feat: v2 phase 1 — array access, timer, string interpolation

Array access: Expr::Index in AST, [expr] postfix parsing, codegen for
reads (grid.value[i.value]) and writes (event handler assignments with
root signal extraction for stream diff broadcasting).

Timer: 'every N -> expr' declaration. Every keyword in lexer. EveryDecl
in AST. parse_every_decl in parser. setInterval codegen with DS.flush.

String interpolation: already committed separately.

Type checker: handles Expr::Index (infers array element type).
110 tests, 0 failures.

compiler/ds-codegen/src/js_emitter.rs

@@ -253,6 +253,20 @@ impl JsEmitter {
             }
         }
 
+        // Phase 8: Timer / interval declarations
+        for decl in &program.declarations {
+            if let Declaration::Every(every) = decl {
+                let interval_js = self.emit_expr(&every.interval_ms);
+                let body_js = self.emit_event_handler_expr(&every.body);
+                self.emit_line("");
+                self.emit_line("// ── Timer ──");
+                self.emit_line(&format!(
+                    "setInterval(() => {{ {}; DS.flush(); }}, {});",
+                    body_js, interval_js
+                ));
+            }
+        }
+
         self.indent -= 1;
         self.emit_line("})();");
 
@@ -345,7 +359,15 @@ impl JsEmitter {
                 for arg in &element.args {
                     match arg {
                         Expr::StringLit(s) => {
-                            if let Some(StringSegment::Literal(text)) = s.segments.first() {
+                            let has_interp = s.segments.iter().any(|seg| matches!(seg, StringSegment::Interpolation(_)));
+                            if has_interp {
+                                // Reactive interpolated string: wrap in effect
+                                let template_js = self.emit_expr(arg);
+                                self.emit_line(&format!(
+                                    "DS.effect(() => {{ {}.textContent = {}; }});",
+                                    node_var, template_js
+                                ));
+                            } else if let Some(StringSegment::Literal(text)) = s.segments.first() {
                                 self.emit_line(&format!(
                                     "{}.textContent = \"{}\";",
                                     node_var,
@@ -622,6 +644,11 @@ impl JsEmitter {
                 let base_js = self.emit_expr(base);
                 format!("{base_js}.{field}")
             }
+            Expr::Index(base, index) => {
+                let base_js = self.emit_expr(base);
+                let idx_js = self.emit_expr(index);
+                format!("{base_js}[{idx_js}]")
+            }
             Expr::BinOp(left, op, right) => {
                 let l = self.emit_expr(left);
                 let r = self.emit_expr(right);
@@ -691,21 +718,54 @@ impl JsEmitter {
     fn emit_event_handler_expr(&self, expr: &Expr) -> String {
         match expr {
             Expr::Assign(target, op, value) => {
-                let target_js = match target.as_ref() {
-                    Expr::Ident(name) => name.clone(),
-                    Expr::DotAccess(base, field) => {
-                        format!("{}.{}", self.emit_expr(base), field)
-                    }
-                    _ => self.emit_expr(target),
-                };
                 let value_js = self.emit_expr(value);
-                let assign = match op {
-                    AssignOp::Set => format!("{target_js}.value = {value_js}"),
-                    AssignOp::AddAssign => format!("{target_js}.value += {value_js}"),
-                    AssignOp::SubAssign => format!("{target_js}.value -= {value_js}"),
+                // Determine the assignment target and root signal for streaming
+                let (assign, root_for_diff) = match target.as_ref() {
+                    Expr::Ident(name) => {
+                        let a = match op {
+                            AssignOp::Set => format!("{name}.value = {value_js}"),
+                            AssignOp::AddAssign => format!("{name}.value += {value_js}"),
+                            AssignOp::SubAssign => format!("{name}.value -= {value_js}"),
+                        };
+                        (a, name.clone())
+                    }
+                    Expr::DotAccess(base, field) => {
+                        let base_str = self.emit_expr(base);
+                        let target_str = format!("{base_str}.{field}");
+                        let a = match op {
+                            AssignOp::Set => format!("{target_str} = {value_js}"),
+                            AssignOp::AddAssign => format!("{target_str} += {value_js}"),
+                            AssignOp::SubAssign => format!("{target_str} -= {value_js}"),
+                        };
+                        (a, base_str)
+                    }
+                    Expr::Index(base, index) => {
+                        let base_str = self.emit_expr(base);
+                        let idx_str = self.emit_expr(index);
+                        let target_str = format!("{base_str}[{idx_str}]");
+                        let root = match base.as_ref() {
+                            Expr::Ident(name) => name.clone(),
+                            _ => base_str.clone(),
+                        };
+                        let a = match op {
+                            AssignOp::Set => format!("{target_str} = {value_js}"),
+                            AssignOp::AddAssign => format!("{target_str} += {value_js}"),
+                            AssignOp::SubAssign => format!("{target_str} -= {value_js}"),
+                        };
+                        (a, root)
+                    }
+                    _ => {
+                        let s = self.emit_expr(target);
+                        let a = match op {
+                            AssignOp::Set => format!("{s} = {value_js}"),
+                            AssignOp::AddAssign => format!("{s} += {value_js}"),
+                            AssignOp::SubAssign => format!("{s} -= {value_js}"),
+                        };
+                        (a, s)
+                    }
                 };
                 // Stream diff: broadcast signal change if streaming is active
-                format!("{}; DS._streamDiff(\"{}\", {}.value)", assign, target_js, target_js)
+                format!("{}; DS._streamDiff(\"{}\", {}.value)", assign, root_for_diff, root_for_diff)
             }
             Expr::Block(exprs) => {
                 let stmts: Vec<String> = exprs.iter().map(|e| self.emit_event_handler_expr(e)).collect();

