handling</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../cycles/fallback.html"><strong aria-hidden="true">7.1.</strong> Recovering via fallback</a></li></ol></li><li class="chapter-item expanded "><li class="part-title">How Salsa works internally</li><li class="chapter-item expanded "><a href="../how_salsa_works.html"><strong aria-hidden="true">8.</strong> How Salsa works</a></li><li class="chapter-item expanded "><a href="../videos.html"><strong aria-hidden="true">9.</strong> Videos</a></li><li class="chapter-item expanded "><a href="../plumbing.html"><strong aria-hidden="true">10.</strong> Plumbing</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../plumbing/jars_and_ingredients.html"><strong aria-hidden="true">10.1.</strong> Jars and ingredients</a></li><li class="chapter-item expanded "><a href="../plumbing/database_and_runtime.html"><strong aria-hidden="true">10.2.</strong> Databases and runtime</a></li><li class="chapter-item expanded "><a href="../plumbing/query_ops.html"><strong aria-hidden="true">10.3.</strong> Query operations</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../plumbing/maybe_changed_after.html"><strong aria-hidden="true">10.3.1.</strong> maybe changed after</a></li><li class="chapter-item expanded "><a href="../plumbing/fetch.html"><strong aria-hidden="true">10.3.2.</strong> Fetch</a></li><li class="chapter-item expanded "><a href="../plumbing/derived_flowchart.html"><strong aria-hidden="true">10.3.3.</strong> Derived queries flowchart</a></li><li class="chapter-item expanded "><a href="../plumbing/cycles.html"><strong aria-hidden="true">10.3.4.</strong> Cycle handling</a></li></ol></li><li class="chapter-item expanded "><a href="../plumbing/terminology.html"><strong aria-hidden="true">10.4.</strong> Terminology</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../plumbing/terminology/backdate.html"><strong aria-hidden="true">10.4.1.</strong> Backdate</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/changed_at.html"><strong aria-hidden="true">10.4.2.</strong> Changed at</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/dependency.html"><strong aria-hidden="true">10.4.3.</strong> Dependency</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/derived_query.html"><strong aria-hidden="true">10.4.4.</strong> Derived query</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/durability.html"><strong aria-hidden="true">10.4.5.</strong> Durability</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/input_query.html"><strong aria-hidden="true">10.4.6.</strong> Input query</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/ingredient.html"><strong aria-hidden="true">10.4.7.</strong> Ingredient</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/LRU.html"><strong aria-hidden="true">10.4.8.</strong> LRU</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/memo.html"><strong aria-hidden="true">10.4.9.</strong> Memo</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/query.html"><strong aria-hidden="true">10.4.10.</strong> Query</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/query_function.html"><strong aria-hidden="true">10.4.11.</strong> Query function</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/revision.html"><strong aria-hidden="true">10.4.12.</strong> Revision</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/salsa_item.html"><strong aria-hidden="true">10.4.13.</strong> Salsa item</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/salsa_struct.html"><strong aria-hidden="true">10.4.14.</strong> Salsa struct</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/untracked.html"><strong aria-hidden="true">10.4.15.</strong> Untracked dependency</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/verified.html"><strong aria-hidden="true">10.4.16.</strong> Verified</a></li></ol></li></ol></li><li class="chapter-item expanded "><li class="part-title">Salsa RFCs</li><li class="chapter-item expanded "><a href="../rfcs.html"><strong aria-hidden="true">11.</strong> RFCs</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../rfcs/template.html"><strong aria-hidden="true">11.1.</strong> Template</a></li><li class="chapter-item expanded "><a href="../rfcs/RFC0001-Query-Group-Traits.html"><strong aria-hidden="true">11.2.</strong> RFC 0001: Query group traits</a></li><li class="chapter-item expanded "><a href="../rfcs/RFC0002-Intern-Queries.html"><strong aria-hidden="true">11.3.</strong> RFC 0002: Intern queries</a></li><li class="chapter-item expanded "><a href="../rfcs/RFC0003-Query-Dependencies.html"><strong aria-hidden="true">11.4.</strong> RFC 0003: Query dependencies</a></li><li class="chapter-item expanded "><a href="../rfcs/RFC0004-LRU.html"><strong aria-hidden="true">11.5.</strong> RFC 0004: LRU</a></li><li class="chapter-item expanded "><a href="../rfcs/RFC0005-Durability.html"><strong aria-hidden="true">11.6.</strong> RFC 0005: Durability</a></li><li class="chapter-item expanded "><a href="../rfcs/RFC0006-Dynamic-Databases.html"><strong aria-hidden="true">11.7.</strong> RFC 0006: Dynamic database</a></li><li class="chapter-item expanded "><a href="../rfcs/RFC0007-Opinionated-Cancelation.html"><strong aria-hidden="true">11.8.</strong> RFC 0007: Opinionated cancelation</a></li><li class="chapter-item expanded "><a href="../rfcs/RFC0008-Remove-Garbage-Collection.html"><strong aria-hidden="true">11.9.</strong> RFC 0008: Remove garbage collection</a></li><li class="chapter-item expanded "><a href="../rfcs/RFC0009-Cycle-recovery.html"><strong aria-hidden="true">11.10.</strong> RFC 0009: Cycle recovery</a></li><li class="chapter-item expanded "><a href="../rfcs/RFC0010-Slot-no-more.html"><strong aria-hidden="true">11.11.</strong> RFC 0010: Slot no more</a></li></ol></li><li class="chapter-item expanded "><li class="part-title">Appendices</li><li class="chapter-item expanded "><a href="../meta.html"><strong aria-hidden="true">12.</strong> Meta: about the book itself</a></li></ol> </div>
