Implement type checking/validation for filters (#4364)

Co-authored-by: pajlada <rasmus.karlsson@pajlada.com>

CHANGELOG.md

@@ -7,6 +7,7 @@
 - Minor: Added support for FrankerFaceZ animated emotes. (#4434)
 - Minor: Added a local backup of the Twitch Badges API in case the request fails. (#4463)
 - Minor: Added the ability to reply to a message by `Shift + Right Click`ing the username. (#4424)
+- Minor: Added better filter validation and error messages. (#4364)
 - Minor: Updated the look of the Black Theme to be more in line with the other themes. (#4523)
 - Bugfix: Fixed an issue where animated emotes would render on top of zero-width emotes. (#4314)
 - Bugfix: Fixed an issue where it was difficult to hover a zero-width emote. (#4314)

src/CMakeLists.txt

@@ -74,12 +74,26 @@ set(SOURCE_FILES
         controllers/filters/FilterRecord.hpp
         controllers/filters/FilterSet.cpp
         controllers/filters/FilterSet.hpp
-        controllers/filters/parser/FilterParser.cpp
-        controllers/filters/parser/FilterParser.hpp
-        controllers/filters/parser/Tokenizer.cpp
-        controllers/filters/parser/Tokenizer.hpp
-        controllers/filters/parser/Types.cpp
-        controllers/filters/parser/Types.hpp
+        controllers/filters/lang/expressions/Expression.cpp
+        controllers/filters/lang/expressions/Expression.hpp
+        controllers/filters/lang/expressions/BinaryOperation.cpp
+        controllers/filters/lang/expressions/BinaryOperation.hpp
+        controllers/filters/lang/expressions/ListExpression.cpp
+        controllers/filters/lang/expressions/ListExpression.hpp
+        controllers/filters/lang/expressions/RegexExpression.cpp
+        controllers/filters/lang/expressions/RegexExpression.hpp
+        controllers/filters/lang/expressions/UnaryOperation.hpp
+        controllers/filters/lang/expressions/UnaryOperation.cpp
+        controllers/filters/lang/expressions/ValueExpression.cpp
+        controllers/filters/lang/expressions/ValueExpression.hpp
+        controllers/filters/lang/Filter.cpp
+        controllers/filters/lang/Filter.hpp
+        controllers/filters/lang/FilterParser.cpp
+        controllers/filters/lang/FilterParser.hpp
+        controllers/filters/lang/Tokenizer.cpp
+        controllers/filters/lang/Tokenizer.hpp
+        controllers/filters/lang/Types.cpp
+        controllers/filters/lang/Types.hpp
 
         controllers/highlights/BadgeHighlightModel.cpp
         controllers/highlights/BadgeHighlightModel.hpp

src/controllers/filters/FilterRecord.cpp

@@ -1,21 +1,40 @@
 #include "controllers/filters/FilterRecord.hpp"
 
+#include "controllers/filters/lang/Filter.hpp"
+
 namespace chatterino {
 
-FilterRecord::FilterRecord(const QString &name, const QString &filter)
-    : name_(name)
-    , filter_(filter)
-    , id_(QUuid::createUuid())
-    , parser_(std::make_unique<filterparser::FilterParser>(filter))
+static std::unique_ptr<filters::Filter> buildFilter(const QString &filterText)
+{
+    using namespace filters;
+    auto result = Filter::fromString(filterText);
+    if (std::holds_alternative<Filter>(result))
+    {
+        auto filter =
+            std::make_unique<Filter>(std::move(std::get<Filter>(result)));
+
+        if (filter->returnType() != Type::Bool)
+        {
+            // Only accept Bool results
+            return nullptr;
+        }
+
+        return filter;
+    }
+
+    return nullptr;
+}
+
+FilterRecord::FilterRecord(QString name, QString filter)
+    : FilterRecord(std::move(name), std::move(filter), QUuid::createUuid())
 {
 }
 
-FilterRecord::FilterRecord(const QString &name, const QString &filter,
-                           const QUuid &id)
-    : name_(name)
-    , filter_(filter)
+FilterRecord::FilterRecord(QString name, QString filter, const QUuid &id)
+    : name_(std::move(name))
+    , filterText_(std::move(filter))
     , id_(id)
-    , parser_(std::make_unique<filterparser::FilterParser>(filter))
+    , filter_(buildFilter(this->filterText_))
 {
 }
 
@@ -26,7 +45,7 @@ const QString &FilterRecord::getName() const
 
 const QString &FilterRecord::getFilter() const
 {
-    return this->filter_;
+    return this->filterText_;
 }
 
 const QUuid &FilterRecord::getId() const
@@ -36,12 +55,13 @@ const QUuid &FilterRecord::getId() const
 
 bool FilterRecord::valid() const
 {
-    return this->parser_->valid();
+    return this->filter_ != nullptr;
 }
 
-bool FilterRecord::filter(const filterparser::ContextMap &context) const
+bool FilterRecord::filter(const filters::ContextMap &context) const
 {
-    return this->parser_->execute(context);
+    assert(this->valid());
+    return this->filter_->execute(context).toBool();
 }
 
 bool FilterRecord::operator==(const FilterRecord &other) const

src/controllers/filters/FilterRecord.hpp

@@ -1,6 +1,6 @@
 #pragma once
 
-#include "controllers/filters/parser/FilterParser.hpp"
+#include "controllers/filters/lang/Filter.hpp"
 #include "util/RapidjsonHelpers.hpp"
 #include "util/RapidJsonSerializeQString.hpp"
 
@@ -16,9 +16,9 @@ namespace chatterino {
 class FilterRecord
 {
 public:
-    FilterRecord(const QString &name, const QString &filter);
+    FilterRecord(QString name, QString filter);
 
-    FilterRecord(const QString &name, const QString &filter, const QUuid &id);
+    FilterRecord(QString name, QString filter, const QUuid &id);
 
     const QString &getName() const;
 
@@ -28,16 +28,16 @@ public:
 
     bool valid() const;
 
-    bool filter(const filterparser::ContextMap &context) const;
+    bool filter(const filters::ContextMap &context) const;
 
     bool operator==(const FilterRecord &other) const;
 
 private:
-    QString name_;
-    QString filter_;
-    QUuid id_;
+    const QString name_;
+    const QString filterText_;
+    const QUuid id_;
 
-    std::unique_ptr<filterparser::FilterParser> parser_;
+    const std::unique_ptr<filters::Filter> filter_;
 };
 
 using FilterRecordPtr = std::shared_ptr<FilterRecord>;

src/controllers/filters/FilterSet.cpp

@@ -38,8 +38,7 @@ bool FilterSet::filter(const MessagePtr &m, ChannelPtr channel) const
     if (this->filters_.size() == 0)
         return true;
 
-    filterparser::ContextMap context =
-        filterparser::buildContextMap(m, channel.get());
+    filters::ContextMap context = filters::buildContextMap(m, channel.get());
     for (const auto &f : this->filters_.values())
     {
         if (!f->valid() || !f->filter(context))

src/controllers/filters/lang/Filter.cpp

@@ -0,0 +1,161 @@
+#include "controllers/filters/lang/Filter.hpp"
+
+#include "Application.hpp"
+#include "common/Channel.hpp"
+#include "controllers/filters/lang/FilterParser.hpp"
+#include "messages/Message.hpp"
+#include "providers/twitch/TwitchBadge.hpp"
+#include "providers/twitch/TwitchChannel.hpp"
+#include "providers/twitch/TwitchIrcServer.hpp"
+
+namespace chatterino::filters {
+
+ContextMap buildContextMap(const MessagePtr &m, chatterino::Channel *channel)
+{
+    auto watchingChannel = chatterino::getApp()->twitch->watchingChannel.get();
+
+    /* 
+     * Looking to add a new identifier to filters? Here's what to do: 
+     *  1. Update validIdentifiersMap in Tokenizer.hpp
+     *  2. Add the identifier to the list below
+     *  3. Add the type of the identifier to MESSAGE_TYPING_CONTEXT in Filter.hpp
+     *  4. Add the value for the identifier to the ContextMap returned by this function
+     * 
+     * List of identifiers:
+     *
+     * author.badges
+     * author.color
+     * author.name
+     * author.no_color
+     * author.subbed
+     * author.sub_length
+     *
+     * channel.name
+     * channel.watching
+     *
+     * flags.highlighted
+     * flags.points_redeemed
+     * flags.sub_message
+     * flags.system_message
+     * flags.reward_message
+     * flags.first_message
+     * flags.elevated_message
+     * flags.cheer_message
+     * flags.whisper
+     * flags.reply
+     * flags.automod
+     *
+     * message.content
+     * message.length
+     *
+     */
+
+    using MessageFlag = chatterino::MessageFlag;
+
+    QStringList badges;
+    badges.reserve(m->badges.size());
+    for (const auto &e : m->badges)
+    {
+        badges << e.key_;
+    }
+
+    bool watching = !watchingChannel->getName().isEmpty() &&
+                    watchingChannel->getName().compare(
+                        m->channelName, Qt::CaseInsensitive) == 0;
+
+    bool subscribed = false;
+    int subLength = 0;
+    for (const auto &subBadge : {"subscriber", "founder"})
+    {
+        if (!badges.contains(subBadge))
+        {
+            continue;
+        }
+        subscribed = true;
+        if (m->badgeInfos.find(subBadge) != m->badgeInfos.end())
+        {
+            subLength = m->badgeInfos.at(subBadge).toInt();
+        }
+    }
+    ContextMap vars = {
+        {"author.badges", std::move(badges)},
+        {"author.color", m->usernameColor},
+        {"author.name", m->displayName},
+        {"author.no_color", !m->usernameColor.isValid()},
+        {"author.subbed", subscribed},
+        {"author.sub_length", subLength},
+
+        {"channel.name", m->channelName},
+        {"channel.watching", watching},
+
+        {"flags.highlighted", m->flags.has(MessageFlag::Highlighted)},
+        {"flags.points_redeemed", m->flags.has(MessageFlag::RedeemedHighlight)},
+        {"flags.sub_message", m->flags.has(MessageFlag::Subscription)},
+        {"flags.system_message", m->flags.has(MessageFlag::System)},
+        {"flags.reward_message",
+         m->flags.has(MessageFlag::RedeemedChannelPointReward)},
+        {"flags.first_message", m->flags.has(MessageFlag::FirstMessage)},
+        {"flags.elevated_message", m->flags.has(MessageFlag::ElevatedMessage)},
+        {"flags.cheer_message", m->flags.has(MessageFlag::CheerMessage)},
+        {"flags.whisper", m->flags.has(MessageFlag::Whisper)},
+        {"flags.reply", m->flags.has(MessageFlag::ReplyMessage)},
+        {"flags.automod", m->flags.has(MessageFlag::AutoMod)},
+
+        {"message.content", m->messageText},
+        {"message.length", m->messageText.length()},
+    };
+    {
+        auto *tc = dynamic_cast<TwitchChannel *>(channel);
+        if (channel && !channel->isEmpty() && tc)
+        {
+            vars["channel.live"] = tc->isLive();
+        }
+        else
+        {
+            vars["channel.live"] = false;
+        }
+    }
+    return vars;
+}
+
+FilterResult Filter::fromString(const QString &str)
+{
+    FilterParser parser(str);
+
+    if (parser.valid())
+    {
+        auto exp = parser.release();
+        auto typ = parser.returnType();
+        return Filter(std::move(exp), typ);
+    }
+
+    return FilterError{parser.errors().join("\n")};
+}
+
+Filter::Filter(ExpressionPtr expression, Type returnType)
+    : expression_(std::move(expression))
+    , returnType_(returnType)
+{
+}
+
+Type Filter::returnType() const
+{
+    return this->returnType_;
+}
+
+QVariant Filter::execute(const ContextMap &context) const
+{
+    return this->expression_->execute(context);
+}
+
+QString Filter::filterString() const
+{
+    return this->expression_->filterString();
+}
+
+QString Filter::debugString(const TypingContext &context) const
+{
+    return this->expression_->debug(context);
+}
+
+}  // namespace chatterino::filters

