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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"><strong aria-hidden="true">3.6.</strong> Defining the parser: reporting errors</a></li><li class="chapter-item expanded "><a href="../tutorial/debug.html" class="active"><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/durability.html"><strong aria-hidden="true">4.1.</strong> Durability</a></li><li class="chapter-item expanded "><a href="../reference/algorithm.html"><strong aria-hidden="true">4.2.</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/on_demand_inputs.html"><strong aria-hidden="true">5.1.</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/db_lifetime.html"><strong aria-hidden="true">10.3.</strong> The db lifetime on tracked/interned structs</a></li><li class="chapter-item expanded "><a href="../plumbing/tracked_structs.html"><strong aria-hidden="true">10.4.</strong> Tracked structures</a></li><li class="chapter-item expanded "><a href="../plumbing/query_ops.html"><strong aria-hidden="true">10.5.</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">10.5.1.</strong> maybe changed after</a></li><li class="chapter-item expanded "><a href="../plumbing/fetch.html"><strong aria-hidden="true">10.5.2.</strong> Fetch</a></li><li class="chapter-item expanded "><a href="../plumbing/derived_flowchart.html"><strong aria-hidden="true">10.5.3.</strong> Derived queries flowchart</a></li><li class="chapter-item expanded "><a href="../plumbing/cycles.html"><strong aria-hidden="true">10.5.4.</strong> Cycle handling</a></li></ol></li><li class="chapter-item expanded "><a href="../plumbing/terminology.html"><strong aria-hidden="true">10.6.</strong> Terminology</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../plumbing/terminology/backdate.html"><strong aria-hidden="true">10.6.1.</strong> Backdate</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/changed_at.html"><strong aria-hidden="true">10.6.2.</strong> Changed at</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/dependency.html"><strong aria-hidden="true">10.6.3.</strong> Dependency</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/derived_query.html"><strong aria-hidden="true">10.6.4.</strong> Derived query</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/durability.html"><strong aria-hidden="true">10.6.5.</strong> Durability</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/input_query.html"><strong aria-hidden="true">10.6.6.</strong> Input query</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/ingredient.html"><strong aria-hidden="true">10.6.7.</strong> Ingredient</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/LRU.html"><strong aria-hidden="true">10.6.8.</strong> LRU</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/memo.html"><strong aria-hidden="true">10.6.9.</strong> Memo</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/query.html"><strong aria-hidden="true">10.6.10.</strong> Query</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/query_function.html"><strong aria-hidden="true">10.6.11.</strong> Query function</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/revision.html"><strong aria-hidden="true">10.6.12.</strong> Revision</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/salsa_item.html"><strong aria-hidden="true">10.6.13.</strong> Salsa item</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/salsa_struct.html"><strong aria-hidden="true">10.6.14.</strong> Salsa struct</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/untracked.html"><strong aria-hidden="true">10.6.15.</strong> Untracked dependency</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/verified.html"><strong aria-hidden="true">10.6.16.</strong> Verified</a></li></ol></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">11.</strong> Meta: about the book itself</a></li></ol> </div>
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<main>
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<h1 id="defining-the-parser-debug-impls-and-testing"><a class="header" href="#defining-the-parser-debug-impls-and-testing">Defining the parser: debug impls and testing</a></h1>
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<p>As the final part of the parser, we need to write some tests.
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To do so, we will create a database, set the input source text, run the parser, and check the result.
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Before we can do that, though, we have to address one question: how do we inspect the value of an interned type like <code>Expression</code>?</p>
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<h2 id="the-debugwithdb-trait"><a class="header" href="#the-debugwithdb-trait">The <code>DebugWithDb</code> trait</a></h2>
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<p>Because an interned type like <code>Expression</code> just stores an integer, the traditional <code>Debug</code> trait is not very useful.
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To properly print a <code>Expression</code>, you need to access the Salsa database to find out what its value is.
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To solve this, <code>salsa</code> provides a <code>DebugWithDb</code> trait that acts like the regular <code>Debug</code>, but takes a database as argument.
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For types that implement this trait, you can invoke the <code>debug</code> method.
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This returns a temporary that implements the ordinary <code>Debug</code> trait, allowing you to write something like</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>eprintln!("Expression = {:?}", expr.debug(db));
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<span class="boring">}
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</span></code></pre></pre>
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<p>and get back the output you expect.</p>
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<p>The <code>DebugWithDb</code> trait is automatically derived for all <code>#[input]</code>, <code>#[interned]</code>, and <code>#[tracked]</code> structs.</p>
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<h2 id="forwarding-to-the-ordinary-debug-trait"><a class="header" href="#forwarding-to-the-ordinary-debug-trait">Forwarding to the ordinary <code>Debug</code> trait</a></h2>
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<p>For consistency, it is sometimes useful to have a <code>DebugWithDb</code> implementation even for types, like <code>Op</code>, that are just ordinary enums. You can do that like so:</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>
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<span class="boring">fn main() {
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</span><span class="boring">}
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</span></code></pre></pre>
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<h2 id="writing-the-unit-test"><a class="header" href="#writing-the-unit-test">Writing the unit test</a></h2>
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<p>Now that we have our <code>DebugWithDb</code> impls in place, we can write a simple unit test harness.
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The <code>parse_string</code> function below creates a database, sets the source text, and then invokes the parser:</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>/// Create a new database with the given source text and parse the result.
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/// Returns the statements and the diagnostics generated.
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#[cfg(test)]
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fn parse_string(source_text: &str) -> String {
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use salsa::debug::DebugWithDb;
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// Create the database
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let db = crate::db::Database::default();
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// Create the source program
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let source_program = SourceProgram::new(&db, source_text.to_string());
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// Invoke the parser
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let statements = parse_statements(&db, source_program);
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// Read out any diagnostics
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let accumulated = parse_statements::accumulated::<Diagnostics>(&db, source_program);
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// Format the result as a string and return it
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format!("{:#?}", (statements.debug(&db), accumulated))
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p>Combined with the <a href="https://crates.io/crates/expect-test"><code>expect-test</code></a> crate, we can then write unit tests like this one:</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>#[test]
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fn parse_print() {
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let actual = parse_string("print 1 + 2");
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let expected = expect_test::expect![[r#"
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(
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Program {
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[salsa id]: 0,
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statements: [
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Statement {
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span: Span {
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[salsa id]: 4,
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start: 0,
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end: 11,
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},
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data: Print(
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Expression {
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span: Span {
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[salsa id]: 3,
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start: 6,
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end: 11,
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},
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data: Op(
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Expression {
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span: Span {
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[salsa id]: 0,
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start: 6,
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end: 7,
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},
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data: Number(
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OrderedFloat(
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1.0,
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),
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),
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},
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Add,
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Expression {
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span: Span {
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[salsa id]: 2,
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start: 10,
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end: 11,
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},
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data: Number(
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OrderedFloat(
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2.0,
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),
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),
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},
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),
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},
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),
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},
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],
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},
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[],
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)"#]];
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expected.assert_eq(&actual);
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}
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<span class="boring">}
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</span></code></pre></pre>
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</body>
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</html>
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