Parse if-expressions

Parser.cs

@@ -143,6 +143,7 @@ class Parser
 	private Expr sum() => binaryLeft(product, TokenType.Minus, TokenType.Plus);
 
 	private Expr product() => binaryLeft(unary, TokenType.Slash, TokenType.Asterisk);
+
 	private Expr unary()
 	{
 		if (match(TokenType.Bang, TokenType.Minus))
@@ -151,6 +152,21 @@ class Parser
 			Expr right = unary();
 			return new Unary { Op = op, Right = right };
 		}
+		return ifExpr();
+	}
+
+	private Expr ifExpr()
+	{
+		if (match(TokenType.If))
+		{
+			Expr condition = expression();
+			consume(TokenType.Then, "Expect 'then' after condition.");
+			Expr thenCase = expression();
+			consume(TokenType.Else, "Expect 'else' after 'then' case.");
+			Expr elseCase = expression();
+			return new If { Condition = condition, Then = thenCase, Else = elseCase };
+		}
+
 		return primary();
 	}