Add Standard ML language

src/modules/languages/standardml.nix

@@ -0,0 +1,27 @@
+{ pkgs, config, lib, ... }:
+
+let
+  cfg = config.languages.standardml;
+in
+{
+  options.languages.standardml = {
+    enable = lib.mkEnableOption "tools for Standard ML development";
+
+    package = lib.mkOption {
+      type = lib.types.package;
+      default = pkgs.mlton;
+      defaultText = "pkgs.mlton";
+      description = ''
+        The Standard ML package to use.
+      '';
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    packages = with pkgs; [
+      cfg.package
+      millet
+      smlfmt
+    ];
+  };
+}