Did you check out the papers I linked to? Because they explain how this "makes sense". Dependently typed languages allow for type-level functions (actually the

Did you check out the papers I linked to? Because they explain how this "makes sense". Dependently typed languages allow for type-level functions (actually they erase the separation between type-level and value-level). This is why you can define a type like "Fin n" meaning a number smaller than n. "Fin n" is a function that takes one argument "n" and returns a type that limits its values to the set of (natural) numbers smaller than n. Arguably, Agda is more often used as a proof assistant, rather than as a programming language (although I've had the pleasure of watching Ulf Norell implement a limited programming language interpreter in Agda, using non-total functions, which was quite nice.) But Idris is a serious effort to create a language that's both dependently typed and performs decently when used as a regular programming language. If you want to see some real world examples of using dependently typed languages, I would suggest looking at the CompCert C Compiler[0]. It is a C Compiler that's been formally verified using the Coq proof assistant, which is a precursor to both Agda and Idris. Coq is as far as I know the first dependently typed language. To be fair, programming in a dependently typed language usually requires more effort upfront, but the ability to encode properties that are usually considered runtime errors into the type system does seem like an interesting future for programming languages. [0] http://compcert.inria.fr/compcert-C.html http://compcert.inria.fr/compcert-C.html (edit: shout out to the excellently named sibling commenter "curryhoward", IMHO one of the most interesting results in programming language technology and type theory research!)