Scope, use, and watch

Learning Echo from scratch?

Finish Lesson 5 — Functions first. This page is advanced: lexical scope, use / use mut, and watch.

Lexical scoping. Functions can read outer variables from the scope where they were defined. Reassigning those outer variables requires use mut. Plain use name; does not permit assignment.

Syntax

use name;
use mut count;
watch count;

Basic Example

count: int = 0;
watch count;

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

bump();

Output

WATCH: count changed to 1 (in bump) at <file>:<line>:<col>

The stable prefix is WATCH: count changed to 1 (in bump). A source location suffix at file:line:col is appended; the path and numbers depend on where you saved the file.

How It Works

Lexical reads

Outer names are visible for reading without use:

name: str = "Echo";

fn greet() {
    say(name);
}

greet();

Output

Echo

use

Optional explicit import of an outer name. It does not grant write access.

use mut

Required before assigning to an outer variable inside a function.

watch

Mark a variable for change reporting. Mutation lines include a source location (at file:line:col). On abort, CLI / elang test also dump watched bindings under Watched: and active call frames under Stack: (when the failure is inside a call). Use trace(value) to print a value without changing it.

Shadowing

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

Common Mistakes

Mutating an outer variable without use mut — E2003

count: int = 0;

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

bump();

Expecting plain use to allow assignment — E2003

count: int = 0;

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

Watching an undefined variable

watch requires a name that already exists in scope.

Current Limitation

Reassignment of outer variables always needs use mut. That is intentional — see Known Limitations.

Related

• Functions
• Errors and Troubleshooting