src/controllers/filters/lang/Filter.hpp

@@ -0,0 +1,77 @@
+#pragma once
+
+#include "controllers/filters/lang/expressions/Expression.hpp"
+#include "controllers/filters/lang/Types.hpp"
+
+#include <QString>
+
+#include <memory>
+#include <variant>
+
+namespace chatterino {
+
+class Channel;
+struct Message;
+using MessagePtr = std::shared_ptr<const Message>;
+
+}  // namespace chatterino
+
+namespace chatterino::filters {
+
+// MESSAGE_TYPING_CONTEXT maps filter variables to their expected type at evaluation.
+// For example, flags.highlighted is a boolean variable, so it is marked as Type::Bool
+// below. These variable types will be used to check whether a filter "makes sense",
+// i.e. if all the variables and operators being used have compatible types.
+static const QMap<QString, Type> MESSAGE_TYPING_CONTEXT = {
+    {"author.badges", Type::StringList},
+    {"author.color", Type::Color},
+    {"author.name", Type::String},
+    {"author.no_color", Type::Bool},
+    {"author.subbed", Type::Bool},
+    {"author.sub_length", Type::Int},
+    {"channel.name", Type::String},
+    {"channel.watching", Type::Bool},
+    {"channel.live", Type::Bool},
+    {"flags.highlighted", Type::Bool},
+    {"flags.points_redeemed", Type::Bool},
+    {"flags.sub_message", Type::Bool},
+    {"flags.system_message", Type::Bool},
+    {"flags.reward_message", Type::Bool},
+    {"flags.first_message", Type::Bool},
+    {"flags.elevated_message", Type::Bool},
+    {"flags.cheer_message", Type::Bool},
+    {"flags.whisper", Type::Bool},
+    {"flags.reply", Type::Bool},
+    {"flags.automod", Type::Bool},
+    {"message.content", Type::String},
+    {"message.length", Type::Int},
+};
+
+ContextMap buildContextMap(const MessagePtr &m, chatterino::Channel *channel);
+
+class Filter;
+struct FilterError {
+    QString message;
+};
+
+using FilterResult = std::variant<Filter, FilterError>;
+
+class Filter
+{
+public:
+    static FilterResult fromString(const QString &str);
+
+    Type returnType() const;
+    QVariant execute(const ContextMap &context) const;
+
+    QString filterString() const;
+    QString debugString(const TypingContext &context) const;
+
+private:
+    Filter(ExpressionPtr expression, Type returnType);
+
+    ExpressionPtr expression_;
+    Type returnType_;
+};
+
+}  // namespace chatterino::filters

src/controllers/filters/lang/FilterParser.cpp

@@ -1,115 +1,21 @@
-#include "FilterParser.hpp"
+#include "controllers/filters/lang/FilterParser.hpp"
 
-#include "Application.hpp"
-#include "common/Channel.hpp"
-#include "controllers/filters/parser/Types.hpp"
-#include "messages/Message.hpp"
-#include "providers/twitch/TwitchBadge.hpp"
-#include "providers/twitch/TwitchChannel.hpp"
-#include "providers/twitch/TwitchIrcServer.hpp"
+#include "controllers/filters/lang/expressions/BinaryOperation.hpp"
+#include "controllers/filters/lang/expressions/Expression.hpp"
+#include "controllers/filters/lang/expressions/ListExpression.hpp"
+#include "controllers/filters/lang/expressions/RegexExpression.hpp"
+#include "controllers/filters/lang/expressions/UnaryOperation.hpp"
+#include "controllers/filters/lang/expressions/ValueExpression.hpp"
+#include "controllers/filters/lang/Filter.hpp"
+#include "controllers/filters/lang/Types.hpp"
 
-namespace filterparser {
+namespace chatterino::filters {
 
-ContextMap buildContextMap(const MessagePtr &m, chatterino::Channel *channel)
+QString explainIllType(const IllTyped &ill)
 {
-    auto watchingChannel = chatterino::getApp()->twitch->watchingChannel.get();
-
-    /* Known Identifiers
-     *
-     * author.badges
-     * author.color
-     * author.name
-     * author.no_color
-     * author.subbed
-     * author.sub_length
-     *
-     * channel.name
-     * channel.watching
-     *
-     * flags.highlighted
-     * flags.points_redeemed
-     * flags.sub_message
-     * flags.system_message
-     * flags.reward_message
-     * flags.first_message
-     * flags.elevated_message
-     * flags.cheer_message
-     * flags.whisper
-     * flags.reply
-     * flags.automod
-     *
-     * message.content
-     * message.length
-     *
-     */
-
-    using MessageFlag = chatterino::MessageFlag;
-
-    QStringList badges;
-    badges.reserve(m->badges.size());
-    for (const auto &e : m->badges)
-    {
-        badges << e.key_;
-    }
-
-    bool watching = !watchingChannel->getName().isEmpty() &&
-                    watchingChannel->getName().compare(
-                        m->channelName, Qt::CaseInsensitive) == 0;
-
-    bool subscribed = false;
-    int subLength = 0;
-    for (const auto &subBadge : {"subscriber", "founder"})
-    {
-        if (!badges.contains(subBadge))
-        {
-            continue;
-        }
-        subscribed = true;
-        if (m->badgeInfos.find(subBadge) != m->badgeInfos.end())
-        {
-            subLength = m->badgeInfos.at(subBadge).toInt();
-        }
-    }
-    ContextMap vars = {
-        {"author.badges", std::move(badges)},
-        {"author.color", m->usernameColor},
-        {"author.name", m->displayName},
-        {"author.no_color", !m->usernameColor.isValid()},
-        {"author.subbed", subscribed},
-        {"author.sub_length", subLength},
-
-        {"channel.name", m->channelName},
-        {"channel.watching", watching},
-
-        {"flags.highlighted", m->flags.has(MessageFlag::Highlighted)},
-        {"flags.points_redeemed", m->flags.has(MessageFlag::RedeemedHighlight)},
-        {"flags.sub_message", m->flags.has(MessageFlag::Subscription)},
-        {"flags.system_message", m->flags.has(MessageFlag::System)},
-        {"flags.reward_message",
-         m->flags.has(MessageFlag::RedeemedChannelPointReward)},
-        {"flags.first_message", m->flags.has(MessageFlag::FirstMessage)},
-        {"flags.elevated_message", m->flags.has(MessageFlag::ElevatedMessage)},
-        {"flags.cheer_message", m->flags.has(MessageFlag::CheerMessage)},
-        {"flags.whisper", m->flags.has(MessageFlag::Whisper)},
-        {"flags.reply", m->flags.has(MessageFlag::ReplyMessage)},
-        {"flags.automod", m->flags.has(MessageFlag::AutoMod)},
-
-        {"message.content", m->messageText},
-        {"message.length", m->messageText.length()},
-    };
-    {
-        using namespace chatterino;
-        auto *tc = dynamic_cast<TwitchChannel *>(channel);
-        if (channel && !channel->isEmpty() && tc)
-        {
-            vars["channel.live"] = tc->isLive();
-        }
-        else
-        {
-            vars["channel.live"] = false;
-        }
-    }
-    return vars;
+    return QString("%1\n\nProblem occurred here:\n%2")
+        .arg(ill.message)
+        .arg(ill.expr->filterString());
 }
 
