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use std::sync::Arc;
// Step 1. Define the query group
// A **query group** is a collection of queries (both inputs and
// functions) that are defined in one particular spot. Each query
// group is defined by a trait decorated with the
// `#[salsa::query_group]` attribute. The trait defines one method per
// query, with the argments to the method being the query **keys** and
// the return value being the query's **value**.
// Along with the trait, each query group has an associated
// "storage struct". The name of this struct is specified in the `query_group`
// attribute -- for a query group `Foo`, it is conventionally `FooStorage`.
// When we define the final database (see below), we will list out the
// storage structs for each query group that it contains. The database
// will then automatically implement the traits.
// Note that one query group can "include" another by listing the
// trait for that query group as a supertrait.
trait HelloWorld: salsa::Database {
// For each query, we give the name, some input keys (here, we
// have one key, `()`) and the output type `Arc<String>`. We can
// use attributes to give other configuration:
// - `salsa::input` indicates that this is an "input" to the system,
// which must be explicitly set. The `salsa::query_group` method
// will autogenerate a `set_input_string` method that can be
// used to set the input.
fn input_string(&self, key: ()) -> Arc<String>;
// This is a *derived query*, meaning its value is specified by
// a function (see Step 2, below).
fn length(&self, key: ()) -> usize;
// Step 2. Define the queries.
// Define the **function** for the `length` query. This function will
// be called whenever the query's value must be recomputed. After it
// is called once, its result is typically memoized, unless we think
// that one of the inputs may have changed. Its first argument (`db`)
// is the "database", which is the type that contains the storage for
// all of the queries in the system -- we never know the concrete type
// here, we only know the subset of methods we care about (defined by
// the `HelloWorld` trait we specified above).
fn length(db: &impl HelloWorld, (): ()) -> usize {
// Read the input string:
let input_string = db.input_string(());
// Return its length:
// Step 3. Define the database struct
// Define the actual database struct. This struct needs to be
// annotated with `#[salsa::database(..)]`. The list `..` will be the
// paths leading to the storage structs for each query group that this
// database supports. This attribute macro will generate the necessary
// impls so that the database implements the corresponding traits as
// well (so, here, `DatabaseStruct` will implement the `HelloWorld`
// trait).
// The database struct can contain basically anything you need, but it
// must have a `runtime` field as shown, and you must implement the
// `salsa::Database` trait (as shown below).
struct DatabaseStruct {
runtime: salsa::Runtime<DatabaseStruct>,
// Tell salsa where to find the runtime in your context.
impl salsa::Database for DatabaseStruct {
fn salsa_runtime(&self) -> &salsa::Runtime<DatabaseStruct> {
// This shows how to use a query.
fn main() {
let mut db = DatabaseStruct::default();
// You cannot access input_string yet, because it does not have a
// value. If you do, it will panic. You could create an Option
// interface by maintaining a HashSet of inserted keys.
// println!("Initially, the length is {}.", db.length(()));
db.set_input_string((), Arc::new(format!("Hello, world")));
println!("Now, the length is {}.", db.length(()));