feat: add ReScript formatter (#293)

README.md

@@ -259,6 +259,7 @@ You can view this list in vim with `:help conform-formatters`
 - [pretty-php](https://github.com/lkrms/pretty-php) - The opinionated PHP code formatter.
 - [puppet-lint](https://github.com/puppetlabs/puppet-lint) - Check that your Puppet manifests conform to the style guide.
 - [reorder-python-imports](https://github.com/asottile/reorder-python-imports) - Rewrites source to reorder python imports
+- [rescript-format](https://github.com/rescript-lang/rescript-compiler) - The built-in ReScript formatter.
 - [rubocop](https://github.com/rubocop/rubocop) - Ruby static code analyzer and formatter, based on the community Ruby style guide.
 - [rubyfmt](https://github.com/fables-tales/rubyfmt) - Ruby Autoformatter! (Written in Rust)
 - [ruff_fix](https://beta.ruff.rs/docs/) - An extremely fast Python linter, written in Rust. Fix lint errors.

lua/conform/formatters/rescript-format.lua

@@ -0,0 +1,40 @@
+-- The formatter expects one of [.res | .resi | .ml | .mli] passed as
+-- the value to the '-stdin' argument.
+local valid_extensions = {
+  res = true,
+  resi = true,
+  ml = true,
+  mli = true,
+}
+
+local default_extension = "res"
+
+---@type conform.FileFormatterConfig
+return {
+  meta = {
+    url = "https://rescript-lang.org/",
+    description = "The built-in ReScript formatter.",
+  },
+  command = "rescript",
+  args = function(self, ctx)
+    local extension = vim.fn.fnamemodify(ctx.filename, ":e")
+
+    local is_invalid_extension = valid_extensions[extension] == nil
+    if is_invalid_extension then
+      extension = default_extension
+    end
+
+    return {
+      "format",
+      "-stdin",
+      "." .. extension,
+    }
+  end,
+  stdin = true,
+
+  require_cwd = true,
+  cwd = require("conform.util").root_file({
+    "rescript.json",
+    "bsconfig.json",
+  }),
+}