 FilterParser::FilterParser(const QString &text)
@@ -117,11 +23,22 @@ FilterParser::FilterParser(const QString &text)
     , tokenizer_(Tokenizer(text))
     , builtExpression_(this->parseExpression(true))
 {
-}
+    if (!this->valid_)
+    {
+        return;
+    }
 
-bool FilterParser::execute(const ContextMap &context) const
-{
-    return this->builtExpression_->execute(context).toBool();
+    // safety: returnType must not live longer than the parsed expression. See
+    // comment on IllTyped::expr.
+    auto returnType =
+        this->builtExpression_->synthesizeType(MESSAGE_TYPING_CONTEXT);
+    if (isIllTyped(returnType))
+    {
+        this->errorLog(explainIllType(std::get<IllTyped>(returnType)));
+        return;
+    }
+
+    this->returnType_ = std::get<TypeClass>(returnType).type;
 }
 
 bool FilterParser::valid() const
@@ -129,6 +46,18 @@ bool FilterParser::valid() const
     return this->valid_;
 }
 
+Type FilterParser::returnType() const
+{
+    return this->returnType_;
+}
+
+ExpressionPtr FilterParser::release()
+{
+    ExpressionPtr ret;
+    this->builtExpression_.swap(ret);
+    return ret;
+}
+
 ExpressionPtr FilterParser::parseExpression(bool top)
 {
     auto e = this->parseAnd();
@@ -379,12 +308,7 @@ const QStringList &FilterParser::errors() const
 
 const QString FilterParser::debugString() const
 {
-    return this->builtExpression_->debug();
+    return this->builtExpression_->debug(MESSAGE_TYPING_CONTEXT);
 }
 
-const QString FilterParser::filterString() const
-{
-    return this->builtExpression_->filterString();
-}
-
-}  // namespace filterparser
+}  // namespace chatterino::filters

src/controllers/filters/lang/FilterParser.hpp

@@ -1,28 +1,22 @@
 #pragma once
 
-#include "controllers/filters/parser/Tokenizer.hpp"
-#include "controllers/filters/parser/Types.hpp"
+#include "controllers/filters/lang/expressions/Expression.hpp"
+#include "controllers/filters/lang/Tokenizer.hpp"
+#include "controllers/filters/lang/Types.hpp"
 
-namespace chatterino {
-
-class Channel;
-
-}  // namespace chatterino
-
-namespace filterparser {
-
-ContextMap buildContextMap(const MessagePtr &m, chatterino::Channel *channel);
+namespace chatterino::filters {
 
 class FilterParser
 {
 public:
     FilterParser(const QString &text);
-    bool execute(const ContextMap &context) const;
+
     bool valid() const;
+    Type returnType() const;
+    ExpressionPtr release();
 
     const QStringList &errors() const;
     const QString debugString() const;
-    const QString filterString() const;
 
 private:
     ExpressionPtr parseExpression(bool top = false);
@@ -41,5 +35,7 @@ private:
     QString text_;
     Tokenizer tokenizer_;
     ExpressionPtr builtExpression_;
+    Type returnType_ = Type::Bool;
 };
-}  // namespace filterparser
+
+}  // namespace chatterino::filters

src/controllers/filters/lang/Tokenizer.cpp

@@ -1,8 +1,79 @@
-#include "controllers/filters/parser/Tokenizer.hpp"
+#include "controllers/filters/lang/Tokenizer.hpp"
 
 #include "common/QLogging.hpp"
 
-namespace filterparser {
+namespace chatterino::filters {
+
+QString tokenTypeToInfoString(TokenType type)
+{
+    switch (type)
+    {
+        case AND:
+            return "And";
+        case OR:
+            return "Or";
+        case LP:
+            return "<left parenthesis>";
+        case RP:
+            return "<right parenthesis>";
+        case LIST_START:
+            return "<list start>";
+        case LIST_END:
+            return "<list end>";
+        case COMMA:
+            return "<comma>";
+        case PLUS:
+            return "Plus";
+        case MINUS:
+            return "Minus";
+        case MULTIPLY:
+            return "Multiply";
+        case DIVIDE:
+            return "Divide";
+        case MOD:
+            return "Mod";
+        case EQ:
+            return "Eq";
+        case NEQ:
+            return "NotEq";
+        case LT:
+            return "LessThan";
+        case GT:
+            return "GreaterThan";
+        case LTE:
+            return "LessThanEq";
+        case GTE:
+            return "GreaterThanEq";
+        case CONTAINS:
+            return "Contains";
+        case STARTS_WITH:
+            return "StartsWith";
+        case ENDS_WITH:
+            return "EndsWith";
+        case MATCH:
+            return "Match";
+        case NOT:
+            return "Not";
+        case STRING:
+            return "<string>";
+        case INT:
+            return "<int>";
+        case IDENTIFIER:
+            return "<identifier>";
+        case CONTROL_START:
+        case CONTROL_END:
+        case BINARY_START:
+        case BINARY_END:
+        case UNARY_START:
+        case UNARY_END:
+        case MATH_START:
+        case MATH_END:
+        case OTHER_START:
+        case NONE:
+        default:
+            return "<unknown>";
+    }
+}
 
 Tokenizer::Tokenizer(const QString &text)
 {
@@ -190,4 +261,4 @@ bool Tokenizer::typeIsMathOp(TokenType token)
     return token > TokenType::MATH_START && token < TokenType::MATH_END;
 }
 
-}  // namespace filterparser
+}  // namespace chatterino::filters

src/controllers/filters/lang/Tokenizer.hpp

@@ -1,12 +1,12 @@
 #pragma once
 
-#include "controllers/filters/parser/Types.hpp"
+#include "controllers/filters/lang/Types.hpp"
 
 #include <QMap>
 #include <QRegularExpression>
 #include <QString>
 
-namespace filterparser {
+namespace chatterino::filters {
 
 static const QMap<QString, QString> validIdentifiersMap = {
     {"author.badges", "author badges"},
@@ -17,7 +17,7 @@ static const QMap<QString, QString> validIdentifiersMap = {
     {"author.sub_length", "author sub length"},
     {"channel.name", "channel name"},
     {"channel.watching", "/watching channel?"},
-    {"channel.live", "Channel live?"},
+    {"channel.live", "channel live?"},
     {"flags.highlighted", "highlighted?"},
     {"flags.points_redeemed", "redeemed points?"},
     {"flags.sub_message", "sub/resub message?"},
@@ -42,6 +42,58 @@ static const QRegularExpression tokenRegex(
 );
 // clang-format on
 
+enum TokenType {
+    // control
+    CONTROL_START = 0,
+    AND = 1,
+    OR = 2,
+    LP = 3,
+    RP = 4,
+    LIST_START = 5,
+    LIST_END = 6,
+    COMMA = 7,
+    CONTROL_END = 19,
+
+    // binary operator
+    BINARY_START = 20,
+    EQ = 21,
+    NEQ = 22,
+    LT = 23,
+    GT = 24,
+    LTE = 25,
+    GTE = 26,
+    CONTAINS = 27,
+    STARTS_WITH = 28,
+    ENDS_WITH = 29,
+    MATCH = 30,
+    BINARY_END = 49,
+
+    // unary operator
+    UNARY_START = 50,
+    NOT = 51,
+    UNARY_END = 99,
+
+    // math operators
+    MATH_START = 100,
+    PLUS = 101,
+    MINUS = 102,
+    MULTIPLY = 103,
+    DIVIDE = 104,
+    MOD = 105,
+    MATH_END = 149,
+
+    // other types
+    OTHER_START = 150,
+    STRING = 151,
+    INT = 152,
+    IDENTIFIER = 153,
+    REGULAR_EXPRESSION = 154,
+
+    NONE = 200
+};
+
+QString tokenTypeToInfoString(TokenType type);
+
 class Tokenizer
 {
 public:
@@ -74,4 +126,4 @@ private:
 
     TokenType tokenize(const QString &text);
 };
-}  // namespace filterparser
+}  // namespace chatterino::filters

src/controllers/filters/lang/Types.cpp

@@ -0,0 +1,101 @@
+#include "controllers/filters/lang/Types.hpp"
+
+#include "controllers/filters/lang/expressions/Expression.hpp"
+#include "controllers/filters/lang/Tokenizer.hpp"
+
+namespace chatterino::filters {
+
+bool isList(const PossibleType &possibleType)
+{
+    using T = Type;
+    if (isIllTyped(possibleType))
+    {
+        return false;
+    }
+
+    auto typ = std::get<TypeClass>(possibleType);
+    return typ == T::List || typ == T::StringList ||
+           typ == T::MatchingSpecifier;
+}
+
+QString typeToString(Type type)
+{
+    using T = Type;
+    switch (type)
+    {
+        case T::String:
+            return "String";
+        case T::Int:
+            return "Int";
+        case T::Bool:
+            return "Bool";
+        case T::Color:
+            return "Color";
+        case T::RegularExpression:
+            return "RegularExpression";
+        case T::List:
+            return "List";
+        case T::StringList:
+            return "StringList";
+        case T::MatchingSpecifier:
+            return "MatchingSpecifier";
+        case T::Map:
+            return "Map";
+        default:
+            return "Unknown";
+    }
+}
+
+QString TypeClass::string() const
+{
+    return typeToString(this->type);
+}
+
+bool TypeClass::operator==(Type t) const
+{
+    return this->type == t;
+}
+
+bool TypeClass::operator==(const TypeClass &t) const
+{
+    return this->type == t.type;
+}
+
+bool TypeClass::operator==(const IllTyped &t) const
+{
+    return false;
+}
+
+bool TypeClass::operator!=(Type t) const
+{
+    return !this->operator==(t);
+}
+
+bool TypeClass::operator!=(const TypeClass &t) const
+{
+    return !this->operator==(t);
+}
+
+bool TypeClass::operator!=(const IllTyped &t) const
+{
+    return true;
+}
+
+QString IllTyped::string() const
+{
+    return "IllTyped";
+}
+
+QString possibleTypeToString(const PossibleType &possible)
+{
+    if (isWellTyped(possible))
+    {
+        return std::get<TypeClass>(possible).string();
+    }
+    else
+    {
+        return std::get<IllTyped>(possible).string();
+    }
+}
+
+}  // namespace chatterino::filters

