feat: add ++ concat operator and auto-invoke main

BUG-004: Add ++ operator for string and list concatenation across all
backends (interpreter, C, JS) with type checking and formatting support.

BUG-001: Auto-invoke top-level `let main = fn () => ...` when main is
a zero-parameter function, instead of just printing the function value.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.lock

@@ -770,7 +770,7 @@ dependencies = [
 
 [[package]]
 name = "lux"
-version = "0.1.1"
+version = "0.1.2"
 dependencies = [
  "lsp-server",
  "lsp-types",

src/ast.rs

@@ -622,6 +622,7 @@ pub enum BinaryOp {
     Or,
     // Other
     Pipe,   // |>
+    Concat, // ++
 }
 
 impl fmt::Display for BinaryOp {
@@ -641,6 +642,7 @@ impl fmt::Display for BinaryOp {
             BinaryOp::And => write!(f, "&&"),
             BinaryOp::Or => write!(f, "||"),
             BinaryOp::Pipe => write!(f, "|>"),
+            BinaryOp::Concat => write!(f, "++"),
         }
     }
 }

src/codegen/c_backend.rs

@@ -730,10 +730,10 @@ impl CBackend {
                 }
 
                 // Check for string concatenation - use lux_string_concat instead of +
-                if matches!(op, BinaryOp::Add) {
+                if matches!(op, BinaryOp::Add | BinaryOp::Concat) {
                     let left_is_string = self.infer_expr_type(left).as_deref() == Some("LuxString");
                     let right_is_string = self.infer_expr_type(right).as_deref() == Some("LuxString");
-                    if left_is_string || right_is_string {
+                    if left_is_string || right_is_string || matches!(op, BinaryOp::Concat) {
                         return Ok(format!("lux_string_concat({}, {})", l, r));
                     }
                 }
@@ -2839,8 +2839,18 @@ impl CBackend {
                     }
                 }
 
+                // String concatenation for ++ and +
+                if matches!(op, BinaryOp::Add | BinaryOp::Concat) {
+                    let left_is_string = self.infer_expr_type(left).as_deref() == Some("LuxString");
+                    let right_is_string = self.infer_expr_type(right).as_deref() == Some("LuxString");
+                    if left_is_string || right_is_string || matches!(op, BinaryOp::Concat) {
+                        return Ok(format!("lux_string_concat({}, {})", l, r));
+                    }
+                }
+
                 let op_str = match op {
                     BinaryOp::Add => "+",
+                    BinaryOp::Concat => unreachable!("handled above"),
                     BinaryOp::Sub => "-",
                     BinaryOp::Mul => "*",
                     BinaryOp::Div => "/",

src/codegen/js_backend.rs

@@ -954,12 +954,17 @@ impl JsBackend {
                 let r = self.emit_expr(right)?;
 
                 // Check for string concatenation
-                if matches!(op, BinaryOp::Add) {
+                if matches!(op, BinaryOp::Add | BinaryOp::Concat) {
                     if self.is_string_expr(left) || self.is_string_expr(right) {
                         return Ok(format!("({} + {})", l, r));
                     }
                 }
 
+                // ++ on lists: use .concat()
+                if matches!(op, BinaryOp::Concat) {
+                    return Ok(format!("{}.concat({})", l, r));
+                }
+
                 let op_str = match op {
                     BinaryOp::Add => "+",
                     BinaryOp::Sub => "-",
@@ -974,6 +979,7 @@ impl JsBackend {
                     BinaryOp::Ge => ">=",
                     BinaryOp::And => "&&",
                     BinaryOp::Or => "||",
+                    BinaryOp::Concat => unreachable!("handled above"),
                     BinaryOp::Pipe => {
                         // Pipe operator: x |> f becomes f(x)
                         return Ok(format!("{}({})", r, l));
@@ -2342,7 +2348,7 @@ impl JsBackend {
                 }
             }
             Expr::BinaryOp { op, left, right, .. } => {
-                matches!(op, BinaryOp::Add)
+                matches!(op, BinaryOp::Add | BinaryOp::Concat)
                     && (self.is_string_expr(left) || self.is_string_expr(right))
             }
             _ => false,

src/formatter.rs

@@ -753,6 +753,7 @@ impl Formatter {
             BinaryOp::Ge => ">=",
             BinaryOp::And => "&&",
             BinaryOp::Or => "||",
+            BinaryOp::Concat => "++",
             BinaryOp::Pipe => "|>",
         }
     }

src/interpreter.rs

@@ -1115,11 +1115,50 @@ impl Interpreter {
     /// Execute a program
     pub fn run(&mut self, program: &Program) -> Result<Value, RuntimeError> {
         let mut last_value = Value::Unit;
+        let mut has_main_let = false;
 
         for decl in &program.declarations {
+            // Track if there's a top-level `let main = ...`
+            if let Declaration::Let(let_decl) = decl {
+                if let_decl.name.name == "main" {
+                    has_main_let = true;
+                }
+            }
             last_value = self.eval_declaration(decl)?;
         }
 
