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
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[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