Capabilities: uses

● Intermediate3 min

A pub function must write its uses clause. A private one may leave it out: the compiler works it out.

A pub function that reads a file without saying so is refused, and the message names what to add:

use std.fs

pub fn load(name: String) -> String ! FsError {
    try fs.read_text(name)
}

kavach run cap-pub.kav

error: `load` is `pub`, so it must say what it uses: add `uses Fs.read` (spec 8.1)

A private function may leave uses out. The compiler works out what it touches, and that still counts for whoever calls it:

fn shout(message: String) {
    print("{message}!")
}

pub fn greet(name: String) uses Console {
    shout("Namaste, {name}")
}

fn main() {
    greet("Asha")
}

kavach run cap-inferred.kav

Namaste, Asha!

So a pub function that calls it must list it too:

fn shout(message: String) {
    print("{message}!")
}

pub fn greet(name: String) {
    shout("Namaste, {name}")
}

fn main() {
    greet("Asha")
}

kavach run cap-inferred-caller.kav

error: `greet` is `pub`, so it must say what it uses: add `uses Console` (through `shout`) (spec 8.1)