src/controllers/filters/lang/Types.hpp

@@ -0,0 +1,95 @@
+#pragma once
+
+#include <QMap>
+#include <QString>
+#include <QVariant>
+
+#include <memory>
+#include <set>
+#include <variant>
+
+namespace chatterino::filters {
+
+class Expression;
+
+enum class Type {
+    String,
+    Int,
+    Bool,
+    Color,
+    RegularExpression,
+    List,
+    StringList,         // List of only strings
+    MatchingSpecifier,  // 2-element list in {RegularExpression, Int} form
+    Map
+};
+
+using ContextMap = QMap<QString, QVariant>;
+using TypingContext = QMap<QString, Type>;
+
+QString typeToString(Type type);
+
+struct IllTyped;
+
+struct TypeClass {
+    Type type;
+
+    QString string() const;
+
+    bool operator==(Type t) const;
+    bool operator==(const TypeClass &t) const;
+    bool operator==(const IllTyped &t) const;
+    bool operator!=(Type t) const;
+    bool operator!=(const TypeClass &t) const;
+    bool operator!=(const IllTyped &t) const;
+};
+
+struct IllTyped {
+    // Important nuance to expr:
+    // During type synthesis, should an error occur and an IllTyped PossibleType be
+    // returned, expr is a pointer to an Expression that exists in the Expression
+    // tree that was parsed. Therefore, you cannot hold on to this pointer longer
+    // than the Expression tree exists. Be careful!
+    const Expression *expr;
+    QString message;
+
+    QString string() const;
+};
+
+using PossibleType = std::variant<TypeClass, IllTyped>;
+
+inline bool isWellTyped(const PossibleType &possible)
+{
+    return std::holds_alternative<TypeClass>(possible);
+}
+
+inline bool isIllTyped(const PossibleType &possible)
+{
+    return std::holds_alternative<IllTyped>(possible);
+}
+
+QString possibleTypeToString(const PossibleType &possible);
+
+bool isList(const PossibleType &possibleType);
+
+inline bool variantIs(const QVariant &a, QMetaType::Type type)
+{
+    return static_cast<QMetaType::Type>(a.type()) == type;
+}
+
+inline bool variantIsNot(const QVariant &a, QMetaType::Type type)
+{
+    return static_cast<QMetaType::Type>(a.type()) != type;
+}
+
+inline bool convertVariantTypes(QVariant &a, QVariant &b, int type)
+{
+    return a.convert(type) && b.convert(type);
+}
+
+inline bool variantTypesMatch(QVariant &a, QVariant &b, QMetaType::Type type)
+{
+    return variantIs(a, type) && variantIs(b, type);
+}
+
+}  // namespace chatterino::filters

src/controllers/filters/lang/expressions/BinaryOperation.cpp

@@ -1,214 +1,8 @@
-#include "controllers/filters/parser/Types.hpp"
+#include "controllers/filters/lang/expressions/BinaryOperation.hpp"
 
-namespace filterparser {
+#include <QRegularExpression>
 
-bool convertVariantTypes(QVariant &a, QVariant &b, int type)
-{
-    return a.convert(type) && b.convert(type);
-}
-
-bool variantTypesMatch(QVariant &a, QVariant &b, QVariant::Type type)
-{
-    return a.type() == type && b.type() == type;
-}
-
-QString tokenTypeToInfoString(TokenType type)
-{
-    switch (type)
-    {
-        case CONTROL_START:
-        case CONTROL_END:
-        case BINARY_START:
-        case BINARY_END:
-        case UNARY_START:
-        case UNARY_END:
-        case MATH_START:
-        case MATH_END:
-        case OTHER_START:
-        case NONE:
-            return "<unknown>";
-        case AND:
-            return "<and>";
-        case OR:
-            return "<or>";
-        case LP:
-            return "<left parenthesis>";
-        case RP:
-            return "<right parenthesis>";
-        case LIST_START:
-            return "<list start>";
-        case LIST_END:
-            return "<list end>";
-        case COMMA:
-            return "<comma>";
-        case PLUS:
-            return "<plus>";
-        case MINUS:
-            return "<minus>";
-        case MULTIPLY:
-            return "<multiply>";
-        case DIVIDE:
-            return "<divide>";
-        case MOD:
-            return "<modulus>";
-        case EQ:
-            return "<equals>";
-        case NEQ:
-            return "<not equals>";
-        case LT:
-            return "<less than>";
-        case GT:
-            return "<greater than>";
-        case LTE:
-            return "<less than equal>";
-        case GTE:
-            return "<greater than equal>";
-        case CONTAINS:
-            return "<contains>";
-        case STARTS_WITH:
-            return "<starts with>";
-        case ENDS_WITH:
-            return "<ends with>";
-        case MATCH:
-            return "<match>";
-        case NOT:
-            return "<not>";
-        case STRING:
-            return "<string>";
-        case INT:
-            return "<int>";
-        case IDENTIFIER:
-            return "<identifier>";
-        default:
-            return "<unknown>";
-    }
-}
-
-// ValueExpression
-
-ValueExpression::ValueExpression(QVariant value, TokenType type)
-    : value_(value)
-    , type_(type){};
-
-QVariant ValueExpression::execute(const ContextMap &context) const
-{
-    if (this->type_ == TokenType::IDENTIFIER)
-    {
-        return context.value(this->value_.toString());
-    }
-    return this->value_;
-}
-
-TokenType ValueExpression::type()
-{
-    return this->type_;
-}
-
-QString ValueExpression::debug() const
-{
-    return this->value_.toString();
-}
-
-QString ValueExpression::filterString() const
-{
-    switch (this->type_)
-    {
-        case INT:
-            return QString::number(this->value_.toInt());
-        case STRING:
-            return QString("\"%1\"").arg(
-                this->value_.toString().replace("\"", "\\\""));
-        case IDENTIFIER:
-            return this->value_.toString();
-        default:
-            return "";
-    }
-}
-
-// RegexExpression
-
-RegexExpression::RegexExpression(QString regex, bool caseInsensitive)
-    : regexString_(regex)
-    , caseInsensitive_(caseInsensitive)
-    , regex_(QRegularExpression(
-          regex, caseInsensitive ? QRegularExpression::CaseInsensitiveOption
-                                 : QRegularExpression::NoPatternOption)){};
-
-QVariant RegexExpression::execute(const ContextMap &) const
-{
-    return this->regex_;
-}
-
-QString RegexExpression::debug() const
-{
-    return this->regexString_;
-}
-
-QString RegexExpression::filterString() const
-{
-    auto s = this->regexString_;
-    return QString("%1\"%2\"")
-        .arg(this->caseInsensitive_ ? "ri" : "r")
-        .arg(s.replace("\"", "\\\""));
-}
-
-// ListExpression
-
-ListExpression::ListExpression(ExpressionList list)
-    : list_(std::move(list)){};
-
-QVariant ListExpression::execute(const ContextMap &context) const
-{
-    QList<QVariant> results;
-    bool allStrings = true;
-    for (const auto &exp : this->list_)
-    {
-        auto res = exp->execute(context);
-        if (allStrings && res.type() != QVariant::Type::String)
-        {
-            allStrings = false;
-        }
-        results.append(res);
-    }
-
-    // if everything is a string return a QStringList for case-insensitive comparison
-    if (allStrings)
-    {
-        QStringList strings;
-        strings.reserve(results.size());
-        for (const auto &val : results)
-        {
-            strings << val.toString();
-        }
-        return strings;
-    }
-    else
-    {
-        return results;
-    }
-}
-
-QString ListExpression::debug() const
-{
-    QStringList debugs;
-    for (const auto &exp : this->list_)
-    {
-        debugs.append(exp->debug());
-    }
-    return QString("{%1}").arg(debugs.join(", "));
-}
-
-QString ListExpression::filterString() const
-{
-    QStringList strings;
-    for (const auto &exp : this->list_)
-    {
-        strings.append(QString("(%1)").arg(exp->filterString()));
-    }
-    return QString("{%1}").arg(strings.join(", "));
-}
-
-// BinaryOperation
+namespace chatterino::filters {
 
