fix(nix_flake_fmt): handle flakes with a `formatter` package #272

jfly · 2025-04-23T00:09:26Z

This fixes a bug with nix_flake_fmt where it doesn't handle flakes with a "formatter" package. I also did some refactoring in separate commits keep things clean. I suggest reading the commits independently.

refactor(fix nix_flake_fmt): split find_nix_fmt into smaller functions
refactor(nix_flake_fmt): pass data as JSON rather than ad-hoc lines
fix(nix_flake_fmt): handle flakes with a formatter package

IMO, this plugin has too much nix specific knowledge in it. I've submitted NixOS/nix#13063 to core nix to add a nix fmt --print-command option. That would allow us to delete a lot of code in this PR.

This should make things a bit more readable without changing the behavior.

This makes the communication between our lua code and the `nix eval` subprocess a bit clearer.

I maintain a vim plugin that automatically runs `nix fmt` on files on save. Since `nix fmt` can be quite slow due to nix evaluation, I choose to cache the `nix fmt `entrypoint. This was very awkward to do, see the implementation for details: https://github.com/nvimtools/none-ls.nvim/blob/786460723170bda9e9f95c55a382d21436575297/lua/null-ls/builtins/formatting/nix_flake_fmt.lua#L83-L110. I recently discovered that my implementation was buggy (it didn't handle flakes that expose a `formatter` package, such as nixpkgs), so I had to rework the implementation: nvimtools/none-ls.nvim#272. With this extra option for `nix fmt`, I'd get to delete all this akward code, and it would be easier for other folks to build performant editor integrations for `nix fmt`.

For example, here's what this does in `nixpkgs`: $ nix fmt --print-command /nix/store/cb9w44vkhk2x4adfxwgdkkf5gjmm856j-treefmt/bin/treefmt Motivation ========== I maintain a vim plugin that automatically runs `nix fmt` on files on save. Since `nix fmt` can be quite slow due to nix evaluation, I choose to cache the `nix fmt `entrypoint. This was very awkward to do, see the implementation for details: https://github.com/nvimtools/none-ls.nvim/blob/786460723170bda9e9f95c55a382d21436575297/lua/null-ls/builtins/formatting/nix_flake_fmt.lua#L83-L110. I recently discovered that my implementation was buggy (it didn't handle flakes that expose a `formatter` package, such as nixpkgs), so I had to rework the implementation: nvimtools/none-ls.nvim#272. With this extra option for `nix fmt`, I'd get to delete all this akward code, and it would be easier for other folks to build performant editor integrations for `nix fmt`.

For example, here's what this does in `nixpkgs`: $ nix fmt --print-command /nix/store/cb9w44vkhk2x4adfxwgdkkf5gjmm856j-treefmt/bin/treefmt Motivation ---------- I maintain a vim plugin that automatically runs `nix fmt` on files on save. Since `nix fmt` can be quite slow due to nix evaluation, I choose to cache the `nix fmt `entrypoint. This was very awkward to do, see the implementation for details: https://github.com/nvimtools/none-ls.nvim/blob/786460723170bda9e9f95c55a382d21436575297/lua/null-ls/builtins/formatting/nix_flake_fmt.lua#L83-L110. I recently discovered that my implementation was buggy (it didn't handle flakes that expose a `formatter` package, such as nixpkgs), so I had to rework the implementation: nvimtools/none-ls.nvim#272. With this extra option for `nix fmt`, I'd get to delete all this akward code, and it would be easier for other folks to build performant editor integrations for `nix fmt`.

nixpkgs [recently add a `nix fmt` entrypoint](NixOS/nixpkgs@374e6bc), and I was excited to be able to use `nix_flake_fmt` in the project. I was disappointed to find that it doesn't actually work in nixpkgs, though :( The problem boils down to how `nix eval .#formatter` behaves if your flake defines a `formatter` package. See: ```console $ nix eval nixpkgs#formatter «derivation /nix/store/nadgq8kci2an9d3ddz1lk662gl53d8yz-formatter-0.4.0.drv» ``` This is *not* the `nix fmt` entrypoint, it's a [completely unrelated package](NixOS/nixpkgs@b99357e). Here's the `nix fmt` entrypoint: ```console $ nix repl nixpkgs nix-repl> formatter.x86_64-linux «derivation /nix/store/rza8g2jxr1h7nd58qq3ryfz2zdqnggga-treefmt.drv» ``` This simple `nix repl` session is quite challenging to port to `nix eval`, though. After quite a bit of futzing around, I found that I could accomplish this with a combination of `nix flake metadata` and `builtins.getFlake`: ``` $ nix flake metadata nixpkgs --json | jq .resolvedUrl "github:NixOS/nixpkgs/nixpkgs-unstable" $ nix eval --expr "(builtins.getFlake "github:NixOS/nixpkgs/nixpkgs-unstable").formatter.x86_64-linux" --impure «derivation /nix/store/rza8g2jxr1h7nd58qq3ryfz2zdqnggga-treefmt.drv» ```

…ation There were 2 bugs here: - In the `drv_path == nil` path, we'd abort execution without returning a result by calling `done(...)`. - In that codepath and another, we'd return a `nil` result without clearing the progress notification. There might be bugs here if we are running multiple formats in parallel. I think that's OK to address in the future if it proves to be a problem.

