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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" class="active"><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/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/tracked_structs.html"><strong aria-hidden="true">10.3.</strong> Tracked structures</a></li><li class="chapter-item expanded "><a href="../plumbing/query_ops.html"><strong aria-hidden="true">10.4.</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">10.4.1.</strong> maybe changed after</a></li><li class="chapter-item expanded "><a href="../plumbing/fetch.html"><strong aria-hidden="true">10.4.2.</strong> Fetch</a></li><li class="chapter-item expanded "><a href="../plumbing/derived_flowchart.html"><strong aria-hidden="true">10.4.3.</strong> Derived queries flowchart</a></li><li class="chapter-item expanded "><a href="../plumbing/cycles.html"><strong aria-hidden="true">10.4.4.</strong> Cycle handling</a></li></ol></li><li class="chapter-item expanded "><a href="../plumbing/terminology.html"><strong aria-hidden="true">10.5.</strong> Terminology</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../plumbing/terminology/backdate.html"><strong aria-hidden="true">10.5.1.</strong> Backdate</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/changed_at.html"><strong aria-hidden="true">10.5.2.</strong> Changed at</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/dependency.html"><strong aria-hidden="true">10.5.3.</strong> Dependency</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/derived_query.html"><strong aria-hidden="true">10.5.4.</strong> Derived query</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/durability.html"><strong aria-hidden="true">10.5.5.</strong> Durability</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/input_query.html"><strong aria-hidden="true">10.5.6.</strong> Input query</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/ingredient.html"><strong aria-hidden="true">10.5.7.</strong> Ingredient</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/LRU.html"><strong aria-hidden="true">10.5.8.</strong> LRU</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/memo.html"><strong aria-hidden="true">10.5.9.</strong> Memo</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/query.html"><strong aria-hidden="true">10.5.10.</strong> Query</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/query_function.html"><strong aria-hidden="true">10.5.11.</strong> Query function</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/revision.html"><strong aria-hidden="true">10.5.12.</strong> Revision</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/salsa_item.html"><strong aria-hidden="true">10.5.13.</strong> Salsa item</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/salsa_struct.html"><strong aria-hidden="true">10.5.14.</strong> Salsa struct</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/untracked.html"><strong aria-hidden="true">10.5.15.</strong> Untracked dependency</a></li><li class="chapter-item expanded "><a href="../plumbing/terminology/verified.html"><strong aria-hidden="true">10.5.16.</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="basic-structure"><a class="header" href="#basic-structure">Basic structure</a></h1>
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<p>Before we do anything with Salsa, let's talk about the basic structure of the calc compiler.
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Part of Salsa's design is that you are able to write programs that feel 'pretty close' to what a natural Rust program looks like.</p>
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<h2 id="example-program"><a class="header" href="#example-program">Example program</a></h2>
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<p>This is our example calc program:</p>
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<pre><code>x = 5
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y = 10
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z = x + y * 3
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print z
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</code></pre>
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<h2 id="parser"><a class="header" href="#parser">Parser</a></h2>
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<p>The calc compiler takes as input a program, represented by a string:</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>struct ProgramSource {
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text: 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>The first thing it does it to parse that string into a series of statements that look something like the following pseudo-Rust:<sup class="footnote-reference"><a href="#lexer">1</a></sup></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>enum Statement {
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/// Defines `fn <name>(<args>) = <body>`
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Function(Function),
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/// Defines `print <expr>`
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Print(Expression),
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}
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/// Defines `fn <name>(<args>) = <body>`
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struct Function {
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name: FunctionId,
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args: Vec<VariableId>,
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body: Expression
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p>where an expression is something like this (pseudo-Rust, because the <code>Expression</code> enum is recursive):</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>enum Expression {
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Op(Expression, Op, Expression),
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Number(f64),
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Variable(VariableId),
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Call(FunctionId, Vec<Expression>),
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}
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enum Op {
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Add,
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Subtract,
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Multiply,
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Divide,
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p>Finally, for function/variable names, the <code>FunctionId</code> and <code>VariableId</code> types will be interned strings:</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>type FunctionId = /* interned string */;
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type VariableId = /* interned string */;
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<span class="boring">}
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</span></code></pre></pre>
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<div class="footnote-definition" id="lexer"><sup class="footnote-definition-label">1</sup>
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<p>Because calc is so simple, we don't have to bother separating out the lexer from the parser.</p>
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</div>
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<h2 id="checker"><a class="header" href="#checker">Checker</a></h2>
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<p>The "checker" has the job of ensuring that the user only references variables that have been defined.
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We're going to write the checker in a "context-less" style,
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which is a bit less intuitive but allows for more incremental re-use.
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The idea is to compute, for a given expression, which variables it references.
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Then there is a function <code>check</code> which ensures that those variables are a subset of those that are already defined.</p>
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<h2 id="interpreter"><a class="header" href="#interpreter">Interpreter</a></h2>
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<p>The interpreter will execute the program and print the result. We don't bother with much incremental re-use here,
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though it's certainly possible.</p>
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