document __init__

[stevengj]

This fixes #8923. @vtjnash, @JeffBezanson, please look over the text to make sure that it is accurate.

cc: @timholy

[JeffBezanson]

👍

[timholy]

Very nice. In practice, I've found that perhaps the most important point is that any global Dicts or Sets need to be rehashed---when that doesn't happen, figuring out what's wrong can be surprisingly tricky. In my recent HDF5 work I forgot to check Sets, and spent about 2 hours on this one-line fix: JuliaIO/HDF5.jl@df75bf9. The absence of the correct hash meant that the correct function was being overwritten by a newly-generated one, which made it a rather non-obvious debugging experience.

rehash is not exported, for good reason. But convenience syntax (for julia 0.4) added in e7ce4cb.

[stevengj]

Isn't this only the case if they are hashed by object_id? Why would a Dict{Int} need to be rehashed, for example?

[timholy]

Correct.

[jakebolewski]

Should we also provide an ObjectIdSet? This distinction could be made more explicit.

[stevengj]

I don't see how ObjectIDSet would help, because Set could still be unsafe (just like Dict can be unsafe even though there is ObjectIdDict). The problem is that the default hash(x,h) method calls object_id(x).

Unfortunately, this includes hashing of strings immutables.

Actually, the story is even more complicated than that, because the object_id appears to be only pointer-based for some types. But it looks like strings immutables are not okay because the hash includes the hash of the pointer to their type? But it seems like the address of the string immutable type does not change from run to run (at least with the same julia binary), so immutable string keys are okay? It's confusing.

[Sean1708]

I think there's a missing backtick here.

[ivarne]

Seems like it does. Fixed in 9743a99

[ivarne]

If you want, you can fix such issues yourself, even without mucking with the git command line, because github provides a Web UI to create pull requests if you click the pencil button above a file.

[stevengj]

Thanks for the fixes.