+        // Auto-invoke main if it was defined as a let binding with a function value
+        if has_main_let {
+            if let Some(main_val) = self.global_env.get("main") {
+                if let Value::Function(ref closure) = main_val {
+                    if closure.params.is_empty() {
+                        let span = Span { start: 0, end: 0 };
+                        let mut result = self.eval_call(main_val.clone(), vec![], span)?;
+                        // Trampoline loop
+                        loop {
+                            match result {
+                                EvalResult::Value(v) => {
+                                    last_value = v;
+                                    break;
+                                }
+                                EvalResult::Effect(req) => {
+                                    last_value = self.handle_effect(req)?;
+                                    break;
+                                }
+                                EvalResult::TailCall { func, args, span } => {
+                                    result = self.eval_call(func, args, span)?;
+                                }
+                                EvalResult::Resume(v) => {
+                                    last_value = v;
+                                    break;
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+        }
+
         Ok(last_value)
     }
 
@@ -1599,6 +1638,18 @@ impl Interpreter {
                     span: Some(span),
                 }),
             },
+            BinaryOp::Concat => match (left, right) {
+                (Value::String(a), Value::String(b)) => Ok(Value::String(a + &b)),
+                (Value::List(a), Value::List(b)) => {
+                    let mut result = a;
+                    result.extend(b);
+                    Ok(Value::List(result))
+                }
+                (l, r) => Err(RuntimeError {
+                    message: format!("Cannot concatenate {} and {}", l.type_name(), r.type_name()),
+                    span: Some(span),
+                }),
+            },
             BinaryOp::Sub => match (left, right) {
                 (Value::Int(a), Value::Int(b)) => Ok(Value::Int(a - b)),
                 (Value::Float(a), Value::Float(b)) => Ok(Value::Float(a - b)),

src/lexer.rs

@@ -70,6 +70,7 @@ pub enum TokenKind {
 
     // Operators
     Plus,       // +
+    PlusPlus,   // ++
     Minus,      // -
     Star,       // *
     Slash,      // /
@@ -160,6 +161,7 @@ impl fmt::Display for TokenKind {
             TokenKind::True => write!(f, "true"),
             TokenKind::False => write!(f, "false"),
             TokenKind::Plus => write!(f, "+"),
+            TokenKind::PlusPlus => write!(f, "++"),
             TokenKind::Minus => write!(f, "-"),
             TokenKind::Star => write!(f, "*"),
             TokenKind::Slash => write!(f, "/"),
@@ -268,7 +270,14 @@ impl<'a> Lexer<'a> {
 
         let kind = match c {
             // Single-character tokens
-            '+' => TokenKind::Plus,
+            '+' => {
+                if self.peek() == Some('+') {
+                    self.advance();
+                    TokenKind::PlusPlus
+                } else {
+                    TokenKind::Plus
+                }
+            }
             '*' => TokenKind::Star,
             '%' => TokenKind::Percent,
             '(' => TokenKind::LParen,

src/parser.rs

@@ -1558,6 +1558,7 @@ impl Parser {
         loop {
             let op = match self.peek_kind() {
                 TokenKind::Plus => BinaryOp::Add,
+                TokenKind::PlusPlus => BinaryOp::Concat,
                 TokenKind::Minus => BinaryOp::Sub,
                 _ => break,
             };

src/typechecker.rs

@@ -1804,6 +1804,29 @@ impl TypeChecker {
                 }
             }
 
+            BinaryOp::Concat => {
+                // Concat (++) supports strings and lists
+                if let Err(e) = unify_with_env(&left_type, &right_type, &self.env) {
+                    self.errors.push(TypeError {
+                        message: format!("Operands of '++' must have same type: {}", e),
+                        span,
+                    });
+                }
+                match &left_type {
+                    Type::String | Type::List(_) | Type::Var(_) => left_type,
+                    _ => {
+                        self.errors.push(TypeError {
+                            message: format!(
+                                "Operator '++' requires String or List operands, got {}",
+                                left_type
+                            ),
+                            span,
+                        });
+                        Type::Error
+                    }
+                }
+            }
+
             BinaryOp::Sub | BinaryOp::Mul | BinaryOp::Div | BinaryOp::Mod => {
                 // Arithmetic: both operands must be same numeric type
                 if let Err(e) = unify_with_env(&left_type, &right_type, &self.env) {