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How Salsa Works (2019.01)

What is Salsa?

Incremental recomputation

fn foo(input1: A, input2: B) -> Output

let x1 = foo(a, b1) // reuse some of the work we did

let x2 = foo(a, b2)

“Automatic”

“Sound / Correct” — you get the same result as if we did not re-use at all

Salsa’s actual model is richer

IDE:

Inputs:

manifest (Cargo.toml)

source files (foo.rs => "…")

Outputs:

Binary executable

Completions at this point?

What should I show the user when the mouse is hovering on this line?

How does it work?

Identify the “base inputs” to your computation

Identify intermediate “derived” values

deterministic, “pure” function that computes them

Inputs:

manifest manifest() → Vec<Path>

source text source_text(x: Path) → String

Derived values:

for each source file X, we might have a derived value ast(x: Path) → AST

completion(path: Path, line: usize, column: usize)

Track:

in the course of computing (say) ast(Path),

which inputs did we access?

which derived values?

When an input changes:

these derived values are still valid (because none of the inputs that they touch changed)

these derived values may be invalid (because some of the inputs that they touch changed)

manifest() <------------------------ whole_program_ast() <-- type-checking

|

source_text("a.rs") <- ast("a.rs") <-------+

|

source_text("b.rs") <- ast("b.rs") <-------+

Key Salsa concepts

Query