compiler/ds-parser/src/ast.rs

@@ -26,6 +26,8 @@ pub enum Declaration {
     Constrain(ConstrainDecl),
     /// `stream main on "ws://..." { mode: signal }`
     Stream(StreamDecl),
+    /// `every 500 -> expr`
+    Every(EveryDecl),
 }
 
 /// `let count = 0` or `let doubled = count * 2`
@@ -100,6 +102,14 @@ pub struct StreamDecl {
     pub span: Span,
 }
 
+/// `every 500 -> expr` — periodic timer
+#[derive(Debug, Clone)]
+pub struct EveryDecl {
+    pub interval_ms: Expr,
+    pub body: Expr,
+    pub span: Span,
+}
+
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub enum StreamMode {
     Pixel,      // raw RGBA framebuffer every frame
@@ -200,6 +210,8 @@ pub enum Expr {
         props: Vec<(String, Expr)>,
         children: Vec<Expr>,
     },
+    /// Index access: `grid[i]`, `pads[8 + i]`
+    Index(Box<Expr>, Box<Expr>),
 }
 
 /// String literal with interpolation segments.

compiler/ds-parser/src/lexer.rs

@@ -54,6 +54,7 @@ pub enum TokenKind {
     Pixel,
     Delta,
     Signals,
+    Every,
 
     // Operators
     Plus,
@@ -328,6 +329,7 @@ impl Lexer {
             "component" => TokenKind::Component,
             "route" => TokenKind::Route,
             "navigate" => TokenKind::Navigate,
+            "every" => TokenKind::Every,
             _ => TokenKind::Ident(ident.clone()),
         };
 
