13 files changed, 940 insertions, 176 deletions

diff --git a/core/CMakeLists.txt b/core/CMakeLists.txt
index 12361f3..b6199cd 100644
--- a/core/CMakeLists.txt
+++ b/core/CMakeLists.txt
@@ -40,7 +40,7 @@ add_source_group(MODEL_MODULE_SOURCES
 	src/Model/Project.cpp
 	src/Model/TransactionsModel.cpp
 	src/Model/Workflow.cpp
-	src/Model/WorkflowSteps.cpp
+	src/Model/WorkflowNodes.cpp
 )
 
 add_source_group(UI_MODULE_SOURCES
diff --git a/core/src/Model/EvaluatedValue.cpp b/core/src/Model/EvaluatedValue.cpp
index c86f00b..685d50f 100644
--- a/core/src/Model/EvaluatedValue.cpp
+++ b/core/src/Model/EvaluatedValue.cpp
@@ -1,15 +1,32 @@
 #include "EvaluatedValue.hpp"
 
-BaseValue::BaseValue(Type type)
-	: mType{ type } {
+#include <charconv>
+
+BaseValue::BaseValue(Kind kind)
+	: mKind{ kind } {
+}
+
+BaseValue::Kind BaseValue::GetKind() const {
+	return mKind;
 }
 
-BaseValue::Type BaseValue::GetType() const {
-	return mType;
+bool NumericValue::IsInstance(const BaseValue* value) {
+	return value->GetKind() == KD_Numeric;
 }
 
 NumericValue::NumericValue()
-	: BaseValue(BaseValue::NumericType) {
+	: BaseValue(BaseValue::KD_Numeric) {
+}
+
+std::string NumericValue::GetString() const {
+	char buf[64];
+	auto res = std::to_chars(buf, buf + std::size(buf), mValue);
+	if (res.ec == std::errc()) {
+		return std::string(buf, res.ptr);
+	} else {
+		// TODO larger buffer
+		return "<err>";
+	}
 }
 
 int64_t NumericValue::GetInt() const {
@@ -24,8 +41,12 @@ void NumericValue::SetValue(double value) {
 	mValue = value;
 }
 
+bool TextValue::IsInstance(const BaseValue* value) {
+	return value->GetKind() == KD_Text;
+}
+
 TextValue::TextValue()
-	: BaseValue(BaseValue::TextType) {
+	: BaseValue(BaseValue::KD_Text) {
 }
 
 const std::string& TextValue::GetValue() const {
@@ -36,8 +57,22 @@ void TextValue::SetValue(const std::string& value) {
 	mValue = value;
 }
 
+bool DateTimeValue::IsInstance(const BaseValue* value) {
+	return value->GetKind() == KD_DateTime;
+}
+
 DateTimeValue::DateTimeValue()
-	: BaseValue(BaseValue::DateTimeType) {
+	: BaseValue(BaseValue::KD_DateTime) {
+}
+
+std::string DateTimeValue::GetString() const {
+	namespace chrono = std::chrono;
+	auto t = chrono::system_clock::to_time_t(mValue);
+
+	char data[32];
+	std::strftime(data, sizeof(data), "%Y-%m-%d %H:%M:%S", std::localtime(&t));
+
+	return std::string(data);
 }
 
 const std::chrono::time_point<std::chrono::system_clock>& DateTimeValue::GetValue() const {
diff --git a/core/src/Model/EvaluatedValue.hpp b/core/src/Model/EvaluatedValue.hpp
index 838fc39..880fd3e 100644
--- a/core/src/Model/EvaluatedValue.hpp
+++ b/core/src/Model/EvaluatedValue.hpp
@@ -6,21 +6,21 @@
 
 class BaseValue {
 public:
-	enum Type {
-		NumericType,
-		TextType,
-		DateTimeType,
+	enum Kind {
+		KD_Numeric,
+		KD_Text,
+		KD_DateTime,
 
 		/// An unspecified type, otherwise known as "any" in some contexts.
-		InvalidType,
-		TypeCount = InvalidType,
+		KindInvalid,
+		KindCount = KindInvalid,
 	};
 
 private:
-	Type mType;
+	Kind mKind;
 
 public:
-	BaseValue(Type type);
+	BaseValue(Kind kind);
 	virtual ~BaseValue() = default;
 
 	BaseValue(const BaseValue&) = delete;
@@ -28,7 +28,7 @@ public:
 	BaseValue(BaseValue&&) = default;
 	BaseValue& operator=(BaseValue&&) = default;
 
-	Type GetType() const;
+	Kind GetKind() const;
 };
 
 class NumericValue : public BaseValue {
@@ -36,8 +36,10 @@ private:
 	double mValue;
 
 public:
+	static bool IsInstance(const BaseValue* value);
 	NumericValue();
 
