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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"><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" class="active"><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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<h1 class="menu-title">Salsa</h1>
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<main>
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<h1 id="how-salsa-works"><a class="header" href="#how-salsa-works">How Salsa works</a></h1>
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<h2 id="video-available"><a class="header" href="#video-available">Video available</a></h2>
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<p>To get the most complete introduction to Salsa's inner workings, check
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out <a href="https://youtu.be/_muY4HjSqVw">the "How Salsa Works" video</a>. If
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you'd like a deeper dive, <a href="https://www.youtube.com/watch?v=i_IhACacPRY">the "Salsa in more depth"
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video</a> digs into the
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details of the incremental algorithm.</p>
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<blockquote>
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<p>If you're in China, watch videos on <a href="https://www.bilibili.com/video/BV1Df4y1A7t3/">"How Salsa Works"</a>, <a href="https://www.bilibili.com/video/BV1AM4y1G7E4/">"Salsa In More Depth"</a>.</p>
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</blockquote>
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<h2 id="key-idea"><a class="header" href="#key-idea">Key idea</a></h2>
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<p>The key idea of <code>salsa</code> is that you define your program as a set of
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<strong>queries</strong>. Every query is used like a function <code>K -> V</code> that maps from
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some key of type <code>K</code> to a value of type <code>V</code>. Queries come in two basic
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varieties:</p>
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<ul>
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<li><strong>Inputs</strong>: the base inputs to your system. You can change these
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whenever you like.</li>
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<li><strong>Functions</strong>: pure functions (no side effects) that transform your
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inputs into other values. The results of queries are memoized to
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avoid recomputing them a lot. When you make changes to the inputs,
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we'll figure out (fairly intelligently) when we can re-use these
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memoized values and when we have to recompute them.</li>
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</ul>
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<h2 id="how-to-use-salsa-in-three-easy-steps"><a class="header" href="#how-to-use-salsa-in-three-easy-steps">How to use Salsa in three easy steps</a></h2>
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<p>Using Salsa is as easy as 1, 2, 3...</p>
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<ol>
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<li>Define one or more <strong>query groups</strong> that contain the inputs
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and queries you will need. We'll start with one such group, but
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later on you can use more than one to break up your system into
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components (or spread your code across crates).</li>
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<li>Define the <strong>query functions</strong> where appropriate.</li>
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<li>Define the <strong>database</strong>, which contains the storage for all
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the inputs/queries you will be using. The query struct will contain
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the storage for all of the inputs/queries and may also contain
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anything else that your code needs (e.g., configuration data).</li>
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</ol>
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<p>To see an example of this in action, check out <a href="https://github.com/salsa-rs/salsa/blob/master/examples/hello_world/main.rs">the <code>hello_world</code>
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example</a>, which has a number of comments explaining how
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things work.</p>
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<h2 id="digging-into-the-plumbing"><a class="header" href="#digging-into-the-plumbing">Digging into the plumbing</a></h2>
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<p>Check out the <a href="plumbing.html">plumbing</a> chapter to see a deeper explanation of the
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code that Salsa generates and how it connects to the Salsa library.</p>
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