Feat/ternary syntax

compiler/src/emit_capability.ts

@@ -41,7 +41,7 @@ function parseExprStr(s: string): Expr {
   }
 
   // Binary operators (check in precedence order, right-to-left to handle left-assoc)
-  const binOps = [" or ", " and ", " == ", " != ", " >= ", " <= ", " > ", " < ", " in ", " contains ", " + ", " - ", " * ", " / "];
+  const binOps = [" ?? ", " or ", " and ", " == ", " != ", " >= ", " <= ", " > ", " < ", " in ", " contains ", " + ", " - ", " * ", " / "];
   for (const op of binOps) {
     const idx = findBinOp(s, op);
     if (idx !== -1) {
@@ -198,6 +198,7 @@ function exprToTsInner(expr: Expr): string {
         case "or": return `(${l} || ${r})`;
         case "in": return `[${r}].flat().includes(${l})`;
         case "contains": return `${l}?.includes(${r})`;
+        case "??": return `(${l} ?? ${r})`;
         case ">": case "<": case ">=": case "<=":
         case "+": case "-": case "*": case "/":
           return `${l} ${expr.op} ${r}`;

compiler/src/lexer.ts

@@ -42,6 +42,7 @@ export enum TokenKind {
   MinusEq = "MinusEq",
   AppendEq = "AppendEq",
   Question = "Question",
+  NullCoalesce = "NullCoalesce",
   Bang = "Bang",
 
   // Literals
@@ -704,6 +705,10 @@ export class Lexer {
       this.advance(); this.advance();
       return { kind: TokenKind.DotDot, value: "..", loc };
     }
+    if (ch === "?" && this.peekAt(1) === "?") {
+      this.advance(); this.advance();
+      return { kind: TokenKind.NullCoalesce, value: "??", loc };
+    }
 
     // Single-character operators
     switch (ch) {

compiler/src/parse_expr.ts

@@ -7,7 +7,7 @@ import { TokenStream, ParseError } from "./parser_base";
 import * as AST from "./ast";
 
 export function parseExpr(s: TokenStream): AST.ExprNode {
-  return parseLogicalOr(s);
+  return parseNullCoalesce(s);
 }
 
 export function parseExprList(s: TokenStream): AST.ExprNode[] {
@@ -19,6 +19,17 @@ export function parseExprList(s: TokenStream): AST.ExprNode[] {
   return exprs;
 }
 
+function parseNullCoalesce(s: TokenStream): AST.ExprNode {
+  let left = parseLogicalOr(s);
+  while (s.check(TokenKind.NullCoalesce)) {
+    const loc = s.peek().loc;
+    s.advance();
+    const right = parseLogicalOr(s);
+    left = { kind: "BinaryExpr", loc, op: "??", left, right };
+  }
+  return left;
+}
+
 function parseLogicalOr(s: TokenStream): AST.ExprNode {
   let left = parseLogicalAnd(s);
   while (s.check(TokenKind.KwOr)) {

compiler/src/parser_base.ts

@@ -38,6 +38,11 @@ export class TokenStream {
     this.loopGuard = 0;
   }
 
+  /** Current position in the token stream (used to detect lack of progress during error recovery) */
+  position(): number {
+    return this.pos;
+  }
+
   peek(offset: number = 0): Token {
     return this.tokens[this.pos + offset] ?? this.tokens[this.tokens.length - 1];
   }

compiler/src/parser_recovery.ts

@@ -57,6 +57,7 @@ export class RecoveringParser {
     const systems: AST.SystemDeclNode[] = [];
 
     while (!this.s.check(TokenKind.EOF)) {
+      const before = this.s.position();
       try {
         systems.push(this.parseSystemDecl());
       } catch (e) {
@@ -67,6 +68,11 @@ export class RecoveringParser {
           throw e;
         }
       }
+      // Guarantee forward progress: if neither parsing nor synchronization
+      // consumed a token, skip one to avoid an infinite recovery loop.
+      if (this.s.position() === before && !this.s.check(TokenKind.EOF)) {
+        this.s.advance();
+      }
     }
 
     if (systems.length === 0 && this.errors.length === 0) {
@@ -107,6 +113,7 @@ export class RecoveringParser {
 
     const declarations: AST.DeclarationNode[] = [];
     while (!this.s.check(TokenKind.RBrace) && !this.s.check(TokenKind.EOF)) {
+      const before = this.s.position();
       try {
         declarations.push(this.parseDeclaration());
       } catch (e) {
@@ -117,6 +124,15 @@ export class RecoveringParser {
           throw e;
         }
       }
+      // Guarantee forward progress to avoid an infinite recovery loop when
+      // synchronization lands on a token the body loop cannot consume.
+      if (
+        this.s.position() === before &&
+        !this.s.check(TokenKind.RBrace) &&
+        !this.s.check(TokenKind.EOF)
+      ) {
+        this.s.advance();
+      }
     }
 
     this.s.expect(TokenKind.RBrace, "system body close");

compiler/src/tests/test_typechecker.ts

@@ -125,6 +125,39 @@ system BadEmit {
 }
 `, "T011");
 
