Debugging Flow

[agentcooper]

I am using Visual Studio Code with vscode-ocaml extension. ./bin/flow is compiled using make build-flow-debug (see also: #4177).

.vscode/launch.json:

{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "OCaml",
      "type": "ocamldebug",
      "request": "launch",
      "program": "${workspaceRoot}/bin/flow",
      "console": "internalConsole",
      "stopOnEntry": false,
      "arguments": [
        "${workspaceRoot}/local-test"
      ]
    }
  ]
}

local-test/test.js:

// @flow

const a: number = 'z' + 'd';

Debugging main seems to work:

However If I set a breakpoint somewhere in src/typings/, for example statement.ml, inside variable_decl function, then the breakpoint is skipped.

I suspect this is because of the server-client architecture - actual type checking happens inside a different process.

Is there any good way around this?

[kogai]

I faced the same problem and I didn't solve it.

For the reference, I tried flow check command.
It seems to be checking type at foreground, but when using ocamlbuild with settings like below,
debugger responds with cannot connect to debugger at 127.0.0.1 error: No such file or directory.

{
    "program": "${workspaceRoot}/bin/flow",
    "arguments": ["check", "path/to/mycode.js"],
}

The error occurred at spawn function in hack/utils/daemon.ml(yes, debugger worked properly), and I guess that the error related to spawning server process.

[kogai]

Still examining, but it seems to get some progress.

As I guessed at the comment above, the problem caused by some process that created behind flow.

Then, I try out arguments "--max-workers" that described with Maximum number of workers to create (capped by number of cores)

I don't understand it in detail, but it seems related to some process behind flow.
So, I modified my settings like follows and executed ocamldebug.

{
    "program": "${workspaceRoot}/bin/flow",
    "arguments": ["check", "--max-workers", "0", "path/to/mycode.js"],
}

And the result looks as I expected.
At least it can execute flow according to breakpoint and process exit normally.

[villesau]

@agentcooper @kogai have you tried to get debugger working with OCaml Debugger? I can get the debugger working with simple example using ocamlc -g helloworld.ml but with Flow pretty much nothing happens, only the loading sign appears below debug menu.

[villesau]

Looks like ocaml debugger runs roughly this kind of command: ocamlrun bin/flow check my-own-test/test.js --max-workers 0, when running the flow with ocamlrun the program throws

Fatal error: unknown C primitive `unix_waitpid'

Maybe this is the culprit?

E: When downgrading ocaml from 4.06.0 to 4.05.0, the error changes to:

Fatal error: unknown C primitive `caml_floatarray_create'

E2: When removing _build and running the make build-flow-debug from scratch with 4.05.0, the error is again

Fatal error: unknown C primitive `unix_waitpid'

E3:

After pulling the latest flow, the error changed to

Fatal error: unknown C primitive `Base_am_testing'

Any hints on how to fix this? would e.g @jbrown215 be able to shed a bit light on the Flow & OCaml debugging setup?

[jbrown215]

I don't use the ocaml debugger, but flow has a few debugging commands that I use to debug type checker behavior. flow check --verbose-indent will show the full list of flows in __flow. The function that adds errors is called add_output, and it also logs when it is called with --verbose-indent.

[villesau]

Nice, this probably already helps a lot, thanks for being so helpful!

[villesau]

Ok, got debugger finally working in VS Code!

Build command

make build-flow-debug

VS Code debugger config

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": "OCaml",
            "type": "ocaml-debugger",
            "request": "launch",
            "program": "${workspaceFolder}/bin/flow",
            "console": "internalConsole",
            "stopOnEntry": false,
            "arguments": ["check", "${workspaceFolder}/local-test/test.js", "--max-workers=0", "--verbose-indent"]
          }
    ]
}

• ${workspaceFolder}/local-test/test.js being the js file to check
• --max-workers=0 apparently runs everything in single process
• --verbose-indent Debugger works also without, but still very useful

Other requirements

• OCaml 4.05.0 <- later versions might work too, I only tested with 4.05.0
• OCaml & ReasonML plugin: https://marketplace.visualstudio.com/items?itemName=freebroccolo.reasonml
• VS Code debugger plugin: https://marketplace.visualstudio.com/items?itemName=hackwaly.ocaml-debugger
• debugger adapter: https://github.com/hackwaly/ocamlearlybird/
  • needs to be installed with opam: opam install earlybird
  • NOTE: Needs to be at least 0.1.3 version, earlier versions does not work

I also recommend to install Merlin: https://github.com/ocaml/merlin

opam install merlin
opam user-setup install

This might be worth documenting to e.g Flow wiki or readme.

[kevinbarabash]

I would also suggest using --merge-timeout=0 as well to avoid timeouts.

[kevinbarabash]

@villesau thanks for the instructions. Being able to step through things is going to be a big help.

[STRML]

Thanks @villesau for such a useful comment.

I was unable to get mine running until I added to my user settings.json:

{
    "reason.path.ocamlmerlin": "_opam/bin/ocamlmerlin",
    "reason.diagnostics.tools": [
        "merlin"
    ],
    "editor.formatOnSave": true,
    "reason.codelens.enabled": true,
}

and to my debug configuration:

  /* start ocamlearlybird --server --port 4711 before running */
  "debugServer": 4711

I then run $ ocamlearlybird --server --port 4711 in the Flow directory before debugging.

[SamChou19815]

Closing since none of our team member uses debuggers regularly, so we also have no ideas...