 BinaryOperation::BinaryOperation(TokenType op, ExpressionPtr left,
                                  ExpressionPtr right)
@@ -225,7 +19,8 @@ QVariant BinaryOperation::execute(const ContextMap &context) const
     switch (this->op_)
     {
         case PLUS:
-            if (left.type() == QVariant::Type::String &&
+            if (static_cast<QMetaType::Type>(left.type()) ==
+                    QMetaType::QString &&
                 right.canConvert(QMetaType::QString))
             {
                 return left.toString().append(right.toString());
@@ -260,14 +55,14 @@ QVariant BinaryOperation::execute(const ContextMap &context) const
                 return left.toBool() && right.toBool();
             return false;
         case EQ:
-            if (variantTypesMatch(left, right, QVariant::Type::String))
+            if (variantTypesMatch(left, right, QMetaType::QString))
             {
                 return left.toString().compare(right.toString(),
                                                Qt::CaseInsensitive) == 0;
             }
             return left == right;
         case NEQ:
-            if (variantTypesMatch(left, right, QVariant::Type::String))
+            if (variantTypesMatch(left, right, QMetaType::QString))
             {
                 return left.toString().compare(right.toString(),
                                                Qt::CaseInsensitive) != 0;
@@ -290,20 +85,20 @@ QVariant BinaryOperation::execute(const ContextMap &context) const
                 return left.toInt() >= right.toInt();
             return false;
         case CONTAINS:
-            if (left.type() == QVariant::Type::StringList &&
+            if (variantIs(left, QMetaType::QStringList) &&
                 right.canConvert(QMetaType::QString))
             {
                 return left.toStringList().contains(right.toString(),
                                                     Qt::CaseInsensitive);
             }
 
-            if (left.type() == QVariant::Type::Map &&
+            if (variantIs(left.type(), QMetaType::QVariantMap) &&
                 right.canConvert(QMetaType::QString))
             {
                 return left.toMap().contains(right.toString());
             }
 
-            if (left.type() == QVariant::Type::List)
+            if (variantIs(left.type(), QMetaType::QVariantList))
             {
                 return left.toList().contains(right);
             }
@@ -317,16 +112,16 @@ QVariant BinaryOperation::execute(const ContextMap &context) const
 
             return false;
         case STARTS_WITH:
-            if (left.type() == QVariant::Type::StringList &&
+            if (variantIs(left.type(), QMetaType::QStringList) &&
                 right.canConvert(QMetaType::QString))
             {
                 auto list = left.toStringList();
                 return !list.isEmpty() &&
                        list.first().compare(right.toString(),
-                                            Qt::CaseInsensitive);
+                                            Qt::CaseInsensitive) == 0;
             }
 
-            if (left.type() == QVariant::Type::List)
+            if (variantIs(left.type(), QMetaType::QVariantList))
             {
                 return left.toList().startsWith(right);
             }
@@ -341,16 +136,16 @@ QVariant BinaryOperation::execute(const ContextMap &context) const
             return false;
 
         case ENDS_WITH:
-            if (left.type() == QVariant::Type::StringList &&
+            if (variantIs(left.type(), QMetaType::QStringList) &&
                 right.canConvert(QMetaType::QString))
             {
                 auto list = left.toStringList();
                 return !list.isEmpty() &&
                        list.last().compare(right.toString(),
-                                           Qt::CaseInsensitive);
+                                           Qt::CaseInsensitive) == 0;
             }
 
-            if (left.type() == QVariant::Type::List)
+            if (variantIs(left.type(), QMetaType::QVariantList))
             {
                 return left.toList().endsWith(right);
             }
@@ -371,14 +166,14 @@ QVariant BinaryOperation::execute(const ContextMap &context) const
 
             auto matching = left.toString();
 
-            switch (right.type())
+            switch (static_cast<QMetaType::Type>(right.type()))
             {
-                case QVariant::Type::RegularExpression: {
+                case QMetaType::QRegularExpression: {
                     return right.toRegularExpression()
                         .match(matching)
                         .hasMatch();
                 }
-                case QVariant::Type::List: {
+                case QMetaType::QVariantList: {
                     auto list = right.toList();
 
                     // list must be two items
@@ -386,19 +181,19 @@ QVariant BinaryOperation::execute(const ContextMap &context) const
                         return false;
 
                     // list must be a regular expression and an int
-                    if (list.at(0).type() !=
-                            QVariant::Type::RegularExpression ||
-                        list.at(1).type() != QVariant::Type::Int)
+                    if (variantIsNot(list.at(0),
+                                     QMetaType::QRegularExpression) ||
+                        variantIsNot(list.at(1), QMetaType::Int))
                         return false;
 
                     auto match =
                         list.at(0).toRegularExpression().match(matching);
 
-                    // if matched, return nth capture group. Otherwise, return false
+                    // if matched, return nth capture group. Otherwise, return ""
                     if (match.hasMatch())
                         return match.captured(list.at(1).toInt());
                     else
-                        return false;
+                        return "";
                 }
                 default:
                     return false;
@@ -409,11 +204,105 @@ QVariant BinaryOperation::execute(const ContextMap &context) const
     }
 }
 
-QString BinaryOperation::debug() const
+PossibleType BinaryOperation::synthesizeType(const TypingContext &context) const
 {
-    return QString("(%1 %2 %3)")
-        .arg(this->left_->debug(), tokenTypeToInfoString(this->op_),
-             this->right_->debug());
+    auto leftSyn = this->left_->synthesizeType(context);
+    auto rightSyn = this->right_->synthesizeType(context);
+
+    // Return if either operand is ill-typed
+    if (isIllTyped(leftSyn))
+    {
+        return leftSyn;
+    }
+    else if (isIllTyped(rightSyn))
+    {
+        return rightSyn;
+    }
+
+    auto left = std::get<TypeClass>(leftSyn);
+    auto right = std::get<TypeClass>(rightSyn);
+
+    switch (this->op_)
+    {
+        case PLUS:
+            if (left == Type::String)
+                return TypeClass{Type::String};  // String concatenation
+            else if (left == Type::Int && right == Type::Int)
+                return TypeClass{Type::Int};
+
+            return IllTyped{this, "Can only add Ints or concatenate a String"};
+        case MINUS:
+        case MULTIPLY:
+        case DIVIDE:
+        case MOD:
+            if (left == Type::Int && right == Type::Int)
+                return TypeClass{Type::Int};
+
+            return IllTyped{this, "Can only perform operation with Ints"};
+        case OR:
+        case AND:
+            if (left == Type::Bool && right == Type::Bool)
+                return TypeClass{Type::Bool};
+
+            return IllTyped{this,
+                            "Can only perform logical operations with Bools"};
+        case EQ:
+        case NEQ:
+            // equals/not equals always produces a valid output
+            return TypeClass{Type::Bool};
+        case LT:
+        case GT:
+        case LTE:
+        case GTE:
+            if (left == Type::Int && right == Type::Int)
+                return TypeClass{Type::Bool};
+
+            return IllTyped{this, "Can only perform comparisons with Ints"};
+        case STARTS_WITH:
+        case ENDS_WITH:
+            if (isList(left))
+                return TypeClass{Type::Bool};
+            if (left == Type::String && right == Type::String)
+                return TypeClass{Type::Bool};
+
+            return IllTyped{
+                this,
+                "Can only perform starts/ends with a List or two Strings"};
+        case CONTAINS:
+            if (isList(left) || left == Type::Map)
+                return TypeClass{Type::Bool};
+            if (left == Type::String && right == Type::String)
+                return TypeClass{Type::Bool};
+
+            return IllTyped{
+                this,
+                "Can only perform contains with a List, a Map, or two Strings"};
+        case MATCH: {
+            if (left != Type::String)
+                return IllTyped{this,
+                                "Left argument of match must be a String"};
+
+            if (right == Type::RegularExpression)
+                return TypeClass{Type::Bool};
+            if (right == Type::MatchingSpecifier)  // group capturing
+                return TypeClass{Type::String};
+
+            return IllTyped{this, "Can only match on a RegularExpression or a "
+                                  "MatchingSpecifier"};
+        }
+        default:
+            return IllTyped{this, "Not implemented"};
+    }
+}
+
+QString BinaryOperation::debug(const TypingContext &context) const
+{
+    return QString("BinaryOp[%1](%2 : %3, %4 : %5)")
+        .arg(tokenTypeToInfoString(this->op_))
+        .arg(this->left_->debug(context))
+        .arg(possibleTypeToString(this->left_->synthesizeType(context)))
+        .arg(this->right_->debug(context))
+        .arg(possibleTypeToString(this->right_->synthesizeType(context)));
 }
 
 QString BinaryOperation::filterString() const
@@ -456,57 +345,14 @@ QString BinaryOperation::filterString() const
             case MATCH:
                 return "match";
             default:
-                return QString();
+                return "";
         }
     }();
 
-    return QString("(%1) %2 (%3)")
+    return QString("(%1 %2 %3)")
         .arg(this->left_->filterString())
         .arg(opText)
         .arg(this->right_->filterString());
 }
 
