A language where every term carries a type, and where the types turn out to be propositions and the terms their proofs. Runs from the simply typed lambda calculus and type inference through System F, dependent types and Martin-Lof's theory, to the homotopy reading where equalities are paths.
Free to start · adaptive placement finds your level · reviews timed to your own forgetting.
Every idea is taught with motivation and a worked example before the drills, and an FSRS spaced-repetition engine schedules each review for the day its model predicts you would forget it. A short placement check finds what you already know, so you start Type Theory exactly where it's useful.