Mark global variables automatically as const

[abraemer]

There was a somewhat recent issue about performance loss triggered by repeated access to a global variable: #2348 (comment)

Of course the observant programmer can avoid this issue but it can happen quite easily. One way to fix this performance is marking the global variable const, which of course has a different semantic generally speaking. But not so in Pluto! When one assigns a value to a global observable, the notebook reruns all the code dependent on that variable and assignment from multiple locations leads to an error. So effectively, global variables in Pluto have the same semantic as a const variable!

Here is the idea: Pluto could mark all global variable const to avoid the slow access in all cases without changing anything for the user! Pluto anyways parses all the code to check for assignments, so it should(?) be relatively simple to modify the global assignments to always be const.

In my mind, this has the benefit of closing a performance trap once and for all with no downsides and could be easy to implement.

[disberd]

I think this was not done at least in part because declaring variables as const prevents them from being deleted/garbage collected when they are removed from the notebook. Pluto deals with adding and removing variables by creating a new module at each execution and eventually deleting all variable from previous modules if they are not in the notebook anymore.

I seem to remember @fonsp telling at some point that if you have a const variable it can't be deleted this way

[fonsp]

Exactly! And because of #720 we are able to get the performance advantage of const already in many cases. ✨

[abraemer]

Ah that's very clever! Thank you both for your answers 😄