-// UnaryOperation
-
-UnaryOperation::UnaryOperation(TokenType op, ExpressionPtr right)
-    : op_(op)
-    , right_(std::move(right))
-{
-}
-
-QVariant UnaryOperation::execute(const ContextMap &context) const
-{
-    auto right = this->right_->execute(context);
-    switch (this->op_)
-    {
-        case NOT:
-            if (right.canConvert<bool>())
-                return !right.toBool();
-            return false;
-        default:
-            return false;
-    }
-}
-
-QString UnaryOperation::debug() const
-{
-    return QString("(%1 %2)").arg(tokenTypeToInfoString(this->op_),
-                                  this->right_->debug());
-}
-
-QString UnaryOperation::filterString() const
-{
-    const auto opText = [&]() -> QString {
-        switch (this->op_)
-        {
-            case NOT:
-                return "!";
-            default:
-                return QString();
-        }
-    }();
-
-    return QString("%1(%2)").arg(opText).arg(this->right_->filterString());
-}
-
-}  // namespace filterparser
+}  // namespace chatterino::filters

src/controllers/filters/lang/expressions/BinaryOperation.hpp

@@ -0,0 +1,24 @@
+#pragma once
+
+#include "controllers/filters/lang/expressions/Expression.hpp"
+#include "controllers/filters/lang/Types.hpp"
+
+namespace chatterino::filters {
+
+class BinaryOperation : public Expression
+{
+public:
+    BinaryOperation(TokenType op, ExpressionPtr left, ExpressionPtr right);
+
+    QVariant execute(const ContextMap &context) const override;
+    PossibleType synthesizeType(const TypingContext &context) const override;
+    QString debug(const TypingContext &context) const override;
+    QString filterString() const override;
+
+private:
+    TokenType op_;
+    ExpressionPtr left_;
+    ExpressionPtr right_;
+};
+
+}  // namespace chatterino::filters

src/controllers/filters/lang/expressions/Expression.cpp

@@ -0,0 +1,25 @@
+#include "controllers/filters/lang/expressions/Expression.hpp"
+
+namespace chatterino::filters {
+
+QVariant Expression::execute(const ContextMap & /*context*/) const
+{
+    return false;
+}
+
+PossibleType Expression::synthesizeType(const TypingContext & /*context*/) const
+{
+    return IllTyped{this, "Not implemented"};
+}
+
+QString Expression::debug(const TypingContext & /*context*/) const
+{
+    return "";
+}
+
+QString Expression::filterString() const
+{
+    return "";
+}
+
+}  // namespace chatterino::filters

src/controllers/filters/lang/expressions/Expression.hpp

@@ -0,0 +1,28 @@
+#pragma once
+
+#include "controllers/filters/lang/Tokenizer.hpp"
+#include "controllers/filters/lang/Types.hpp"
+
+#include <QString>
+#include <QVariant>
+
+#include <memory>
+#include <vector>
+
+namespace chatterino::filters {
+
+class Expression
+{
+public:
+    virtual ~Expression() = default;
+
+    virtual QVariant execute(const ContextMap &context) const;
+    virtual PossibleType synthesizeType(const TypingContext &context) const;
+    virtual QString debug(const TypingContext &context) const;
+    virtual QString filterString() const;
+};
+
+using ExpressionPtr = std::unique_ptr<Expression>;
+using ExpressionList = std::vector<std::unique_ptr<Expression>>;
+
+}  // namespace chatterino::filters

src/controllers/filters/lang/expressions/ListExpression.cpp

@@ -0,0 +1,94 @@
+#include "controllers/filters/lang/expressions/ListExpression.hpp"
+
+namespace chatterino::filters {
+
+ListExpression::ListExpression(ExpressionList &&list)
+    : list_(std::move(list)){};
+
+QVariant ListExpression::execute(const ContextMap &context) const
+{
+    QList<QVariant> results;
+    bool allStrings = true;
+    for (const auto &exp : this->list_)
+    {
+        auto res = exp->execute(context);
+        if (allStrings && variantIsNot(res.type(), QMetaType::QString))
+        {
+            allStrings = false;
+        }
+        results.append(res);
+    }
+
+    // if everything is a string return a QStringList for case-insensitive comparison
+    if (allStrings)
+    {
+        QStringList strings;
+        strings.reserve(results.size());
+        for (const auto &val : results)
+        {
+            strings << val.toString();
+        }
+        return strings;
+    }
+
+    return results;
+}
+
+PossibleType ListExpression::synthesizeType(const TypingContext &context) const
+{
+    std::vector<TypeClass> types;
+    types.reserve(this->list_.size());
+    bool allStrings = true;
+    for (const auto &exp : this->list_)
+    {
+        auto typSyn = exp->synthesizeType(context);
+        if (isIllTyped(typSyn))
+        {
+            return typSyn;  // Ill-typed
+        }
+
+        auto typ = std::get<TypeClass>(typSyn);
+
+        if (typ != Type::String)
+        {
+            allStrings = false;
+        }
+
+        types.push_back(typ);
+    }
+
+    if (types.size() == 2 && types[0] == Type::RegularExpression &&
+        types[1] == Type::Int)
+    {
+        // Specific {RegularExpression, Int} form
+        return TypeClass{Type::MatchingSpecifier};
+    }
+
+    return allStrings ? TypeClass{Type::StringList} : TypeClass{Type::List};
+}
+
+QString ListExpression::debug(const TypingContext &context) const
+{
+    QStringList debugs;
+    for (const auto &exp : this->list_)
+    {
+        debugs.append(
+            QString("%1 : %2")
+                .arg(exp->debug(context))
+                .arg(possibleTypeToString(exp->synthesizeType(context))));
+    }
+
+    return QString("List(%1)").arg(debugs.join(", "));
+}
+
+QString ListExpression::filterString() const
+{
+    QStringList strings;
+    for (const auto &exp : this->list_)
+    {
+        strings.append(exp->filterString());
+    }
+    return QString("{%1}").arg(strings.join(", "));
+}
+
+}  // namespace chatterino::filters

src/controllers/filters/lang/expressions/ListExpression.hpp

@@ -0,0 +1,22 @@
+#pragma once
+
+#include "controllers/filters/lang/expressions/Expression.hpp"
+#include "controllers/filters/lang/Types.hpp"
+
+namespace chatterino::filters {
+
+class ListExpression : public Expression
+{
+public:
+    ListExpression(ExpressionList &&list);
+
+    QVariant execute(const ContextMap &context) const override;
+    PossibleType synthesizeType(const TypingContext &context) const override;
+    QString debug(const TypingContext &context) const override;
+    QString filterString() const override;
+
+private:
+    ExpressionList list_;
+};
+
+}  // namespace chatterino::filters

src/controllers/filters/lang/expressions/RegexExpression.cpp

@@ -0,0 +1,36 @@
+#include "controllers/filters/lang/expressions/RegexExpression.hpp"
+
+namespace chatterino::filters {
+
+RegexExpression::RegexExpression(const QString &regex, bool caseInsensitive)
+    : regexString_(regex)
+    , caseInsensitive_(caseInsensitive)
+    , regex_(QRegularExpression(
+          regex, caseInsensitive ? QRegularExpression::CaseInsensitiveOption
+                                 : QRegularExpression::NoPatternOption)){};
+
+QVariant RegexExpression::execute(const ContextMap & /*context*/) const
+{
+    return this->regex_;
+}
+
+PossibleType RegexExpression::synthesizeType(
+    const TypingContext & /*context*/) const
+{
+    return TypeClass{Type::RegularExpression};
+}
+
+QString RegexExpression::debug(const TypingContext & /*context*/) const
+{
+    return QString("RegEx(%1)").arg(this->regexString_);
+}
+
+QString RegexExpression::filterString() const
+{
+    auto s = this->regexString_;
+    return QString("%1\"%2\"")
+        .arg(this->caseInsensitive_ ? "ri" : "r")
+        .arg(s.replace("\"", "\\\""));
+}
+
+}  // namespace chatterino::filters

src/controllers/filters/lang/expressions/RegexExpression.hpp

@@ -0,0 +1,26 @@
+#pragma once
+
+#include "controllers/filters/lang/expressions/Expression.hpp"
+#include "controllers/filters/lang/Types.hpp"
+
+#include <QRegularExpression>
+
+namespace chatterino::filters {
+
+class RegexExpression : public Expression
+{
+public:
+    RegexExpression(const QString &regex, bool caseInsensitive);
+
+    QVariant execute(const ContextMap &context) const override;
+    PossibleType synthesizeType(const TypingContext &context) const override;
+    QString debug(const TypingContext &context) const override;
+    QString filterString() const override;
+
+private:
+    QString regexString_;
+    bool caseInsensitive_;
+    QRegularExpression regex_;
+};
+
+}  // namespace chatterino::filters

src/controllers/filters/lang/expressions/UnaryOperation.cpp

@@ -0,0 +1,69 @@
+#include "controllers/filters/lang/expressions/UnaryOperation.hpp"
+
+namespace chatterino::filters {
+
+UnaryOperation::UnaryOperation(TokenType op, ExpressionPtr right)
+    : op_(op)
+    , right_(std::move(right))
+{
+}
+
+QVariant UnaryOperation::execute(const ContextMap &context) const
+{
+    auto right = this->right_->execute(context);
+    switch (this->op_)
+    {
+        case NOT:
+            return right.canConvert<bool>() && !right.toBool();
+        default:
+            return false;
+    }
+}
+
+PossibleType UnaryOperation::synthesizeType(const TypingContext &context) const
+{
+    auto rightSyn = this->right_->synthesizeType(context);
+    if (isIllTyped(rightSyn))
+    {
+        return rightSyn;
+    }
+
+    auto right = std::get<TypeClass>(rightSyn);
+
+    switch (this->op_)
+    {
+        case NOT:
+            if (right == Type::Bool)
+            {
+                return TypeClass{Type::Bool};
+            }
+            return IllTyped{this, "Can only negate boolean values"};
+        default:
+            return IllTyped{this, "Not implemented"};
+    }
+}
+
+QString UnaryOperation::debug(const TypingContext &context) const
+{
+    return QString("UnaryOp[%1](%2 : %3)")
+        .arg(tokenTypeToInfoString(this->op_))
+        .arg(this->right_->debug(context))
+        .arg(possibleTypeToString(this->right_->synthesizeType(context)));
+}
+
+QString UnaryOperation::filterString() const
+{
+    const auto opText = [&]() -> QString {
+        switch (this->op_)
+        {
+            case NOT:
+                return "!";
+            default:
+                return "";
+        }
+    }();
+
+    return QString("(%1%2)").arg(opText).arg(this->right_->filterString());
+}
+
+}  // namespace chatterino::filters

