Unable to inspect when debugging object extending App trait

[chrisbarker]

Bug
I can't inspect variables when I hit a breakpoint in an object which extends the App trait.

Steps to reproduce the behavior:

1. Set break point in code
2. Run debugger
3. Hit breakpoint
4. Unable to inspect variables

Expected behavior

I expect to be able to debug and inspect variables in an object which extends the App trait in the same way that I can with an object that implements a main entry point.

Screenshots
Expected

Bug

Installation:

• Operating system: Linux - Ubuntu 20.04.2 LTS

• Editor: Visual Studio Code v1.53.0

• Metals version: v1.9.13

Search terms
debug debugging inspect

[tgodzik]

Thanks for reporting! The behaviour is expected. The values in an object are not really local values due to have App trait works, which is why they are not shown in the second example. You should still be able to see them in this value:

[chrisbarker]

I can't see them in this This is what I see. New project using seed, then open proj folder in vscode and import project into metals, then debug.
This is not expandable to see variables it contains

[tgodzik]

It seems I can confirm this in a clean project. Not exactly sure why, the static variables shouldn't actually be currently seen unless we specify an additional setting, which however I did not set.

So what is unexpected is that it works in my case. We would really need to specify the additional setting showStaticVariables here:

https://github.com/scalacenter/bloop/blob/6e43cbfda8c67ea96a9595f6d768f4b5f8d2874b/frontend/src/main/scala/bloop/dap/DebugServer.scala#L33

though I worry about providing too much information really. I think it would better to add expression evalautor that would just allow to see it on demand.

[tgodzik]

Och, I think it may be the Scala version. It does work for me on 2.12.13, but doesn't on 2.13.4. What version are you using @chrisbarker ?

[chrisbarker]

@tgodzik 2.13.4

[tgodzik]

Och yeah, that fits. Though why it's happening on 2.13.4 is a mystery to me :O

[adpi2]

Since 2.13.x the fields of Scala objects are represented by static fields in bytecode (scala/scala#7270). It's the same in Scala 3.

So maybe you are right @tgodzik, we should enable the showStaticVariables setting. And if there is too much noise in it we can always filter...

[tgodzik]

I am not sure if I ever managed to check if it work with that option. I got kind of confused with the different Scala versions. Did you manage to confirm that it helps here?

[adpi2]

I confirm that the showStaticVariables works as expected. It shows the object fields, the trait fields and MODULE$ (that is synthetized by the compiler).

Here for instance, in an object extending App:

MODULE$ is a recursive reference to this (not very useful):

executionStart, scala$App$$_args and scala$App$initCode are coming from the App trait:

It is okay to show them but we should find a proper name for scala$App$$_args and scala$App$initCode as requested in scalacenter/scala-debug-adapter#38. They have weird names because they are private in a trait.

[tgodzik]

I think we can turn it on by default and make the names nicer later on. It's better to have more info and in this case Scala usage of static members is more prevalent than in Java I think.