Bogus "Duplicate instance declarations:" error

[mouse07410]

Your environment

• MacOS Monterey 12.6 with Xcode-14
• GHC-9.4.2
• Cabal-3.8.1.0
• HLS 1.8.0.0 installed via ghcup
• hlint v3.4.1
• VS Code 1.71.2 with vscode-haskell (Haskell for VIsual Studio Code) v2.2.1

Steps to reproduce and Actual behavior

At some point in time, HLS complains about "Duplicate instance declarations" which don't make sense because both refer to the same definition - same file and line number (aka, the same place).

Here's an example:

Duplicate instance declarations:
  instance Functor Point3D
     -- Defined at /Users/ur20980/src/stepic-tests/src/Lib.hs:523:10
  instance Functor Point3D
     -- Defined at /Users/ur20980/src/stepic-tests/src/Lib.hs:523:10  typecheck

Expected behaviour

Understand that one entry in one file is not a duplicate of itself?

Debug information

This is an intermittent problem. Usually it clears up when I restart HLS within VS Code (don't even need to restart VS Code itself).

[lf-]

I've also hit this recently on ghc-9.4.2. My wild speculation is that it might be something like is described in this section, related to mishandling of multiple home units: https://well-typed.com/blog/2022/01/multiple-home-units/#closure-property-for-home-units

[mouse07410]

. . . mishandling of multiple home units . . .

I don't know - but I doubt that "multiple home units" is the cause, because in my case there are no such dependencies.

[michaelpj]

It sounds an issue with our hacked up support for multiple home units indeed, we should revisit this after the proper support is merged.

[mouse07410]

It sounds an issue with our hacked up support for multiple home units indeed

But I do not have multiple homes...? Or are you saying that "multiple home units" mistakenly triggers here?

[lf-]

likewise; in my project i hit this in, there's no home unit depending on non home unit depending on home unit problem. A and B are both home units.

dependency graph looks like A -> B, and the alleged duplicate instance was in B, from recollection.

[michaelpj]

There is an ambiguity. We currently have some hacks to load multiple components together "as if" they were multiple home units. This is not 100% reliable and can result in this kind of issue. You typically see it when working on a test or exe component as that's a point where we need to load multiple components.

"True" support for multiple home units is indeed not implemented yet.

[mouse07410]

@michaelpj thank you, now I understand.

In your opinion, how near are we to having this solved?

[michaelpj]

I'm not actually sure where we are with that, @fendor do we have a tracking issue?

[fendor]

Depends.
Fully correctly? Still pretty far.
Best effort, sort-of-kind-of correct? Actually very close, maybe by the end of the year?

[soulomoon]

I have another one which is off by a line?

  instance Hashable KeyValue
    -- Defined at /Users/ares/src/haskell-language-server/hls-graph/src/Development/IDE/Graph/Internal/Types.hs:145:10
  instance Hashable KeyValue
    -- Defined at /Users/ares/src/haskell-language-server/hls-graph/src/Development/IDE/Graph/Internal/Types.hs:146:10typecheck

also related to #3738?

[soulomoon]

The problem does goes away, swtiching to cabal head, with cabal's multiple home unites feature enabled

[PaulJohnson]

Just getting rid of most of these would be a benefit. Depending on which files I'm editing I can get hundreds of them, and real error messages are just buried.

[soulomoon]

Just getting rid of most of these would be a benefit. Depending on which files I'm editing I can get hundreds of them, and real error messages are just buried.

Is switching to cabal head help in your case? @PaulJohnson

[fendor]

It did in my tests

[michaelpj]

This should be fixed with the new multi-component support.