When most practitioners think of "typechecking", they typically think about proving properties about programs statically. This project seems equivalent to adding a runtime check at each call site to ensure the arguments and return values are the correct type.

This is certainly useful sometimes, as it gives programs the desirable "fail fast" property. But it isn't "typechecking" as most engineers understand it. Or at least, it should be clarified that this is run-time typechecking. As such, it negatively impacts runtime performance, unlike compile-time typechecking.

This project also seems to miss the primary opportunity of run-time type checking: checking properties that are difficult to prove statically! For example, checking that a number is even, that a string is lowercase, that an index is within the bounds of an array, etc. These exotic types require a dependent type system to be checked statically, but in a dynamic environment they are trivial to verify.

Two suggestions for improvement: 1) add "sum" types (i.e., discriminated unions), and 2) let the user define their own types via lambdas, such as PrimeNumber.