[ocamlformat]: Version detection

programs/ocamlformat.nix

@@ -4,17 +4,46 @@
 , ...
 }:
 let
+  l = lib // builtins;
   cfg = config.programs.ocamlformat;
+
+  detectVersion = configFile: pkgSet:
+    let
+      optionValue = list:
+        assert l.assertMsg (list != [ ]) "treefmt-nix: Unable to detect ocamlformat version from file";
+        l.elemAt list (l.length list - 1);
+    in
+    l.getAttr "ocamlformat_${
+      l.replaceStrings ["."] ["_"]
+      (optionValue (l.findFirst (option: l.head option == "version") []
+          (l.map (n: l.splitString "=" n) (l.splitString "\n" (l.readFile configFile)))))
+    }"
+      pkgSet;
 in
 {
   options.programs.ocamlformat = {
-    enable = lib.mkEnableOption "ocamlformat";
-    package = lib.mkPackageOption pkgs "ocamlformat" { };
+    enable = l.mkEnableOption "ocamlformat";
+    package = l.mkPackageOption pkgs "ocamlformat" { };
+
+    pkgs = l.mkOption {
+      type = l.types.lazyAttrsOf l.types.raw;
+      description = "The package set used to get the ocamlformat package at a specific version. defaults to nixpkgs.";
+      default = pkgs;
+    };
+
+    configFile = l.mkOption {
+      type = l.types.nullOr l.types.path;
+      description = "Path to the .ocamlformat file. Used to pick the right version of ocamlformat if passed.";
+      default = null;
+    };
   };
 
-  config = lib.mkIf cfg.enable {
+  config = l.mkIf cfg.enable {
     settings.formatter.ocamlformat = {
-      command = cfg.package;
+      command =
+        if l.isNull cfg.configFile
+        then cfg.package
+        else detectVersion cfg.configFile cfg.pkgs;
       options = [ "-i" ];
       includes = [ "*.ml" "*.mli" ];
     };