src/controllers/filters/lang/expressions/UnaryOperation.hpp

@@ -0,0 +1,23 @@
+#pragma once
+
+#include "controllers/filters/lang/expressions/Expression.hpp"
+#include "controllers/filters/lang/Types.hpp"
+
+namespace chatterino::filters {
+
+class UnaryOperation : public Expression
+{
+public:
+    UnaryOperation(TokenType op, ExpressionPtr right);
+
+    QVariant execute(const ContextMap &context) const override;
+    PossibleType synthesizeType(const TypingContext &context) const override;
+    QString debug(const TypingContext &context) const override;
+    QString filterString() const override;
+
+private:
+    TokenType op_;
+    ExpressionPtr right_;
+};
+
+}  // namespace chatterino::filters

src/controllers/filters/lang/expressions/ValueExpression.cpp

@@ -0,0 +1,70 @@
+#include "controllers/filters/lang/expressions/ValueExpression.hpp"
+
+#include "controllers/filters/lang/Tokenizer.hpp"
+
+namespace chatterino::filters {
+
+ValueExpression::ValueExpression(QVariant value, TokenType type)
+    : value_(std::move(value))
+    , type_(type)
+{
+}
+
+QVariant ValueExpression::execute(const ContextMap &context) const
+{
+    if (this->type_ == TokenType::IDENTIFIER)
+    {
+        return context.value(this->value_.toString());
+    }
+    return this->value_;
+}
+
+PossibleType ValueExpression::synthesizeType(const TypingContext &context) const
+{
+    switch (this->type_)
+    {
+        case TokenType::IDENTIFIER: {
+            auto it = context.find(this->value_.toString());
+            if (it != context.end())
+            {
+                return TypeClass{it.value()};
+            }
+
+            return IllTyped{this, "Unbound identifier"};
+        }
+        case TokenType::INT:
+            return TypeClass{Type::Int};
+        case TokenType::STRING:
+            return TypeClass{Type::String};
+        default:
+            return IllTyped{this, "Invalid value type"};
+    }
+}
+
+TokenType ValueExpression::type()
+{
+    return this->type_;
+}
+
+QString ValueExpression::debug(const TypingContext & /*context*/) const
+{
+    return QString("Val(%1)").arg(this->value_.toString());
+}
+
+QString ValueExpression::filterString() const
+{
+    switch (this->type_)
+    {
+        case INT:
+            return QString::number(this->value_.toInt());
+        case STRING:
+            return QString("\"%1\"").arg(
+                this->value_.toString().replace("\"", "\\\""));
+        case IDENTIFIER:
+            return this->value_.toString();
+        default:
+            return "";
+    }
+}
+
+}  // namespace chatterino::filters

src/controllers/filters/lang/expressions/ValueExpression.hpp

@@ -0,0 +1,24 @@
+#pragma once
+
+#include "controllers/filters/lang/expressions/Expression.hpp"
+#include "controllers/filters/lang/Types.hpp"
+
+namespace chatterino::filters {
+
+class ValueExpression : public Expression
+{
+public:
+    ValueExpression(QVariant value, TokenType type);
+    TokenType type();
+
+    QVariant execute(const ContextMap &context) const override;
+    PossibleType synthesizeType(const TypingContext &context) const override;
+    QString debug(const TypingContext &context) const override;
+    QString filterString() const override;
+
+private:
+    QVariant value_;
+    TokenType type_;
+};
+
+}  // namespace chatterino::filters

src/controllers/filters/parser/Types.hpp

@@ -1,168 +0,0 @@
-#pragma once
-
-#include <QRegularExpression>
-
-#include <memory>
-
-namespace chatterino {
-
-struct Message;
-
-}
-
-namespace filterparser {
-
-using MessagePtr = std::shared_ptr<const chatterino::Message>;
-using ContextMap = QMap<QString, QVariant>;
-
-enum TokenType {
-    // control
-    CONTROL_START = 0,
-    AND = 1,
-    OR = 2,
-    LP = 3,
-    RP = 4,
-    LIST_START = 5,
-    LIST_END = 6,
-    COMMA = 7,
-    CONTROL_END = 19,
-
-    // binary operator
-    BINARY_START = 20,
-    EQ = 21,
-    NEQ = 22,
-    LT = 23,
-    GT = 24,
-    LTE = 25,
-    GTE = 26,
-    CONTAINS = 27,
-    STARTS_WITH = 28,
-    ENDS_WITH = 29,
-    MATCH = 30,
-    BINARY_END = 49,
-
-    // unary operator
-    UNARY_START = 50,
-    NOT = 51,
-    UNARY_END = 99,
-
-    // math operators
-    MATH_START = 100,
-    PLUS = 101,
-    MINUS = 102,
-    MULTIPLY = 103,
-    DIVIDE = 104,
-    MOD = 105,
-    MATH_END = 149,
-
-    // other types
-    OTHER_START = 150,
-    STRING = 151,
-    INT = 152,
-    IDENTIFIER = 153,
-    REGULAR_EXPRESSION = 154,
-
-    NONE = 200
-};
-
-bool convertVariantTypes(QVariant &a, QVariant &b, int type);
-QString tokenTypeToInfoString(TokenType type);
-
-class Expression
-{
-public:
-    virtual ~Expression() = default;
-
-    virtual QVariant execute(const ContextMap &) const
-    {
-        return false;
-    }
-
-    virtual QString debug() const
-    {
-        return "(false)";
-    }
-
-    virtual QString filterString() const
-    {
-        return "";
-    }
-};
-
-using ExpressionPtr = std::unique_ptr<Expression>;
-
-class ValueExpression : public Expression
-{
-public:
-    ValueExpression(QVariant value, TokenType type);
-    TokenType type();
-
-    QVariant execute(const ContextMap &context) const override;
-    QString debug() const override;
-    QString filterString() const override;
-
-private:
-    QVariant value_;
-    TokenType type_;
-};
-
-class RegexExpression : public Expression
-{
-public:
-    RegexExpression(QString regex, bool caseInsensitive);
-
-    QVariant execute(const ContextMap &context) const override;
-    QString debug() const override;
-    QString filterString() const override;
-
-private:
-    QString regexString_;
-    bool caseInsensitive_;
-    QRegularExpression regex_;
-};
-
-using ExpressionList = std::vector<std::unique_ptr<Expression>>;
-
-class ListExpression : public Expression
-{
-public:
-    ListExpression(ExpressionList list);
-
-    QVariant execute(const ContextMap &context) const override;
-    QString debug() const override;
-    QString filterString() const override;
-
-private:
-    ExpressionList list_;
-};
-
-class BinaryOperation : public Expression
-{
-public:
-    BinaryOperation(TokenType op, ExpressionPtr left, ExpressionPtr right);
-
-    QVariant execute(const ContextMap &context) const override;
-    QString debug() const override;
-    QString filterString() const override;
-
-private:
-    TokenType op_;
-    ExpressionPtr left_;
-    ExpressionPtr right_;
-};
-
-class UnaryOperation : public Expression
-{
-public:
-    UnaryOperation(TokenType op, ExpressionPtr right);
-
-    QVariant execute(const ContextMap &context) const override;
-    QString debug() const override;
-    QString filterString() const override;
-
-private:
-    TokenType op_;
-    ExpressionPtr right_;
-};
-
-}  // namespace filterparser

src/widgets/dialogs/ChannelFilterEditorDialog.cpp

@@ -1,6 +1,6 @@
-#include "ChannelFilterEditorDialog.hpp"
+#include "widgets/dialogs/ChannelFilterEditorDialog.hpp"
 
-#include "controllers/filters/parser/FilterParser.hpp"
+#include "controllers/filters/lang/Tokenizer.hpp"
 
 #include <QLabel>
 #include <QPushButton>
@@ -99,7 +99,8 @@ ChannelFilterEditorDialog::ValueSpecifier::ValueSpecifier()
 
     this->typeCombo_->insertItems(
         0, {"Constant Text", "Constant Number", "Variable"});
-    this->varCombo_->insertItems(0, filterparser::validIdentifiersMap.values());
+
+    this->varCombo_->insertItems(0, filters::validIdentifiersMap.values());
 
     this->layout_->addWidget(this->typeCombo_);
     this->layout_->addWidget(this->varCombo_, 1);
@@ -141,7 +142,7 @@ void ChannelFilterEditorDialog::ValueSpecifier::setValue(const QString &value)
     if (this->typeCombo_->currentIndex() == 2)
     {
         this->varCombo_->setCurrentText(
-            filterparser::validIdentifiersMap.value(value));
+            filters::validIdentifiersMap.value(value));
     }
     else
     {
@@ -164,7 +165,7 @@ QString ChannelFilterEditorDialog::ValueSpecifier::expressionText()
         case 1:  // number
             return this->valueInput_->text();
         case 2:  // variable
-            return filterparser::validIdentifiersMap.key(
+            return filters::validIdentifiersMap.key(
                 this->varCombo_->currentText());
         default:
             return "";
@@ -221,7 +222,7 @@ QString ChannelFilterEditorDialog::BinaryOperationSpecifier::expressionText()
         return this->left_->expressionText();
     }
 
-    return QString("(%1) %2 (%3)")
+    return QString("(%1 %2 %3)")
         .arg(this->left_->expressionText())
         .arg(opText)
         .arg(this->right_->expressionText());

src/widgets/settingspages/FiltersPage.cpp

@@ -91,10 +91,14 @@ void FiltersPage::tableCellClicked(const QModelIndex &clicked,
     {
         QMessageBox popup(this->window());
 
