> you could use types and would not need these tests anymore. No. These are not internal contracts, these are contracts with user input. In a statically type

> you could use types and would not need these tests anymore. No. These are not internal contracts, these are contracts with user input. In a statically typed language, You are still advised to write tests that your marshalling technique provides the expected error (and downstream effects) that you plan for, if say the user inputs the string "1.", For a string that should be marshalled as an integer. > A type is literally a description of a set of valid values. That is generally not the case. There are, for example cases where certain floating points are invalid inputs (either outside of the domain of the function or a singular points), and I don't know of a mainstream PL that lets you define subsets of the reals in their type system. In go, or c, c++, or rust, you could have a situation where a subset of integers are valid values (perhaps you need "positive, nonzero integers", because you are going to do an integer divide at some point) and that is not an expressible set of value in that type system. Ironically, that is a scenario that IS typable in Elixir and erlang, which are dynamically typed languages.