[kenjackson]

In particular, dynamicity is necessary whenever data live longer than the code manipulating them. If you want to be able to change the program arbitrarily while not losing the data you're working with, you need dynamicity. In dynamic languages, the data can remain live in the program's address space while you modify and recompile the code. With static languages, what you have to do is write the data into files, change your program, and read them back in.

Not completely true. While I don't know of many use cases for doing what you're talking about, there is at least one that I know of, which is debugging.

In the case of debugging, you can change code with static languages almost arbitrarily while the data stays live ine program's address space. Although one thing that is not possible to do, with current implementations, is change the type definition. But otherwise changes can be made to code w/ data never leaving main memory (I probably do this 10 times per day).

Now it is theoretically possible to also change type definitions, but that would require a notion of constructor that builds the new type from the old type. In some cases this can effectively just be an interface, then its really just a noop.