feat: when/else conditional branching

- AST: When(cond, body) -> When(cond, body, Option<else_body>)
- Parser: optional 'else -> expr' after when body
- Codegen: reactive DOM swap with anchor comments
- Signal graph + type checker updated for 3-arg When
- Component prop signal wrapping: .value compatible accessors
- Added examples/when-else-demo.ds

compiler/ds-analyzer/src/signal_graph.rs

@@ -321,9 +321,12 @@ fn collect_deps(expr: &Expr, deps: &mut Vec<String>) {
                 collect_deps(item, deps);
             }
         }
-        Expr::When(cond, body) => {
+        Expr::When(cond, body, else_body) => {
             collect_deps(cond, deps);
             collect_deps(body, deps);
+            if let Some(eb) = else_body {
+                collect_deps(eb, deps);
+            }
         }
         Expr::Match(scrutinee, arms) => {
             collect_deps(scrutinee, deps);
@@ -439,13 +442,16 @@ fn collect_bindings(expr: &Expr, bindings: &mut Vec<DomBinding>) {
                 }
             }
         }
-        Expr::When(cond, body) => {
+        Expr::When(cond, body, else_body) => {
             let deps = extract_dependencies(cond);
             bindings.push(DomBinding {
                 kind: BindingKind::Conditional { condition_signals: deps.clone() },
                 dependencies: deps,
             });
             collect_bindings(body, bindings);
+            if let Some(eb) = else_body {
+                collect_bindings(eb, bindings);
+            }
         }
         _ => {}
     }

compiler/ds-codegen/src/js_emitter.rs

@@ -743,7 +743,8 @@ impl JsEmitter {
                 node_var
             }
 
-            Expr::When(cond, body) => {
+            Expr::When(cond, body, else_body) => {
+                let else_expr = else_body.clone();
                 let anchor_var = self.next_node_id();
                 let container_var = self.next_node_id();
                 let cond_js = self.emit_expr(cond);
@@ -751,12 +752,25 @@ impl JsEmitter {
                 self.emit_line(&format!("const {} = document.createComment('when');", anchor_var));
                 self.emit_line(&format!("let {} = null;", container_var));
 
-                // Build the conditional child
+                let has_else = else_expr.is_some();
+                let else_container = if has_else {
+                    let ec = self.next_node_id();
+                    self.emit_line(&format!("let {} = null;", ec));
+                    Some(ec)
+                } else {
+                    None
+                };
+
                 self.emit_line("DS.effect(() => {");
                 self.indent += 1;
                 self.emit_line(&format!("const show = {};", cond_js));
+
+                // Show when: condition true and not already showing
                 self.emit_line(&format!("if (show && !{}) {{", container_var));
                 self.indent += 1;
+                if let Some(ref ec) = else_container {
+                    self.emit_line(&format!("if ({}) {{ {}.remove(); {} = null; }}", ec, ec, ec));
+                }
                 let child_var = self.emit_view_expr(body, graph);
                 self.emit_line(&format!("{} = {};", container_var, child_var));
                 self.emit_line(&format!(
@@ -764,11 +778,39 @@ impl JsEmitter {
                     anchor_var, container_var, anchor_var
                 ));
                 self.indent -= 1;
+
+                // Hide when: condition false and currently showing
                 self.emit_line(&format!("}} else if (!show && {}) {{", container_var));
                 self.indent += 1;
                 self.emit_line(&format!("{}.remove();", container_var));
                 self.emit_line(&format!("{} = null;", container_var));
+                if let Some(eb) = &else_expr {
+                    if let Some(ec) = &else_container {
+                        let else_child = self.emit_view_expr(eb, graph);
+                        self.emit_line(&format!("{} = {};", ec, else_child));
+                        self.emit_line(&format!(
+                            "{}.parentNode.insertBefore({}, {}.nextSibling);",
+                            anchor_var, ec, anchor_var
+                        ));
+                    }
+                }
                 self.indent -= 1;
+
+                // Initial else: show=false and nothing rendered yet
+                if let Some(eb) = &else_expr {
+                    if let Some(ec) = &else_container {
+                        self.emit_line(&format!("}} else if (!show && !{} && !{}) {{", container_var, ec));
+                        self.indent += 1;
+                        let else_child2 = self.emit_view_expr(eb, graph);
+                        self.emit_line(&format!("{} = {};", ec, else_child2));
+                        self.emit_line(&format!(
+                            "{}.parentNode.insertBefore({}, {}.nextSibling);",
+                            anchor_var, ec, anchor_var
+                        ));
+                        self.indent -= 1;
+                    }
+                }
+
                 self.emit_line("}");
                 self.indent -= 1;
                 self.emit_line("});");

compiler/ds-parser/src/ast.rs

@@ -261,8 +261,8 @@ pub enum Expr {
     Element(Element),
     /// Container: `column [ ... ]`, `row [ ... ]`
     Container(Container),
-    /// When conditional: `when count > 10 -> ...`
-    When(Box<Expr>, Box<Expr>),
+    /// When conditional: `when count > 10 -> ...` with optional `else -> ...`
+    When(Box<Expr>, Box<Expr>, Option<Box<Expr>>),
 
     /// Each loop: `each item in list => template`
     Each(String, Box<Expr>, Box<Expr>),

compiler/ds-parser/src/parser.rs

@@ -891,7 +891,17 @@ impl Parser {
                 self.expect(&TokenKind::Arrow)?;
                 self.skip_newlines();
                 let body = self.parse_expr()?;
-                Ok(Expr::When(Box::new(cond), Box::new(body)))
+                // Optional else branch
+                self.skip_newlines();
+                let else_body = if self.check(&TokenKind::Else) {
+                    self.advance();
+                    self.expect(&TokenKind::Arrow)?;
+                    self.skip_newlines();
+                    Some(Box::new(self.parse_expr()?))
+                } else {
+                    None
+                };
+                Ok(Expr::When(Box::new(cond), Box::new(body), else_body))
             }
 
             // Each loop: `each item in list => template`

compiler/ds-types/src/checker.rs

@@ -391,7 +391,7 @@ impl TypeChecker {
                     });
                 }
             }
-            Expr::When(condition, body) => {
+            Expr::When(condition, body, else_body) => {
                 let cond_type = self.infer_expr(condition);
                 let inner = cond_type.unwrap_reactive().clone();
                 if inner != Type::Bool && !matches!(inner, Type::Var(_)) && inner != Type::Error {
@@ -634,8 +634,12 @@ impl TypeChecker {
                 self.infer_expr(then_br)
             }
 
-            Expr::When(_, body) => {
-                self.infer_expr(body)
+            Expr::When(_, body, else_body) => {
+                let body_ty = self.infer_expr(body);
+                if let Some(eb) = else_body {
+                    let _ = self.infer_expr(eb);
+                }
+                body_ty
             }
 
             Expr::Block(exprs) => {

examples/when-else-demo.ds

@@ -0,0 +1,17 @@
+-- DreamStack When/Else Demo
+
+let loggedIn = false
+let count = 0
+
+view main = column [
+  text "🔀 When/Else Demo" { variant: "title" }
+  button "Toggle Login" { click: loggedIn = !loggedIn }
+
+  when loggedIn ->
+    text "Welcome back! ✅"
+  else ->
+    text "Please log in 🔒"
+
+  text "Count: {count}"
+  button "+1" { click: count += 1 }
+]