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88 lines
3.6 KiB
Rust
88 lines
3.6 KiB
Rust
use std::sync::Arc;
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///////////////////////////////////////////////////////////////////////////
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// Step 1. Define the query group
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// A **query group** is a collection of queries (both inputs and
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// functions) that are defined in one particular spot. Each query group
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// represents some subset of the full set of queries you will use in your
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// application. Query groups can also depend on one another: so you might
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// have some basic query group A and then another query group B that uses
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// the queries from A and adds a few more. (These relationships must form
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// a DAG at present, but that is due to Rust's restrictions around
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// supertraits, which are likely to be lifted.)
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#[salsa::query_group]
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trait HelloWorldDatabase: salsa::Database {
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// For each query, we give the name, some input keys (here, we
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// have one key, `()`) and the output type `Arc<String>`. We can
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// use attributes to give other configuration:
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//
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// - `salsa::input` indicates that this is an "input" to the system,
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// which must be explicitly set. The `salsa::query_group` method
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// will autogenerate a `set_input_string` method that can be
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// used to set the input.
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#[salsa::input]
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fn input_string(&self, key: ()) -> Arc<String>;
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// This is a *derived query*, meaning its value is specified by
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// a function (see Step 2, below).
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fn length(&self, key: ()) -> usize;
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}
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///////////////////////////////////////////////////////////////////////////
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// Step 2. Define the queries.
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// Define the **function** for the `length` query. This function will
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// be called whenever the query's value must be recomputed. After it
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// is called once, its result is typically memoized, unless we think
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// that one of the inputs may have changed. Its first argument (`db`)
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// is the "database", which is the type that contains the storage for
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// all of the queries in the system -- we never know the concrete type
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// here, we only know the subset of methods we care about (defined by
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// the `HelloWorldDatabase` trait we specified above).
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fn length(db: &impl HelloWorldDatabase, (): ()) -> usize {
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// Read the input string:
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let input_string = db.input_string(());
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// Return its length:
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input_string.len()
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}
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///////////////////////////////////////////////////////////////////////////
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// Step 3. Define the database struct
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// Define the actual database struct. This struct needs to be
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// annotated with `#[salsa::database(..)]`, which contains a list of
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// query groups that this database supports. This attribute macro will
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// generate the necessary impls so that the database implements all of
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// those traits.
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//
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// The database struct can contain basically anything you need, but it
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// must have a `runtime` field as shown, and you must implement the
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// `salsa::Database` trait (as shown below).
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#[salsa::database(HelloWorldDatabase)]
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#[derive(Default)]
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struct DatabaseStruct {
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runtime: salsa::Runtime<DatabaseStruct>,
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}
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// Tell salsa where to find the runtime in your context.
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impl salsa::Database for DatabaseStruct {
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fn salsa_runtime(&self) -> &salsa::Runtime<DatabaseStruct> {
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&self.runtime
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}
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}
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// This shows how to use a query.
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fn main() {
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let mut db = DatabaseStruct::default();
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// You cannot access input_string yet, because it does not have a
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// value. If you do, it will panic. You could create an Option
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// interface by maintaining a HashSet of inserted keys.
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// println!("Initially, the length is {}.", db.length(()));
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db.set_input_string((), Arc::new(format!("Hello, world")));
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println!("Now, the length is {}.", db.length(()));
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}
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