Enable formatter for explicit use only

[MoritzBoehme]

Hey, I was wondering if there is a possibility to enable a formatter for explicit use only.
For example if I want to format a nix file I want to use nixpkgs-fmt as the default, but only sometimes I want to use alejandra (not always, as it is more opinionated than the default formatter).
In the cases where I want to use the non default formatter, my ideal usage would just be to call :Format alejandra. When calling :Format on a nix file it would ideally ignore alejandra, because I didn't explicitly choose it.
Is this or something alike possible?
Could I perhaps wrap the default config for alejandra to detect if it was called explicitly?

[akanmuratcimen]

You can use something like this:

vim.api.nvim_create_user_command("Fmt", function(opts)
  local params = vim.split(opts.args, "%s+", { trimempty = true })
  vim.cmd("set filetype=" .. params[1] .. " | :Format")
end, {
  bang = true,
  nargs = 1,
  complete = function()
    return { "json", "xml", "yaml" }
  end,
})

example usage: :Fmt json

[MoritzBoehme]

Maybe I could just add a new file type with the name "alejandra" and then use your function to format the current file with this. And using the completion maybe I could even make it smarter such that nix files get only the completion options for the fake formatter languages for nix.
I'll try this and update as soon as possible.

[MoritzBoehme]

Great, this works well enough.
For anyone wanting to replicate this:

require("formatter").setup({
  filetype = {
    -- HACK to use specific formatters only when specified
    alejandra = {
      require("formatter.filetypes.nix").alejandra,
    },
  },
})

vim.api.nvim_create_user_command("Fmt", function(opts)
  local params = vim.split(opts.args, "%s+", { trimempty = true })
  local filetype = vim.bo.filetype
  vim.cmd("set filetype=" .. params[1]) -- fake filetype
  vim.cmd(":Format")
  vim.cmd("set filetype=" .. filetype) -- restore original filetype
end, {
  nargs = 1,
  complete = function()
    local languages = {
      nix = { "alejandra" },
    }
    return languages[vim.bo.filetype] or {}
  end,
})

[akanmuratcimen]

Thanks for sharing your solution.

[MoritzBoehme]

Considering this works well enough for me, I think this issue is closed.