`recursive_bottom_eltype` breaks when given non-numeric type or value

[janlucaklees]

Hi all,

I'd like to discuss the implementation of the recursive_bottom_eltype function found in utils.jl on line 79 - 80.

I don't think the context is really necessary here. If you are curious, I added it at the end. I just happened to stumble across this.

So here's what I don't like:
The recursive_bottom_eltype seems to be written with only Numbers in mind, while at the same time not explicitly disallowing non-numeric values.
This leaves us with a function that works for numbers, but for Strings (and I suspect all other non-numeric type) it fails badly in a StackOverflowError. Because there is no other definition for non-numeric types other than the one that call itself recursively.

Steps to reproduce this error:
Run recursive_bottom_eltype("I love to eat up stack space") in your Julia shell.

How I would like it:
Instead of failing with a StackOverflowError the function should either directly throw an adequate error for non-numeric values, or it should be extended to also work for other types as well.

And here is where I would like to hear your suggestions. I don't know this Package really at all, so I can't tell which way would be the better one.

I would offer to implement a solution myself, after hearing your feedback on which way would be more suitable for this package.

As promised, the context:
I am trying to implement a dynamic solver (using DiffEqFlux) that uses a custom (not yet build) DSL. The idea is that this way the end-user does not need to learn Julia and how to handle any of the Packages involved, but just can model their problem using this DSL.
So I am at the beginning of this and rather than using a DLS I just use strings that will be evaluated during the solving with the Meta package. (I know, I know, performance and security; This is just to get things going and figure out other stuff in my project.) And so I ended up passing arrays of strings into the solving function of DiffEqFlux (more specifically in the problem for the solving function) rather than a arrays of numbers. And somehow diffeq_fd ends up calling recursive_bottom_eltype on the u0 or u array (not too sure form looking at their code). But as explained above, strings cause the recursive_bottom_eltype function to recurse infinitely. And thus, here I am complaining ;P

[ChrisRackauckas]

Yeah, a base case for strings would be accepted. I'm not sure how you're going to solve a differential equation of strings, but I guess * is defined so 🤷‍♂️ I'm not judging.

[janlucaklees]

Thank you for the feedback! I'll try and implement a solution right away.

I'm not sure how you're going to solve a differential equation of strings

I have some code that loops over the array of strings and evaluates them inside the callback for the problem.

[janlucaklees]

Done. See #55

EDIT
Never mind, let me figure out how to make my tests work for different platforms.

UPDATE
Fixed it. Thought the nightly builds fail as they cannot reach the Julia server for some reason. Trying the url in my browser it works. Not sure how to fix this.

[ChrisRackauckas]

Thanks!