How exactly do I use Metals and VS Code Debugger? - scala

Question

Metals announced that "It is now possible to run and test directly from VS Code using the new "Run", "Test", "Debug" and "Debug test" buttons." There is a nice gif showing what it can do, and I don't know how to get to that point.
I tried to launch VS Code debugger with the following configurations in launch.json
{
// Use IntelliSense to learn about possible attributes.
// Hover to view descriptions of existing attributes.
// For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
"version": "0.2.0",
"configurations": [
{
"type": "scala",
"request": "launch",
"name": "Untitled",
"mainClass": "com.playZip.Unzip",
"args": [],
"jvmOptions": []
}
]
}
and got this error message:
Couldn't find a debug adapter descriptor for debug type 'scala' (extension might have failed to activate)
Somebody on Gitter scalameta/metals had this problem and the answer was that he needs Bloop to support utest and I think mine does because there's a file .bloop/play-zip-test.json in my sbt project, but I'm not 100% if my Bloop supports utest and what to do if it doesn't. I tried running bloop utest and it failed because I don't have Bloop CLI installed. I have Bloop that comes with Metals.

Answer 1

Document how to run or debug applications #2005 added official debugging documentation at Running and debugging your code which documents two approaches
via code lenses run | debug
via a launch.json configuration
Here is a hello world example how to debug a test using VSC and Metals via launch.json approach. We will use
lihaoyi/utest library and set a breakpoint in a test.
Execute sbt new scala/scala-seed.g8 to create correct project structure
Open... sbt project with VSC or simply cd into project and execute code .
Replace ScalaTest with utest in build.sbt
libraryDependencies += "com.lihaoyi" %% "utest" % "0.7.2" % "test",
testFrameworks += new TestFramework("utest.runner.Framework")
Replace test/scala/example/HelloSpec.scala with HelloTests.scala
package example
import utest._
object HelloTests extends TestSuite{
val tests = Tests{
test("test1"){
1
}
}
}
Import sbt build with View | Command Palette... | Metals: Import Build
Put a breakpoint at line 8 and click Run and Debug
Select Test Suite for Pick the kind of class to debug
Leave empty for Enter the name of the build target
Write example.HelloTests for Enter the name of the class to debug
Write Debug example.HelloTests for Enter the name of configuration
This should create .vscode/launch.json
{
"version": "0.2.0",
"configurations": [
{
"type": "scala",
"name": "Debug example.HelloTests",
"request": "launch",
"testClass": "example.HelloTests"
}
]
}
Now you should be able to Start Debugging by clicking the green triangle and stop at the breakpoint

Answer 2

Not sure if your problem is solved, but I did see the same issue before. To get more information about the error, you can check the Metals output. See the picture below:
From the output tab, select Metals. More detail of the error should be available.
In my case, the reason I got this error (Couldn't find a debug adapter descriptor for debug type 'scala' (extension might have failed to activate) ) is because the Java installed in my machine does not support JDI.
Message: Debugging is not supported because bloop server is running on a JRE /usr/lib/jvm/java-8-openjdk-amd64/jre with no support for Java Debug Interface: 'JDI implementation is not provided by the vendor'. To enable debugging, install a JDK and restart the bloop server.
I guess your case is probably the same. To solve it, just install a Java implementation supports JDI. For example, openjdk version "11.0.8" 2020-07-14 works fine with Metals on Ubuntu. You can do this to install it.
$ sudo apt install openjdk-11-jdk
If it still not works, make sure Metals: Java Home in VS Code settings points to the right Java version.

Answer 3

I ran into this same problem and it came down to the buildTarget. I have a multiple module project. When I looked at the metals logs this is what I saw:
Caused by: scala.MatchError: scala.meta.internal.metals.debug.BuildTargetNotFoundException: Build target not found: (of class scala.meta.internal.metals.debug.BuildTargetNotFoundException)
My Scala project
/client_accounts
/migrations
/app
Updated the launch.json to "buildTarget": "app", and it worked. Error reporting could be a bit better.
So if you get this error look at the logs for the root cause.