LibSQL: Parse EXISTS expressions

The EXISTS expression is a bit of an odd-man-out because it can appear
as any of the following forms:

    EXISTS (select-stmt)
    NOT EXISTS (select-stmt)
    (select-stmt)

Which makes it the only keyword expression that doesn't require its
keyword to actually be used. The consequence is that we might come
across an EXISTS expression while parsing another expression type;
NOT would have triggered a unary operator expression, and an opening
parentheses would have triggered an expression chain.

Userland/Libraries/LibSQL/AST.h

@@ -537,6 +537,22 @@ private:
     RefPtr<Expression> m_else_expression;
 };
 
+class ExistsExpression : public Expression {
+public:
+    ExistsExpression(NonnullRefPtr<Select> select_statement, bool invert_expression)
+        : m_select_statement(move(select_statement))
+        , m_invert_expression(invert_expression)
+    {
+    }
+
+    const NonnullRefPtr<Select>& select_statement() const { return m_select_statement; }
+    bool invert_expression() const { return m_invert_expression; }
+
+private:
+    NonnullRefPtr<Select> m_select_statement;
+    bool m_invert_expression;
+};
+
 class CollateExpression : public NestedExpression {
 public:
     CollateExpression(NonnullRefPtr<Expression> expression, String collation_name)

Userland/Libraries/LibSQL/Parser.cpp

@@ -215,7 +215,6 @@ NonnullRefPtr<Expression> Parser::parse_expression()
 
     // FIXME: Parse 'bind-parameter'.
     // FIXME: Parse 'function-name'.
-    // FIXME: Parse 'exists'.
     // FIXME: Parse 'raise-function'.
 
     return expression;
@@ -241,6 +240,9 @@ NonnullRefPtr<Expression> Parser::parse_primary_expression()
     if (auto expression = parse_case_expression(); expression.has_value())
         return move(expression.value());
 
+    if (auto expression = parse_exists_expression(false); expression.has_value())
+        return move(expression.value());
+
     expected("Primary Expression");
     consume();
 
@@ -381,8 +383,12 @@ Optional<NonnullRefPtr<Expression>> Parser::parse_unary_operator_expression()
     if (consume_if(TokenType::Tilde))
         return create_ast_node<UnaryOperatorExpression>(UnaryOperator::BitwiseNot, parse_expression());
 
-    if (consume_if(TokenType::Not))
+    if (consume_if(TokenType::Not)) {
+        if (match(TokenType::Exists))
+            return parse_exists_expression(true);
+        else
             return create_ast_node<UnaryOperatorExpression>(UnaryOperator::Not, parse_expression());
+    }
 
     return {};
 }
@@ -448,11 +454,15 @@ Optional<NonnullRefPtr<Expression>> Parser::parse_binary_operator_expression(Non
 
 Optional<NonnullRefPtr<Expression>> Parser::parse_chained_expression()
 {
-    if (!match(TokenType::ParenOpen))
+    if (!consume_if(TokenType::ParenOpen))
         return {};
 
+    if (match(TokenType::Select))
+        return parse_exists_expression(false, TokenType::Select);
+
     NonnullRefPtrVector<Expression> expressions;
-    parse_comma_separated_list(true, [&]() { expressions.append(parse_expression()); });
+    parse_comma_separated_list(false, [&]() { expressions.append(parse_expression()); });
+    consume(TokenType::ParenClose);
 
     return create_ast_node<ChainedExpression>(move(expressions));
 }
@@ -506,6 +516,21 @@ Optional<NonnullRefPtr<Expression>> Parser::parse_case_expression()
     return create_ast_node<CaseExpression>(move(case_expression), move(when_then_clauses), move(else_expression));
 }
 
+Optional<NonnullRefPtr<Expression>> Parser::parse_exists_expression(bool invert_expression, TokenType opening_token)
+{
+    VERIFY((opening_token == TokenType::Exists) || (opening_token == TokenType::Select));
+
+    if ((opening_token == TokenType::Exists) && !consume_if(TokenType::Exists))
+        return {};
+
+    if (opening_token == TokenType::Exists)
+        consume(TokenType::ParenOpen);
+    auto select_statement = parse_select_statement({});
+    consume(TokenType::ParenClose);
+
+    return create_ast_node<ExistsExpression>(move(select_statement), invert_expression);
+}
+
 Optional<NonnullRefPtr<Expression>> Parser::parse_collate_expression(NonnullRefPtr<Expression> expression)
 {
     if (!match(TokenType::Collate))

Userland/Libraries/LibSQL/Parser.h

@@ -68,6 +68,7 @@ private:
     Optional<NonnullRefPtr<Expression>> parse_chained_expression();
     Optional<NonnullRefPtr<Expression>> parse_cast_expression();
     Optional<NonnullRefPtr<Expression>> parse_case_expression();
+    Optional<NonnullRefPtr<Expression>> parse_exists_expression(bool invert_expression, TokenType opening_token = TokenType::Exists);
     Optional<NonnullRefPtr<Expression>> parse_collate_expression(NonnullRefPtr<Expression> expression);
     Optional<NonnullRefPtr<Expression>> parse_is_expression(NonnullRefPtr<Expression> expression);
     Optional<NonnullRefPtr<Expression>> parse_match_expression(NonnullRefPtr<Expression> lhs, bool invert_expression);

Userland/Libraries/LibSQL/Tests/TestSqlExpressionParser.cpp

@@ -338,6 +338,39 @@ TEST_CASE(case_expression)
     validate("CASE 15 WHEN 16 THEN 17 WHEN 18 THEN 19 ELSE 20 END", true, 2, true);
 }
 
+TEST_CASE(exists_expression)
+{
+    EXPECT(parse("EXISTS").is_error());
+    EXPECT(parse("EXISTS (").is_error());
+    EXPECT(parse("EXISTS (SELECT").is_error());
+    EXPECT(parse("EXISTS (SELECT)").is_error());
+    EXPECT(parse("EXISTS (SELECT * FROM table").is_error());
+    EXPECT(parse("NOT EXISTS").is_error());
+    EXPECT(parse("NOT EXISTS (").is_error());
+    EXPECT(parse("NOT EXISTS (SELECT").is_error());
+    EXPECT(parse("NOT EXISTS (SELECT)").is_error());
+    EXPECT(parse("NOT EXISTS (SELECT * FROM table").is_error());
+    EXPECT(parse("(").is_error());
+    EXPECT(parse("(SELECT").is_error());
+    EXPECT(parse("(SELECT)").is_error());
+    EXPECT(parse("(SELECT * FROM table").is_error());
+
+    auto validate = [](StringView sql, bool expected_invert_expression) {
+        auto result = parse(sql);
+        EXPECT(!result.is_error());
+
+        auto expression = result.release_value();
+        EXPECT(is<SQL::ExistsExpression>(*expression));
+
+        const auto& exists = static_cast<const SQL::ExistsExpression&>(*expression);
+        EXPECT_EQ(exists.invert_expression(), expected_invert_expression);
+    };
+
+    validate("EXISTS (SELECT * FROM table)", false);
+    validate("NOT EXISTS (SELECT * FROM table)", true);
+    validate("(SELECT * FROM table)", false);
+}
+
 TEST_CASE(collate_expression)
 {
     EXPECT(parse("COLLATE").is_error());