Fix finding the standard library for pnpm

Author: cknitt

compiler/ext/config.ml

@@ -1,23 +1,64 @@
 let version = "4.06.1+BS"
 
-(* FIXME: Unreliable resolution *)
+exception Cannot_find_standard_library of string
+
 let standard_library =
+  (* This resolves the location of the standard library starting from the location of bsc.exe.
+     Unfortunately it is rather hacky, as we have to detect and handle several cases here:
+     1.) Local development in the `rescript` repo
+     2.) Packages installed via npm
+     3.) Packages installed via pnpm *)
+  let bin_dir = Filename.dirname Sys.executable_name in
+  let dir_sep = Filename.dir_sep in
   let ( // ) = Filename.concat in
-  let exe_path = Sys.executable_name in
-  if Ext_string.contain_substring exe_path ("node_modules" // "@rescript") then
-    (* node_modules/@rescript/{platform}/bin *)
-    Filename.dirname exe_path // Filename.parent_dir_name
-    // Filename.parent_dir_name // Filename.parent_dir_name // "rescript"
-    // "lib" // "ocaml"
-  else if Ext_string.contain_substring exe_path ("node_modules" // "rescript")
-  then
-    (* node_modules/rescript/{platform} *)
-    Filename.dirname exe_path // Filename.parent_dir_name // "lib" // "ocaml"
-  else
-    (* git repo: rescript/packages/@rescript/{platform}/bin *)
-    Filename.dirname exe_path // Filename.parent_dir_name
-    // Filename.parent_dir_name // Filename.parent_dir_name
-    // Filename.parent_dir_name // "lib" // "ocaml"
+
+  (* Find position of /node_modules/ in bin_dir *)
+  match Ext_string.find ~sub:(dir_sep ^ "node_modules" ^ dir_sep) bin_dir with
+  | -1 ->
+    (* 1. Not installed in node_modules => local development in the `rescript` repo
+     - bin:    <repo>/packages/@rescript/{platform}/bin
+     - stdlib: <repo>/lib/ocaml
+    *)
+    let up = Filename.parent_dir_name in
+    bin_dir // up // up // up // up // "lib" // "ocaml"
+  | i -> (
+    (* to account for the / before node_modules *)
+    let i = i + 1 in
+    let project_path = String.sub bin_dir 0 i in
+    let subpath = String.sub bin_dir i (String.length bin_dir - i) in
+
+    match subpath |> String.split_on_char dir_sep.[0] with
+    (* 2. Packages installed via npm
+     - bin:    node_modules/@rescript/{platform}/bin
+     - stdlib: node_modules/rescript/lib/ocaml
+    *)
+    | ["node_modules"; "@rescript"; _platform; "bin"] ->
+      project_path // "node_modules" // "rescript" // "lib" // "ocaml"
+    (* 3. Packages installed via pnpm
+     - bin:    node_modules/.pnpm/@[email protected]/node_modules/@rescript/darwin-arm64/bin
+     - stdlib: node_modules/.pnpm/[email protected]/node_modules/rescript/lib/ocaml
+    *)
+    | [
+     "node_modules";
+     ".pnpm";
+     platform_package;
+     "node_modules";
+     "@rescript";
+     _platform;
+     "bin";
+    ] -> (
+      match platform_package |> String.split_on_char '@' with
+      | [""; _prefix; version] ->
+        project_path // "node_modules" // ".pnpm" // ("rescript@" ^ version)
+        // "node_modules" // "rescript" // "lib" // "ocaml"
+      | _ ->
+        raise
+          (Cannot_find_standard_library
+             ("Cannot parse pnpm platform package: " ^ platform_package)))
+    | _ ->
+      raise
+        (Cannot_find_standard_library
+           ("Unrecognized node_modules structure: " ^ subpath)))
 
 let standard_library_default = standard_library