Free public beta until December 4, 2026: every course is open, no card needed. After that, the first chapter of each course stays free and full access is a one-time $39 ($19 for up to 100 founding members chosen for helpful feedback). Your progress is kept. How the beta works

mutable

Allowed to change after it is set.

A mutable binding (let mut) may be assigned to again; bindings are immutable by default.

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Examples

score = 15; after let score = 10; fails with E0384; writing let mut score = 10; allows the change.