+// ─── Null-coalescing operator (??) ───────────────────────────────────────────
+
+expect("?? allowed in capability effects", `
+system NCOk {
+  entity Product {
+    owns: [title: string]
+  }
+  capability update_product(p: Product, title: optional<string>) {
+    requires: [p.title != ""]
+    effects: [p.title = title ?? p.title]
+    sync: eventual
+  }
+}
+`, null);
+
+expect("T013: ?? rejected in entity constraint", `
+system NCBadEntity {
+  entity Product {
+    owns: [price_cents: optional<uint>]
+    constraints: [(price_cents ?? 0) > 0]
+  }
+}
+`, "T013");
+
+expect("T013: ?? rejected in top-level constraint", `
+system NCBadTop {
+  entity Product {
+    owns: [price_cents: optional<uint>]
+  }
+  constraint nonneg: (Product.price_cents ?? 0) >= 0
+}
+`, "T013");
+
 // ─── Summary ─────────────────────────────────────────────────────────────────
 
 console.log(`\n═══════════════════════════════════════`);

compiler/src/typechecker.ts

@@ -325,6 +325,9 @@ export class TypeChecker {
     if (entitySym) {
       const ctx = new TypeContext(entitySym.type.fields, this.symbols);
       for (const constraint of decl.constraints) {
+        if (this.exprUsesNullCoalesce(constraint)) {
+          this.addError("T013", "The '??' operator is not allowed in entity constraints (constraints compile to SQL)", constraint.loc);
+        }
         const ctype = this.inferExprType(constraint, ctx);
         if (ctype && ctype.tag !== "primitive") {
           this.addError("T005", `Constraint expression must type to bool, got ${typeToString(ctype)}`, constraint.loc);
@@ -522,12 +525,36 @@ export class TypeChecker {
   private checkConstraint(decl: AST.ConstraintDeclNode) {
     // Top-level constraints are checked in a global context
     const globalCtx = new TypeContext(new Map(), this.symbols);
+    if (this.exprUsesNullCoalesce(decl.expr)) {
+      this.addError("T013", `The '??' operator is not allowed in constraint '${decl.name}' (constraints compile to SQL)`, decl.loc);
+    }
     const ctype = this.inferExprType(decl.expr, globalCtx);
     if (ctype && !this.isBoolish(ctype)) {
       this.addError("T005", `Top-level constraint '${decl.name}' must type to bool`, decl.loc);
     }
   }
 
+  /** Recursively detect the null-coalescing operator anywhere in an expression. */
+  private exprUsesNullCoalesce(expr: AST.ExprNode): boolean {
+    switch (expr.kind) {
+      case "BinaryExpr":
+        return expr.op === "??" || this.exprUsesNullCoalesce(expr.left) || this.exprUsesNullCoalesce(expr.right);
+      case "UnaryExpr":
+        return this.exprUsesNullCoalesce(expr.operand);
+      case "CallExpr":
+        return expr.args.some(a => this.exprUsesNullCoalesce(a));
+      case "TernaryExpr":
+        return this.exprUsesNullCoalesce(expr.condition) || this.exprUsesNullCoalesce(expr.consequent) || this.exprUsesNullCoalesce(expr.alternate);
+      case "Literal":
+        if (expr.type === "list" && Array.isArray(expr.value)) {
+          return (expr.value as AST.ExprNode[]).some(e => this.exprUsesNullCoalesce(e));
+        }
+        return false;
+      default:
+        return false;
+    }
+  }
+
   // ─── Expression Type Inference ─────────────────────────────────────────────
 
   private inferExprType(expr: AST.ExprNode, ctx: TypeContext): CVType | null {
@@ -651,6 +678,13 @@ export class TypeChecker {
       case "-":
         return prim("int"); // subtraction may produce negative
 
+      // Null-coalescing: `a ?? b` yields the unwrapped (non-optional) type
+      case "??": {
+        const unwrap = (t: CVType | null): CVType | null =>
+          t && t.tag === "generic" && t.name === "optional" ? t.args[0] : t;
+        return unwrap(left) ?? unwrap(right) ?? prim("json");
+      }
+
       default:
         return prim("bool");
     }