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<h1 id="defining-the-parser-reporting-errors"><a class="header" href="#defining-the-parser-reporting-errors">Defining the parser: reporting errors</a></h1>
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<p>The last interesting case in the parser is how to handle a parse error.
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Because salsa functions are memoized and may not execute, they should not have side-effects,
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so we don't just want to call <code>eprintln!</code>.
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If we did so, the error would only be reported the first time the function was called.</p>
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<p>Salsa defines a mechanism for managing this called an <strong>accumulator</strong>.
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In our case, we define an accumulator struct called <code>Diagnostics</code> in the <code>ir</code> module:</p>
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<pre><pre class="playground"><code class="language-rust">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>#[salsa::accumulator]
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pub struct Diagnostics(Diagnostic);
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#[derive(Clone, Debug)]
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pub struct Diagnostic {
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pub position: usize,
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pub message: String,
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p>Accumulator structs are always newtype structs with a single field, in this case of type <code>Diagnostic</code>.
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Memoized functions can <em>push</em> <code>Diagnostic</code> values onto the accumulator.
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Later, you can invoke a method to find all the values that were pushed by the memoized functions
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or any function that it called
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(e.g., we could get the set of <code>Diagnostic</code> values produced by the <code>parse_statements</code> function).</p>
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<p>The <code>Parser::report_error</code> method contains an example of pushing a diagnostic:</p>
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<pre><pre class="playground"><code class="language-rust">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span> /// Report an error diagnostic at the current position.
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fn report_error(&self) {
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Diagnostics::push(
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self.db,
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Diagnostic {
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position: self.position,
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message: "unexpected character".to_string(),
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},
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);
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p>To get the set of diagnostics produced by <code>parse_errors</code>, or any other memoized function,
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we invoke the associated <code>accumulated</code> function:</p>
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<pre><pre class="playground"><code class="language-rust">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>let accumulated: Vec<Diagnostic> =
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parse_statements::accumulated::<Diagnostics>(db);
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// -----------
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// Use turbofish to specify
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// the diagnostics type.
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<span class="boring">}
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</span></code></pre></pre>
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<p><code>accumulated</code> takes the database <code>db</code> as argument and returns a <code>Vec</code>.</p>
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