Scope, use, and watch

Lexical scoping. Functions can read outer variables from the scope where they were defined. Reassigning those outer variables still requires use mut.

Syntax

use name;
use mut count;
watch count;

Example

count: int = 0;
watch count;

fn bump() {
    use mut count;
    count = count + 1;
}

bump();

WATCH: count changed to 1 (in bump)

Notes

use

Import an outer variable for reading inside a function.

use mut

Import an outer variable for mutation inside a function.

watch

Mark a variable for change reporting.

Shadowing

A local typed variable with the same name as a parent variable can trigger a warning.

Common Mistakes

• Reading outer variables without use
• Mutating outer variables without use mut
• Watching an undefined variable

Current Limitation

Function scope is stricter than typical lexical closures. That is intentional.

See Also

• Functions
• Errors and Troubleshooting