3 ms·

I agree with what you said but I don't see how a heterogeneous dynamic structure is "pretty useless" though

• colanderman 13y ago
  In a dynamic structure, you're adding and removing key/value pairs at runtime, when you generally don't know what the keys are. When you later access the structure, since you don't generally know what's in it, you can't know the type of any of the values, and hence you can't know what operations are valid on them. The way around this of course is to tag the values with their type. This is usually done implicitly in dynamically typed languages (e.g. Javascript), and always must be done explicitly in statically typed languages (e.g. OCaml). But once you do that, you have a single type for all the things in the structure (a tagged union type) and it's now a homogeneous structure. Either that, or you can split your mapping into multiple mappings, one for each type of thing. That has the benefit of permitting better optimization (you don't need to store per-value tags, among other benefits). (Even in dynamically typed languages, it's better to explicitly tag things anyway. If you have a structure that can legitimately contain two kinds of things, one which happens to be an integer, and one which happens to be a string, then it's pretty likely it might later need to contain a third kind of string, which, if it happens to be an integer or a string, well, you're screwed.)