gistfile1.rs

/* a macro that defines a context variable that is stored in thread local
   storage.  It's implemented as a module that exports a `get`, `set` and
   `set_to` function.  `set_to` acts as a stack. */
macro_rules! context_var (($name:ident : $t:ty) => (
    mod $name {
        /* internal tls key helper */
        fn key(_x: @$t) {}

        /* checks if the variable is set */
        pub fn is_set() -> bool { unsafe {
            match task::local_data::local_data_get(key) {
                Some(ref _x) => true,
                None => false
            }
        } }

        /* returns the current value of that variable or fails with an error */
        pub fn get() -> @$t { unsafe {
            match move task::local_data::local_data_get(key) {
                Some(move x) => move x,
                None => {
                    die!(fmt!("%s::get() tried to return value \
                               but no value was set",
                              stringify!($name)))
                }
            }
        } }

        /* sets the variable to a new value */
        pub fn set(value: @$t) { unsafe {
            task::local_data::local_data_set(key, value);
        } }

        /* unsets the value */
        pub fn unset() { unsafe {
            task::local_data::local_data_pop(key);
        }}

        /* temporarily overrides the variable with a new value */
        pub fn set_to(value: @$t, cb: fn()) { unsafe {
            let old = move task::local_data::local_data_pop(key);
            task::local_data::local_data_set(key, value);
            cb();
            match (old) {
                None => {}
                Some(ref x) => {
                    task::local_data::local_data_set(key, *x);
                }
            }
        } }
    }
))

context_var!(language: ~str)

fn print_language() {
    if language::is_set() {
        io::println(fmt!("Your language is: %?", language::get()));
    } else {
        io::println("No language is set");
    }
}

fn main() {
    io::print("main task before setting: ");
    print_language();
    do language::set_to(@~"en_US") {
        io::print("main task in block: ");
        print_language();
        do task::spawn {
            io::print("in another task: ");
            print_language();
        }
    }
}