@@ -428,6 +428,18 @@ $ node
 > eval(result.js_code);
 ```
 
+### Lambda debug outputs (API v9)
+
+`comp.rescript.compileWithDebug(source)` returns `lambda` before the backend's
+Lambda optimization passes and `lambda_optimized` after the whole-term passes,
+before export grouping and JavaScript lowering. Both use the same Lambda printer
+so their text can be compared directly. JavaScript lowering performs further
+transformations; this snapshot is not the final JavaScript IR.
+
+Regular `comp.rescript.compile(source)` does not capture these debug outputs.
+Older bundles do not provide `lambda_optimized`; consumers should treat it as
+optional when supporting multiple API versions.
+
 ### Testing the Playground bundle
 
 Run `yarn workspace playground test` for a quick sanity check to see if all the build artifacts are working together correctly. When releasing the playground bundle, the test will always be executed before publishing to catch regressions.
 
@@ -0,0 +1 @@
+- Add before/after optimization views and inline diffs to the developer playground's Lambda tab, with optimized Lambda output in playground API v9, deferred hidden-panel updates, and debounced comparisons while editing. https://github.com/rescript-lang/rescript/pull/8636
@@ -254,7 +254,8 @@ let required_modules (lam : Lambda.t) : unit Lam_module_ident.Hash_set.t =
   collect lam;
   required
 
-let compile (output_prefix : string) export_idents hoisted (lam : Lambda.t) =
+let compile ?on_optimized_lambda (output_prefix : string) export_idents hoisted
+    (lam : Lambda.t) =
   let debug_ir = !((Js_config.current ()).debug_ir) in
   let diagnostics =
     if debug_ir then Some (Ir_diagnostics.create ~output_prefix) else None
@@ -329,6 +330,9 @@ let compile (output_prefix : string) export_idents hoisted (lam : Lambda.t) =
     lam
   in
 
+  (* Capture the final whole-term optimization result before export grouping
+     and JavaScript lowering. The callback is absent in normal compilation. *)
+  Option.iter (fun capture -> capture lam) on_optimized_lambda;
   let ({Lam_coercion.groups} as coerced_input), meta =
     Lam_coercion.coerce_and_group_big_lambda meta lam
   in
 
@@ -28,13 +28,17 @@
 *)
 
 val compile :
+  ?on_optimized_lambda:(Lambda.t -> unit) ->
   string ->
   Ident.t list ->
   Lambda.hoisted_function list ->
   Lambda.t ->
   J.deps_program
 (** For toplevel, [filename] is [""] which is the same as
     {!Env.get_unit_name ()}
+
+    [on_optimized_lambda], when supplied, observes the whole-term Lambda after
+    optimization and before export grouping and JavaScript lowering.
 *)
 
 val lambda_as_module : J.deps_program -> string -> unit
@@ -53,8 +53,9 @@
  * v6: Added `config.experimental_features` and `config.jsx_preserve_mode` to the BundleConfig.
  * v7: Added debug dump output APIs for developer playground tooling.
  * v8: Added genType and source map configuration and compilation outputs.
+ * v9: Added optimized Lambda output to debug compilation.
  * *)
-let api_version = "8"
+let api_version = "9"
 
 module Js = Js_of_ocaml.Js
 
@@ -670,8 +671,18 @@ module Compile = struct
       let {Translmod.lambda; exports; hoisted_functions} =
         Translmod.transl_implementation modulename typed_tree
       in
+      let optimized_lambda = ref None in
+      let on_optimized_lambda =
+        if include_debug_outputs then
+          Some
+            (fun lambda ->
+              optimized_lambda :=
+                Some (Printer.to_string Printlambda.lambda lambda))
+        else None
+      in
       let lambda_output =
-        Lam_compile_main.compile "" exports hoisted_functions lambda
+        Lam_compile_main.compile ?on_optimized_lambda "" exports
+          hoisted_functions lambda
       in
       let js_code, source_map =
         render_javascript ~module_system ~filename ~source:str ~source_map_mode
@@ -728,7 +739,15 @@ module Compile = struct
               ("lam", inject @@ Js.string lam);
             |]
         in
-        Js.Unsafe.obj (Array.concat [attrs; debug_attrs; gentype_attrs])
+        let optimized_lambda_attrs =
+          match !optimized_lambda with
+          | None -> [||]
+          | Some output ->
+            Js.Unsafe.[|("lambda_optimized", inject @@ Js.string output)|]
+        in
+        Js.Unsafe.obj
+          (Array.concat
+             [attrs; debug_attrs; gentype_attrs; optimized_lambda_attrs])
       else Js.Unsafe.obj attrs
     with e -> (
       match e with
 
@@ -9,7 +9,7 @@
     "prepare-pages-site": "node scripts/prepare-pages-site.mjs",
     "res:build": "rescript",
     "res:watch": "rescript -w",
-    "test": "rescript && node --test scripts/source-map-navigation.test.mjs scripts/pane-layout.test.mjs",
+    "test": "rescript && node --test scripts/source-map-navigation.test.mjs scripts/pane-layout.test.mjs scripts/lambda-diff.test.mjs",
     "dev": "vite --host 127.0.0.1",
     "build": "rescript && vite build",
     "preview": "vite preview --host 127.0.0.1"
Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	`+- Add before/after optimization views and inline diffs to the developer playground's Lambda tab, with optimized Lambda output in playground API v9, deferred hidden-panel updates, and debounced comparisons while editing. https://github.com/rescript-lang/rescript/pull/8636`