-        filterparser::FilterParser f(
-            view->getModel()->data(clicked.siblingAtColumn(1)).toString());
+        auto filterText =
+            view->getModel()->data(clicked.siblingAtColumn(1)).toString();
+        auto filterResult = filters::Filter::fromString(filterText);
 
-        if (f.valid())
+        if (std::holds_alternative<filters::Filter>(filterResult))
+        {
+            auto f = std::move(std::get<filters::Filter>(filterResult));
+            if (f.returnType() == filters::Type::Bool)
             {
                 popup.setIcon(QMessageBox::Icon::Information);
                 popup.setWindowTitle("Valid filter");
@@ -104,10 +108,22 @@ void FiltersPage::tableCellClicked(const QModelIndex &clicked,
             }
             else
             {
+                popup.setIcon(QMessageBox::Icon::Warning);
+                popup.setWindowTitle("Invalid filter");
+                popup.setText(QString("Unexpected filter return type"));
+                popup.setInformativeText(
+                    QString("Expected %1 but got %2")
+                        .arg(filters::typeToString(filters::Type::Bool))
+                        .arg(filters::typeToString(f.returnType())));
+            }
+        }
+        else
+        {
+            auto err = std::move(std::get<filters::FilterError>(filterResult));
             popup.setIcon(QMessageBox::Icon::Warning);
             popup.setWindowTitle("Invalid filter");
             popup.setText(QString("Parsing errors occurred:"));
-            popup.setInformativeText(f.errors().join("\n"));
+            popup.setInformativeText(err.message);
         }
 
         popup.exec();

tests/CMakeLists.txt

@@ -24,6 +24,7 @@ set(test_SOURCES
     ${CMAKE_CURRENT_LIST_DIR}/src/SeventvEventAPI.cpp
     ${CMAKE_CURRENT_LIST_DIR}/src/BttvLiveUpdates.cpp
     ${CMAKE_CURRENT_LIST_DIR}/src/Updates.cpp
+    ${CMAKE_CURRENT_LIST_DIR}/src/Filters.cpp
     ${CMAKE_CURRENT_LIST_DIR}/src/LinkParser.cpp
     # Add your new file above this line!
     )

tests/src/Filters.cpp

@@ -0,0 +1,172 @@
+#include "controllers/filters/lang/Filter.hpp"
+#include "controllers/filters/lang/Types.hpp"
+
+#include <gtest/gtest.h>
+#include <QColor>
+#include <QVariant>
+
+using namespace chatterino;
+using namespace chatterino::filters;
+
+TypingContext typingContext = MESSAGE_TYPING_CONTEXT;
+
+namespace chatterino::filters {
+
+std::ostream &operator<<(std::ostream &os, Type t)
+{
+    os << qUtf8Printable(typeToString(t));
+    return os;
+}
+
+}  // namespace chatterino::filters
+
+TEST(Filters, Validity)
+{
+    struct TestCase {
+        QString input;
+        bool valid;
+    };
+
+    // clang-format off
+    std::vector<TestCase> tests{
+        {"", false},
+        {R".(1 + 1).", true},
+        {R".(1 + ).", false},
+        {R".(1 + 1)).", false},
+        {R".((1 + 1).", false},
+        {R".(author.name contains "icelys").", true},
+        {R".(author.color == "#ff0000").", true},
+        {R".(author.name - 5).", false},  // can't perform String - Int
+        {R".(message.content match {r"(\d\d)/(\d\d)/(\d\d\d\d)", 3}).", true},
+        {R".("abc" + 123 == "abc123").", true},
+        {R".(123 + "abc" == "hello").", false},
+        {R".(flags.reply && flags.automod).", true},
+        {R".(unknown.identifier).", false},
+        {R".(channel.name == "forsen" && author.badges contains "moderator").", true},
+    };
+    // clang-format on
+
+    for (const auto &[input, expected] : tests)
+    {
+        auto filterResult = Filter::fromString(input);
+        bool isValid = std::holds_alternative<Filter>(filterResult);
+        EXPECT_EQ(isValid, expected)
+            << "Filter::fromString( " << qUtf8Printable(input)
+            << " ) should be " << (expected ? "valid" : "invalid");
+    }
+}
+
+TEST(Filters, TypeSynthesis)
+{
+    using T = Type;
+    struct TestCase {
+        QString input;
+        T type;
+    };
+
+    // clang-format off
+    std::vector<TestCase> tests
+    {
+        {R".(1 + 1).", T::Int},
+        {R".(author.color).", T::Color},
+        {R".(author.name).", T::String},
+        {R".(!author.subbed).", T::Bool},
+        {R".(author.badges).", T::StringList},
+        {R".(channel.name == "forsen" && author.badges contains "moderator").", T::Bool},
+        {R".(message.content match {r"(\d\d)/(\d\d)/(\d\d\d\d)", 3}).", T::String},
+    };
+    // clang-format on
+
+    for (const auto &[input, expected] : tests)
+    {
+        auto filterResult = Filter::fromString(input);
+        bool isValid = std::holds_alternative<Filter>(filterResult);
+        ASSERT_TRUE(isValid) << "Filter::fromString( " << qUtf8Printable(input)
+                             << " ) is invalid";
+
+        auto filter = std::move(std::get<Filter>(filterResult));
+        T type = filter.returnType();
+        EXPECT_EQ(type, expected)
+            << "Filter{ " << qUtf8Printable(input) << " } has type " << type
+            << " instead of " << expected << ".\nDebug: "
+            << qUtf8Printable(filter.debugString(typingContext));
+    }
+}
+
+TEST(Filters, Evaluation)
+{
+    struct TestCase {
+        QString input;
+        QVariant output;
+    };
+
+    ContextMap contextMap = {
+        {"author.name", QVariant("icelys")},
+        {"author.color", QVariant(QColor("#ff0000"))},
+        {"author.subbed", QVariant(false)},
+        {"message.content", QVariant("hey there :) 2038-01-19 123 456")},
+        {"channel.name", QVariant("forsen")},
+        {"author.badges", QVariant(QStringList({"moderator", "staff"}))}};
+
+    // clang-format off
+    std::vector<TestCase> tests
+    {
+        // Evaluation semantics
+        {R".(1 + 1).", QVariant(2)},
+        {R".(!(1 == 1)).", QVariant(false)},
+        {R".(2 + 3 * 4).", QVariant(20)},  // math operators have the same precedence
+        {R".(1 > 2 || 3 >= 3).", QVariant(true)},
+        {R".(1 > 2 && 3 > 1).", QVariant(false)},
+        {R".("abc" + 123).", QVariant("abc123")},
+        {R".("abc" + "456").", QVariant("abc456")},
+        {R".(3 - 4).", QVariant(-1)},
+        {R".(3 * 4).", QVariant(12)},
+        {R".(8 / 3).", QVariant(2)},
+        {R".(7 % 3).", QVariant(1)},
+        {R".(5 == 5).", QVariant(true)},
+        {R".(5 == "5").", QVariant(true)},
+        {R".(5 != 7).", QVariant(true)},
+        {R".(5 == "abc").", QVariant(false)},
+        {R".("ABC123" == "abc123").", QVariant(true)},  // String comparison is case-insensitive
+        {R".("Hello world" contains "Hello").", QVariant(true)},
+        {R".("Hello world" contains "LLO W").", QVariant(true)},  // Case-insensitive
+        {R".({"abc", "def"} contains "abc").", QVariant(true)},
+        {R".({"abc", "def"} contains "ABC").", QVariant(true)},  // Case-insensitive when list is all strings
+        {R".({123, "def"} contains "DEF").", QVariant(false)},  // Case-sensitive if list not all strings
+        {R".({"a123", "b456"} startswith "a123").", QVariant(true)},
+        {R".({"a123", "b456"} startswith "A123").", QVariant(true)},
+        {R".({} startswith "A123").", QVariant(false)},
+        {R".("Hello world" startswith "Hello").", QVariant(true)},
+        {R".("Hello world" startswith "world").", QVariant(false)},
+        {R".({"a123", "b456"} endswith "b456").", QVariant(true)},
+        {R".({"a123", "b456"} endswith "B456").", QVariant(true)},
+        {R".("Hello world" endswith "world").", QVariant(true)},
+        {R".("Hello world" endswith "Hello").", QVariant(false)},
+        // Context map usage
+        {R".(author.name).", QVariant("icelys")},
+        {R".(!author.subbed).", QVariant(true)},
+        {R".(author.color == "#ff0000").", QVariant(true)},
+        {R".(channel.name == "forsen" && author.badges contains "moderator").", QVariant(true)},
+        {R".(message.content match {r"(\d\d\d\d)\-(\d\d)\-(\d\d)", 3}).", QVariant("19")},
+        {R".(message.content match r"HEY THERE").", QVariant(false)},
+        {R".(message.content match ri"HEY THERE").", QVariant(true)},
+    };
+    // clang-format on
+
+    for (const auto &[input, expected] : tests)
+    {
+        auto filterResult = Filter::fromString(input);
+        bool isValid = std::holds_alternative<Filter>(filterResult);
+        ASSERT_TRUE(isValid) << "Filter::fromString( " << qUtf8Printable(input)
+                             << " ) is invalid";
+
+        auto filter = std::move(std::get<Filter>(filterResult));
+        auto result = filter.execute(contextMap);
+
+        EXPECT_EQ(result, expected)
+            << "Filter{ " << qUtf8Printable(input) << " } evaluated to "
+            << qUtf8Printable(result.toString()) << " instead of "
+            << qUtf8Printable(expected.toString()) << ".\nDebug: "
+            << qUtf8Printable(filter.debugString(typingContext));
+    }
+}