for more information, see https://pre-commit.ci

`nix formatter run` is an alias for `nix fmt`. Nothing new there. `nix formatter build` is sort of like `nix build`: it builds, links, and prints a path to the formatter program: $ nix formatter build /nix/store/cb9w44vkhk2x4adfxwgdkkf5gjmm856j-treefmt/bin/treefmt Note that unlike `nix build`, this prints the full path to the program, not just the store path (in the example above that would be `/nix/store/cb9w44vkhk2x4adfxwgdkkf5gjmm856j-treefmt`). Motivation ---------- I maintain a vim plugin that automatically runs `nix fmt` on files on save. Since `nix fmt` can be quite slow due to nix evaluation, I choose to cache the `nix fmt `entrypoint. This was very awkward to do, see the implementation for details: https://github.com/nvimtools/none-ls.nvim/blob/786460723170bda9e9f95c55a382d21436575297/lua/null-ls/builtins/formatting/nix_flake_fmt.lua#L83-L110. I recently discovered that my implementation was buggy (it didn't handle flakes that expose a `formatter` package, such as nixpkgs), so I had to rework the implementation: nvimtools/none-ls.nvim#272. With the new `nix formatter build` command, I can delete all this akward code, and it will be easier for other folks to build performant editor integrations for `nix fmt`.

`nix formatter build` is sort of like `nix build`: it builds, links, and prints a path to the formatter program: $ nix formatter build /nix/store/cb9w44vkhk2x4adfxwgdkkf5gjmm856j-treefmt/bin/treefmt Note that unlike `nix build`, this prints the full path to the program, not just the store path (in the example above that would be `/nix/store/cb9w44vkhk2x4adfxwgdkkf5gjmm856j-treefmt`). Motivation ---------- I maintain a vim plugin that automatically runs `nix fmt` on files on save. Since `nix fmt` can be quite slow due to nix evaluation, I choose to cache the `nix fmt `entrypoint. This was very awkward to do, see the implementation for details: https://github.com/nvimtools/none-ls.nvim/blob/786460723170bda9e9f95c55a382d21436575297/lua/null-ls/builtins/formatting/nix_flake_fmt.lua#L83-L110. I recently discovered that my implementation was buggy (it didn't handle flakes that expose a `formatter` package, such as nixpkgs), so I had to rework the implementation: nvimtools/none-ls.nvim#272. With the new `nix formatter build` command, I can delete all this akward code, and it will be easier for other folks to build performant editor integrations for `nix fmt`.

barrett-ruth · 2025-04-27T01:05:58Z

NGL, feel like this should go into extras if it 300 LOC for a formatter.

jfly · 2025-04-27T03:11:54Z

That's fair, @barrett-ruth. I am hopeful that nix will accept my PR, which will allow us to delete a lot of code here. I don't think this builtin has a lot of users (yet ;)), so I could see doing any of the following (in no particular order):

Wait for my PR to nix to land.
Merge this more or less as is, knowing that we'll get to delete a lot of code once my PR lands.
Put this in none-ls-extras.

My preference would be 2: I joined the nix team meeting this week, and it seemed likely they'll accept my PR. But I'm happy to tackle this however you prefer!

barrett-ruth · 2025-04-27T21:23:18Z

Super cool that you joined there. I'll merge this for now. Please delete the code once merged!

jfly · 2025-04-27T21:30:28Z

Thanks, @barrett-ruth!

`nix formatter build` is sort of like `nix build`: it builds, links, and prints a path to the formatter program: $ nix formatter build /nix/store/cb9w44vkhk2x4adfxwgdkkf5gjmm856j-treefmt/bin/treefmt Note that unlike `nix build`, this prints the full path to the program, not just the store path (in the example above that would be `/nix/store/cb9w44vkhk2x4adfxwgdkkf5gjmm856j-treefmt`). Motivation ---------- I maintain a vim plugin that automatically runs `nix fmt` on files on save. Since `nix fmt` can be quite slow due to nix evaluation, I choose to cache the `nix fmt `entrypoint. This was very awkward to do, see the implementation for details: https://github.com/nvimtools/none-ls.nvim/blob/786460723170bda9e9f95c55a382d21436575297/lua/null-ls/builtins/formatting/nix_flake_fmt.lua#L83-L110. I recently discovered that my implementation was buggy (it didn't handle flakes that expose a `formatter` package, such as nixpkgs), so I had to rework the implementation: nvimtools/none-ls.nvim#272. With the new `nix formatter build` command, I can delete all this akward code, and it will be easier for other folks to build performant editor integrations for `nix fmt`.

jfly · 2025-05-01T20:03:13Z

@barrett-ruth, my PR landed! I've reworked things over in #279

jfly added 2 commits April 22, 2025 17:12

refactor(fix nix_flake_fmt): split find_nix_fmt into smaller functions

b75f346

This should make things a bit more readable without changing the behavior.

refactor(nix_flake_fmt): pass data as JSON rather than ad-hoc lines

4f76cb4

This makes the communication between our lua code and the `nix eval` subprocess a bit clearer.

jfly force-pushed the fix-nix-flake-fmt branch from bec4823 to 5727f0b Compare April 23, 2025 00:12

jfly mentioned this pull request Apr 23, 2025

Add nix formatter build and nix formatter run commands NixOS/nix#13063

Merged

jfly force-pushed the fix-nix-flake-fmt branch from 5727f0b to 128c131 Compare April 23, 2025 17:09

jfly and others added 2 commits April 23, 2025 10:20

[pre-commit.ci] auto fixes from pre-commit.com hooks

b77872c

for more information, see https://pre-commit.ci

barrett-ruth merged commit 04213ab into nvimtools:main Apr 27, 2025
3 checks passed

jfly deleted the fix-nix-flake-fmt branch April 27, 2025 21:30

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fix(nix_flake_fmt): handle flakes with a `formatter` package #272

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fix(nix_flake_fmt): handle flakes with a `formatter` package #272

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