How to debug Julia simulation codes (ODEs, optimization, etc.!)

The ODE solver spit out dt<dtmin, what do you do? MethodError Dual{...}, what does it mean? Plenty of people ask these questions every day. In this talk I'll walk through the steps of debugging Julia simulation codes and help you get something working!

Debugging simulation codes can be very different from "standard" or "simple" codes. There's many details that can show up that the user needs to be aware of. Thus while there have been many beginner tutorials in using Julia, and many tutorials on how to using SciML ecosystem tools like DifferentialEquations.jl, there has never been a tutorial that says "okay, I got this error when using Optim.jl, what do I do now?". Some major pieces have been written which condense such information, such as the DifferentialEquations.jl PSA on Discourse (https://discourse.julialang.org/t/psa-how-to-help-yourself-debug-differential-equation-solving-issues/62489), but we believe there remains many things to say.

And also, a video walkthrough is simply the best way to "show some how I do it" so to speak.

So let's do it! But what would this entail? There are many topics to cover, including:

• How to read the gigantic stack traces that arise from dual number issues. Why does f(du,u::Array{Float64},p,t) fail with this error? Why can dual numbers cause issues in some mutating code? How do you use https://github.com/SciML/PreallocationTools.jl to solve these Dual number issues?
• When trying to debug code deep within some package context, how do you do it in a "nice" way (i.e. without the slow interpreted mode of Debugger.jl)? The answer is using Revise with tools like @show and x=Ref{Any}() and then in the package you can do Main.x[] = .... Never seen this trick before? Well then you'll be interested in this talk. We'll showcase how to use these tricks in real-world contexts where such debugging arises.
• When you get dt<dtmin or other ODE solver exit warnings? What are you supposed to do? u'=u^2-u with u(0)=2, oh wait analytically that should error? How do I find out if my model is written incorrectly (it is) and how do I figure out what I should be changing?
• When doing optimization, say using GalacticOptim.jl or Flux.jl, what are these "Zygote does not support mutation" errors? Why do they exist and how do I work around them?
• Everyone is asking for an MWE. How do I make a good MWE? How do I figure out what in the package that is likely causing the issue, and use this to help developers help me?

I will continue to grow the list by keeping tabs on what comes up the most often in Github issues and Discourse posts. At the end of the day, I hope this can be a video that is pasted onto thousands of Discourse questions to give people a much more in-depth view of how to fix issues, and potentially train the next generation of "Discourse answerers".