Harmonise our use of !!

[mhauru]

We are currently inconsistent in our use of methods with !! at the end of the name. We sometimes call them but discard the return value, assuming they mutate; some methods which return their value are only marked width !; there may be a few that have !! in the name but don't in fact always return a value; etc. Once #555 is done I would like to take a pass over the codebase and make it such that

• ! methods never return a value. [EDIT: See comments]
• !! methods always return a value.
• Whenever we call !! methods we make use of the return value and don't assume mutation.

I would also like to switch to using !! methods in many places where we currently use ! methods. For instance, in #555 I'm building towards VarInfo being such that you can push!! new values to it that don't fit the element or VarName type of the current VarInfo, and if need be a new object is returned with expanded types, although most often that's not necessary and the old object is returned mutated. This might help simplify building VarInfos that with concrete types.

[devmotion]

! methods never return a value.

It's quite common in Julia that ! functions return the argument that is mutated since this makes it easier to compose functions.

The other two points though are a requirement of the bang-bang convention: Since they may mutate but don't have to, they must return the result and one must not assume mutation.

[mhauru]

It's quite common in Julia that ! functions return the argument that is mutated since this makes it easier to compose functions.

Fair point. I first thought, if that's the case, why not call it a !! function. But I guess sometimes you may want a guarantee that the returned object isn't a new one. Happy to strike the "! methods never return a value" request of the list, the other two are the important ones.

[torfjelde]

Very much in favour of this 👍

[mhauru]

@AoifeHughes, this could be a candidate for something that requires you to tour the DynamicPPL codebase without having to understand all that is happening. The context is that in Julia functions that mutate their arguments should, by convention, have a ! at the end of their name. By BangBang.jl functions that may or may not mutate their inputs, but commit to always returning the object you need, should end in !!. We are not following this convention properly.

Some of this is already getting fixed on breaking in #901, because the work there requires respecting the convention better, but I haven't systematically gone and fixed this on the whole codebase.