[kd0amg]

One can exist without the other and most statically typed languages either forbid or strongly discourage dynamic typing.

It never seemed like that much of a prohibition to me. Dynamic types take one grand universe of "values" and divide it up in ways that (ideally) reflect differences in those values -- the number six is a different kind of thing than the string with the three letters s i x -- but what the types are is sort of arbitrary. Is an int/string pair a different type than a float/float pair? Is positive-integer a type in its own right? Is every int a rational, or just convertible into a rational? What if you have union types? After using enough dynamically typed languages, the only common factor that I'm confident holds across the whole design space is that a dynamic type is a set of values. That means static typing still leaves you free to define dynamic types that refine the classification of values imposed by your static types, and people do program with pre-/postconditions not stated in types. You just don't get the compiler's help ensuring your code is safe with regard to your own distinctions (unless maybe you build your own refinement type system on top of your language of choice).

By a similar process, dynamic typing leaves you free to define and follow your own static discipline even if a lot of programmers prefer not to. This is more or less why How to Design Programs is written/taught using a dynamic language. The static type-like discipline is a huge aspect of the curriculum, but the authors don't want to force students to commit to one specific language's typing discipline.