Developer playground: compare Lambda before and after optimization

CONTRIBUTING.md

@@ -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.

changes/developer-playground-lambda-diff.internal.md

@@ -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. https://github.com/rescript-lang/rescript/pull/8636

compiler/core/lam_compile_main.ml

@@ -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.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

compiler/core/lam_compile_main.mli

@@ -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

compiler/jsoo/jsoo_playground_main.ml

@@ -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
 
@@ -669,8 +670,18 @@ 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
@@ -727,7 +738,15 @@ 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

packages/dev-playground/package.json

@@ -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"