`nix fmt` support

[opeik]

Hi there,

I'm using vscode-nix-ide which breaks if you ask nil to format with nix fmt. Instead of formatting the file, it clears it instead. This seems to be caused by nix fmt not accepting input from stdin.

On a side note, it would be cool if nil could use self.formatter."${system}" by default.

VSCode configuration

"nix.enableLanguageServer": true,
"nix.serverPath": "nil",
"nix.serverSettings": {
  "nil": {
    "formatting": {
      "command": ["nix", "fmt"]
    }
  }
},

App versions

❯ nix --version
nix (Nix) 2.11.1

❯ nil --version
nil 2022-10-03 release

❯ code --list-extensions --show-versions
alefragnani.Bookmarks@13.3.1
eamodio.gitlens@13.3.2
esbenp.prettier-vscode@9.10.4
golang.go@0.38.0
GraphQL.vscode-graphql@0.8.6
GraphQL.vscode-graphql-syntax@1.1.0
Gruntfuggly.todo-tree@0.0.224
jnoortheen.nix-ide@0.2.1
marp-team.marp-vscode@2.5.0
monokai.theme-monokai-pro-vscode@1.2.0
ms-azuretools.vscode-docker@1.24.0
ms-python.python@2023.4.1
ms-python.vscode-pylance@2023.3.20
ms-vscode-remote.remote-ssh@0.98.0
ms-vscode-remote.remote-ssh-edit@0.84.0
ms-vscode.remote-explorer@0.2.0
ms-vsliveshare.vsliveshare@1.0.5834
quicktype.quicktype@12.0.46
redhat.vscode-yaml@1.12.1
rust-lang.rust-analyzer@0.3.1426
skellock.just@2.0.0
streetsidesoftware.code-spell-checker@2.19.0
tamasfe.even-better-toml@0.19.0
timonwong.shellcheck@0.29.4
usernamehw.errorlens@3.7.0
vadimcn.vscode-lldb@1.8.1
vscodevim.vim@1.25.2

[oxalica]

This seems to be caused by nix fmt not accepting input from stdin.

nix fmt automatically add an argument . to the underlying formatter if there is none, which makes it format the whole directory instead of using stdio interface.

After some digging, I found that you setting formatting command to ["nix", "fmt", "--", "--"] work for me, as long as the underlying formatter supports stdio interface.

On a side note, it would be cool if nil could use self.formatter."${system}" by default.

I'm afraid not. nix fmt requires dumping the whole directory, evaluation and maybe building on every change. It would cause unacceptable delay/hanging with format-on-save.

My suggestion is to set fixed and responsive formatter for nil instead, and use nix fmt only in CI or pre-commit hooks.

[figsoda]

maybe nil can evaluate .#formatter.${system} once at startup and use that as a fallback to nil.formatting.command if it exists

[oxalica]

maybe nil can evaluate .#formatter.${system} once at startup and use that as a fallback to nil.formatting.command if it exists

formatter interface uses file IO by design, which is not compatible with ours. So we cannot do it in general. If only Nix Flakes can enforce the stdio interface by using, to say nix fmt -- -, then it can be guaranteed to work.

[oxalica]

XRef: NixOS/nix#8063

[nyabinary]

This seems to be caused by nix fmt not accepting input from stdin.

nix fmt automatically add an argument . to the underlying formatter if there is none, which makes it format the whole directory instead of using stdio interface.

After some digging, I found that you setting formatting command to ["nix", "fmt", "--", "--"] work for me, as long as the underlying formatter supports stdio interface.

On a side note, it would be cool if nil could use self.formatter."${system}" by default.

I'm afraid not. nix fmt requires dumping the whole directory, evaluation and maybe building on every change. It would cause unacceptable delay/hanging with format-on-save.

My suggestion is to set fixed and responsive formatter for nil instead, and use nix fmt only in CI or pre-commit hooks.

["nix", "fmt", "--", "-"] also works :3

[losnappas]

nix fmt (with nix fmt -- - using nixfmt-rfc-style) is incredibly slow, any methods of speeding it up?

[sellout]

nix fmt (with nix fmt -- - using nixfmt-rfc-style) is incredibly slow, any methods of speeding it up?

My trick for speeding up nix fmt is to get an alias into my environment. E.g.,

devShells.${system}.default = pkgs.mkShell {
  shellHook = ''
    alias flake-fmt=${self.formatter.${system}.meta.mainProgram}
  '';
};

this avoids the need to evaluate the flake on each execution of the formatter and now I can set up flake-fmt to run-on-save in my editor and it’s very speedy.

Unfortunately, it doesn’t work quite as smoothly with direnv/use flake, but direnv/direnv#73 has a lot of workarounds for getting aliases into the shell from direnv.

[sellout]

I’ve modified this a bit. My infra is a bit more involved, but it boils down to

devShells.${system}.default = pkgs.mkShell {
  shellHook = ''
    export PROJECT_FORMATTER=${lib.getExe self.formatter.${system}}
  '';
};

and then I have a script on the PATH that does roughly

"$PROJECT_FORMATTER" "$@"

And this now has no issues with using Direnv.

[oxalica]

As mentioned by #70 (comment), setting formatting command to ["nix", "fmt", "--", "-"] should work if the formatter obeys the CLI convention and that's your intention.

I lean towards NOT making this the default, since:

1. The CLI of .#formatter is not standardized. The formatter may accept stdin, or requires --, or requires to be a file, nobody knows.
2. It's slow to build from user flake. Note that currently, any flake operation for nix does a tree dump, effectively copying the whole working-tree to store every time when you save.
3. It's a security risk to run an arbitrary package specified by the repository content (usually on every save!), especially when you run LSP in a foreign one.
4. Since [RFC 0166] Nix formatting, take two NixOS/rfcs#166, we already has an "official formatter" which serves as a better default choice (89e5a4e).

Thus I don't think this issue is actionable now.