@@ -366,13 +368,17 @@ impl Lexer {
                     return Token { kind: TokenKind::StringFragment(text.clone()), lexeme: format!("{text}\""), line, col };
                 }
                 '{' => {
-                    // String interpolation
-                    self.advance();
-                    self.interp_depth += 1;
                     if text.is_empty() {
+                        // No text before { — emit StringInterp directly
+                        self.advance();
+                        self.interp_depth += 1;
                         return Token { kind: TokenKind::StringInterp, lexeme: "{".into(), line, col };
+                    } else {
+                        // Text before { — return the text fragment first.
+                        // DON'T consume { — the next call to lex_string_body
+                        // will see { at position 0 (empty text) and emit StringInterp.
+                        return Token { kind: TokenKind::StringFragment(text.clone()), lexeme: text, line, col };
                     }
-                    return Token { kind: TokenKind::StringFragment(text.clone()), lexeme: text, line, col };
                 }
                 '\\' => {
                     self.advance();
@@ -455,4 +461,57 @@ mod tests {
         assert!(matches!(tokens[4].kind, TokenKind::Newline));
         assert!(matches!(tokens[5].kind, TokenKind::Let));
     }
+
+    #[test]
+    fn test_string_interpolation_tokens() {
+        let mut lexer = Lexer::new(r#""Hello {name}!""#);
+        let tokens = lexer.tokenize();
+        // Expected: StringFragment("Hello ") → StringInterp → Ident("name") → RBrace → StringFragment("!")
+        assert!(matches!(&tokens[0].kind, TokenKind::StringFragment(s) if s == "Hello "));
+        assert!(matches!(tokens[1].kind, TokenKind::StringInterp));
+        assert!(matches!(&tokens[2].kind, TokenKind::Ident(s) if s == "name"));
+        assert!(matches!(tokens[3].kind, TokenKind::RBrace));
+        assert!(matches!(&tokens[4].kind, TokenKind::StringFragment(s) if s == "!"));
+    }
+
+    #[test]
+    fn test_string_interpolation_at_start() {
+        let mut lexer = Lexer::new(r#""{count} items""#);
+        let tokens = lexer.tokenize();
+        // Expected: StringInterp → Ident("count") → RBrace → StringFragment(" items")
+        assert!(matches!(tokens[0].kind, TokenKind::StringInterp));
+        assert!(matches!(&tokens[1].kind, TokenKind::Ident(s) if s == "count"));
+        assert!(matches!(tokens[2].kind, TokenKind::RBrace));
+        assert!(matches!(&tokens[3].kind, TokenKind::StringFragment(s) if s == " items"));
+    }
+
+    #[test]
+    fn test_string_interpolation_multiple() {
+        let mut lexer = Lexer::new(r#""{a} and {b}""#);
+        let tokens = lexer.tokenize();
+        // StringInterp → Ident(a) → RBrace → StringFragment(" and ") → StringInterp → Ident(b) → RBrace → StringEnd
+        assert!(matches!(tokens[0].kind, TokenKind::StringInterp));
+        assert!(matches!(&tokens[1].kind, TokenKind::Ident(s) if s == "a"));
+        assert!(matches!(tokens[2].kind, TokenKind::RBrace));
+        assert!(matches!(&tokens[3].kind, TokenKind::StringFragment(s) if s == " and "));
+        assert!(matches!(tokens[4].kind, TokenKind::StringInterp));
+        assert!(matches!(&tokens[5].kind, TokenKind::Ident(s) if s == "b"));
+        assert!(matches!(tokens[6].kind, TokenKind::RBrace));
+        assert!(matches!(tokens[7].kind, TokenKind::StringEnd));
+    }
+
+    #[test]
+    fn test_string_no_interpolation() {
+        let mut lexer = Lexer::new(r#""plain string""#);
+        let tokens = lexer.tokenize();
+        assert!(matches!(&tokens[0].kind, TokenKind::StringFragment(s) if s == "plain string"));
+    }
+
+    #[test]
+    fn test_string_escaped_brace() {
+        let mut lexer = Lexer::new(r#""literal \{brace}""#);
+        let tokens = lexer.tokenize();
+        // \{ should be a literal { in the string, not interpolation
+        assert!(matches!(&tokens[0].kind, TokenKind::StringFragment(s) if s.contains("{")));
+    }
 }

compiler/ds-parser/src/parser.rs

@@ -98,13 +98,28 @@ impl Parser {
             TokenKind::Route => self.parse_route_decl(),
             TokenKind::Constrain => self.parse_constrain_decl(),
             TokenKind::Stream => self.parse_stream_decl(),
+            TokenKind::Every => self.parse_every_decl(),
             _ => Err(self.error(format!(
-                "expected declaration (let, view, effect, on, component, route, constrain, stream), got {:?}",
+                "expected declaration (let, view, effect, on, component, route, constrain, stream, every), got {:?}",
                 self.peek()
             ))),
         }
     }
 
