From 7984f7d1e8ebad92ebb6311e6369bf5c62aca864 Mon Sep 17 00:00:00 2001
From: Kovacsics Robert <kovirobi@gmail.com>
Date: Thu, 13 Feb 2025 23:26:33 +0000
Subject: [PATCH] WIP: Add scoped expressions inspired by OCaml

The parsing works but it has the semantics of normal `with` currently.

I am currently calling it `scoped dot`, because calling it `innermost
with` would maybe make people confuse them with the current `outermost
with` expressions
---
 src/libexpr/eval.cc                           |  4 +++-
 src/libexpr/nixexpr.hh                        | 11 +++++++++-
 src/libexpr/parser.y                          |  5 ++++-
 tests/functional/lang/eval-okay-scope-dot.exp |  1 +
 tests/functional/lang/eval-okay-scope-dot.nix | 20 +++++++++++++++++++
 5 files changed, 38 insertions(+), 3 deletions(-)
 create mode 100644 tests/functional/lang/eval-okay-scope-dot.exp
 create mode 100644 tests/functional/lang/eval-okay-scope-dot.nix

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index dee764429e9..8f96a5661ba 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -697,7 +697,9 @@ void mapStaticEnvBindings(const SymbolTable & st, const StaticEnv & se, const En
     if (env.up && se.up) {
         mapStaticEnvBindings(st, *se.up, *env.up, vm);
 
-        if (se.isWith && !env.values[0]->isThunk()) {
+        if (se.isWith &&
+            se.isWith->binding == ExprWith::Binding::Outermost &&
+            !env.values[0]->isThunk()) {
             // add 'with' bindings.
             for (auto & j : *env.values[0]->attrs())
                 vm.insert_or_assign(std::string(st[j.name]), j.value);
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index a7ad580d2df..fc98185cddd 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -381,12 +381,21 @@ struct ExprLet : Expr
 
 struct ExprWith : Expr
 {
+    enum class Binding {
+        /** Classic `with` behaviour */
+        Outermost,
+        /** `with` behaviour mimicking OCaml's `foo.( <expr> )` */
+        Innermost,
+    };
+
     PosIdx pos;
     Expr * attrs, * body;
     size_t prevWith;
     ExprWith * parentWith;
-    ExprWith(const PosIdx & pos, Expr * attrs, Expr * body) : pos(pos), attrs(attrs), body(body) { };
+    ExprWith(const PosIdx & pos, Expr * attrs, Expr * body, Binding binding)
+          : pos(pos), attrs(attrs), body(body), binding(binding) { };
     PosIdx getPos() const override { return pos; }
+    const Binding binding;
     COMMON_METHODS
 };
 
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index bde72140114..493bda2af42 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -211,7 +211,7 @@ expr_function
   | ASSERT expr ';' expr_function
     { $$ = new ExprAssert(CUR_POS, $2, $4); }
   | WITH expr ';' expr_function
-    { $$ = new ExprWith(CUR_POS, $2, $4); }
+    { $$ = new ExprWith(CUR_POS, $2, $4, ExprWith::Binding::Outermost); }
   | LET binds IN_KW expr_function
     { if (!$2->dynamicAttrs.empty())
         throw ParseError({
@@ -286,6 +286,9 @@ expr_select
        be used to emit a warning when an affected expression is parsed. */
     expr_simple OR_KW
     { $$ = new ExprCall(CUR_POS, $1, {new ExprVar(CUR_POS, state->s.or_)}, state->positions.add(state->origin, @$.endOffset)); }
+  | expr_simple '.' '(' expr ')'      { $$ = new ExprWith(CUR_POS, $1, $4, ExprWith::Binding::Innermost); }
+  | expr_simple '.' '[' expr_list ']' { $$ = new ExprWith(CUR_POS, $1, $4, ExprWith::Binding::Innermost); }
+  | expr_simple '.' '{' binds '}'     { $$ = new ExprWith(CUR_POS, $1, $4, ExprWith::Binding::Innermost); }
   | expr_simple
   ;
 
diff --git a/tests/functional/lang/eval-okay-scope-dot.exp b/tests/functional/lang/eval-okay-scope-dot.exp
new file mode 100644
index 00000000000..6d89b96c4e2
--- /dev/null
+++ b/tests/functional/lang/eval-okay-scope-dot.exp
@@ -0,0 +1 @@
+"x: (as-a as-b) y: (bs-a bs-b)"
diff --git a/tests/functional/lang/eval-okay-scope-dot.nix b/tests/functional/lang/eval-okay-scope-dot.nix
new file mode 100644
index 00000000000..69be9186f7b
--- /dev/null
+++ b/tests/functional/lang/eval-okay-scope-dot.nix
@@ -0,0 +1,20 @@
+let {
+
+  a = "let-a";
+
+  as = {
+    a = "as-a";
+    b = "as-b";
+  };
+
+  bs = {
+    a = "bs-a";
+    b = "bs-b";
+  };
+
+  x = as.(a + " " + b);
+
+  y = as.(bs.(a + " " + b));
+
+  body = "x: (" + x + ") y: (" + y + ")";
+}