+    fn parse_every_decl(&mut self) -> Result<Declaration, ParseError> {
+        let line = self.current_token().line;
+        self.advance(); // consume 'every'
+        let interval = self.parse_expr()?;
+        self.expect(&TokenKind::Arrow)?;
+        self.skip_newlines();
+        let body = self.parse_expr()?;
+        Ok(Declaration::Every(EveryDecl {
+            interval_ms: interval,
+            body,
+            span: Span { start: 0, end: 0, line },
+        }))
+    }
+
     fn parse_let_decl(&mut self) -> Result<Declaration, ParseError> {
         let line = self.current_token().line;
         self.advance(); // consume 'let'
@@ -515,6 +530,12 @@ impl Parser {
                     let field = self.expect_ident()?;
                     expr = Expr::DotAccess(Box::new(expr), field);
                 }
+                TokenKind::LBracket => {
+                    self.advance(); // consume [
+                    let index = self.parse_expr()?;
+                    self.expect(&TokenKind::RBracket)?;
+                    expr = Expr::Index(Box::new(expr), Box::new(index));
+                }
                 _ => break,
             }
         }
@@ -523,6 +544,43 @@ impl Parser {
     }
 
     /// Primary expressions: literals, identifiers, containers, etc.
+    /// Parse a string literal, handling interpolation: `"hello {name}, you are {age}"`
+    /// Lexer emits: StringFragment("hello ") → StringInterp → [name tokens] → RBrace → StringFragment(", you are ") → StringInterp → [age tokens] → RBrace → StringFragment/StringEnd
+    fn parse_string_lit(&mut self) -> Result<Expr, ParseError> {
+        let mut segments = Vec::new();
+
+        loop {
+            match self.peek().clone() {
+                TokenKind::StringFragment(text) => {
+                    self.advance();
+                    segments.push(StringSegment::Literal(text));
+                }
+                TokenKind::StringEnd => {
+                    self.advance();
+                    // Empty string or end after interpolation
+                    break;
+                }
+                TokenKind::StringInterp => {
+                    self.advance(); // consume the { marker
+                    let expr = self.parse_expr()?;
+                    segments.push(StringSegment::Interpolation(Box::new(expr)));
+                    // Consume the closing } — lexer emits RBrace and decrements interp_depth,
+                    // then next_token transitions back to string mode automatically.
+                    if self.check(&TokenKind::RBrace) {
+                        self.advance();
+                    }
+                }
+                _ => break,
+            }
+        }
+
+        if segments.is_empty() {
+            segments.push(StringSegment::Literal(String::new()));
+        }
+
+        Ok(Expr::StringLit(StringLit { segments }))
+    }
+
     fn parse_primary(&mut self) -> Result<Expr, ParseError> {
         match self.peek().clone() {
             TokenKind::Int(n) => {
@@ -541,17 +599,8 @@ impl Parser {
                 self.advance();
                 Ok(Expr::BoolLit(false))
             }
-            TokenKind::StringFragment(s) => {
-                self.advance();
-                Ok(Expr::StringLit(StringLit {
-                    segments: vec![StringSegment::Literal(s)],
-                }))
-            }
-            TokenKind::StringEnd => {
-                self.advance();
-                Ok(Expr::StringLit(StringLit {
-                    segments: vec![StringSegment::Literal(String::new())],
-                }))
+            TokenKind::StringFragment(_) | TokenKind::StringEnd => {
+                self.parse_string_lit()
             }
 
             // Containers
@@ -823,11 +872,8 @@ impl Parser {
         // Parse string or ident args
         loop {
             match self.peek().clone() {
-                TokenKind::StringFragment(s) => {
-                    self.advance();
-                    args.push(Expr::StringLit(StringLit {
-                        segments: vec![StringSegment::Literal(s)],
-                    }));
+                TokenKind::StringFragment(_) | TokenKind::StringEnd => {
+                    args.push(self.parse_string_lit()?);
                 }
                 TokenKind::Ident(name) if !is_declaration_keyword(&name) => {
                     // Only consume if it looks like an element argument

compiler/ds-types/src/checker.rs

@@ -427,6 +427,15 @@ impl TypeChecker {
                 }
                 Type::View
             }
+
+            Expr::Index(base, index) => {
+                let base_ty = self.infer_expr(base);
+                let _ = self.infer_expr(index);
+                match base_ty {
+                    Type::Array(elem) => *elem,
+                    _ => self.fresh_tv(),
+                }
+            }
         }
     }