+	std::string GetString() const;
 	int64_t GetInt() const;
 	double GetValue() const;
 	void SetValue(double value);
@@ -48,7 +50,9 @@ private:
 	std::string mValue;
 
 public:
+	static bool IsInstance(const BaseValue* value);
 	TextValue();
+
 	const std::string& GetValue() const;
 	void SetValue(const std::string& value);
 };
@@ -58,7 +62,10 @@ private:
 	std::chrono::time_point<std::chrono::system_clock> mValue;
 
 public:
+	static bool IsInstance(const BaseValue* value);
 	DateTimeValue();
+
+	std::string GetString() const;
 	const std::chrono::time_point<std::chrono::system_clock>& GetValue() const;
 	void SetValue(const std::chrono::time_point<std::chrono::system_clock>& value);
 };
diff --git a/core/src/Model/Workflow.cpp b/core/src/Model/Workflow.cpp
index 96031c1..385544e 100644
--- a/core/src/Model/Workflow.cpp
+++ b/core/src/Model/Workflow.cpp
@@ -1,98 +1,298 @@
 #include "Workflow.hpp"
 
-bool WorkflowStep::InputNode::IsConnected() const {
-	return ConnectedNodeIndex != -1;
+#include <cassert>
+#include <utility>
+
+WorkflowConnection::WorkflowConnection()
+	: Source{ WorkflowNode::kInvalidId }
+	, Destination{ WorkflowNode::kInvalidId }
+	, SourcePin{ -1 }
+	, DestinationPin{ -1 } {
+}
+
+bool WorkflowConnection::IsValid() const {
+	return Source != WorkflowNode::kInvalidId;
+}
+
+bool WorkflowNode::InputPin::IsConstantConnection() const {
+	return ConnectionToConst;
 }
 
-bool WorkflowStep::OutputNode::IsConnected() const {
-	return ConnectedNodeIndex != -1;
+bool WorkflowNode::InputPin::IsConnected() const {
+	return Connection != WorkflowConnection::kInvalidId;
 }
 
-size_t WorkflowStep::GetId() const {
+bool WorkflowNode::OutputPin::IsConnected() const {
+	return Connection != WorkflowConnection::kInvalidId;
+}
+
+WorkflowNode::WorkflowNode(Type type, Kind kind)
+	: mType{ type }
+	, mKind{ kind } {
+}
+
+size_t WorkflowNode::GetId() const {
 	return mId;
 }
 
-void WorkflowStep::ConnectInput(int nodeId, size_t outputId, int outputNodeId) {
-	mWorkflow->Connect(mId, nodeId, outputId, outputNodeId);
+WorkflowNode::Type WorkflowNode::GetType() const {
+	return mType;
+}
+
+WorkflowNode::Kind WorkflowNode::GetKind() const {
+	return mKind;
+}
+
+void WorkflowNode::ConnectInput(int nodeId, WorkflowNode& output, int outputNodeId) {
+	mWorkflow->Connect(*this, nodeId, output, outputNodeId);
 }
 
-void WorkflowStep::DisconnectInput(int nodeId) {
-	mWorkflow->DisconnectByDestination(mId, nodeId);
+void WorkflowNode::DisconnectInput(int nodeId) {
+	mWorkflow->DisconnectByDestination(*this, nodeId);
 }
 
-bool WorkflowStep::IsInputConnected(int nodeId) const {
+bool WorkflowNode::IsInputConnected(int nodeId) const {
 	return mInputs[nodeId].IsConnected();
 }
 
-void WorkflowStep::ConnectOutput(int nodeId, size_t inputId, int inputNodeId) {
-	mWorkflow->Connect(inputId, inputNodeId, mId, nodeId);
+void WorkflowNode::ConnectOutput(int nodeId, WorkflowNode& input, int inputNodeId) {
+	mWorkflow->Connect(input, inputNodeId, *this, nodeId);
 }
 
-void WorkflowStep::DisconnectOutput(int nodeId) {
-	mWorkflow->DisconnectBySource(mId, nodeId);
+void WorkflowNode::DisconnectOutput(int nodeId) {
+	mWorkflow->DisconnectBySource(*this, nodeId);
 }
 
-bool WorkflowStep::IsOutputConnected(int nodeId) const {
+bool WorkflowNode::IsOutputConnected(int nodeId) const {
 	return mOutputs[nodeId].IsConnected();
 }
 
+WorkflowNode::InputPin& WorkflowNode::InsertInputPin(int atIdx) {
+	assert(atIdx >= 0 && atIdx < mInputs.size());
+
+	mInputs.push_back(InputPin{});
+	for (int i = (int)mInputs.size() - 1, end = atIdx + 1; i >= end; --i) {
+		SwapInputPin(i, i + 1);
+	}
+
+	return mInputs[atIdx];
+}
+
+void WorkflowNode::RemoveInputPin(int pin) {
+	DisconnectInput(pin);
+	for (int i = 0, end = (int)mInputs.size() - 1; i < end; ++i) {
+		SwapInputPin(i, i + 1);
+	}
+	mInputs.resize(mInputs.size() - 1);
+}
+
+void WorkflowNode::SwapInputPin(int a, int b) {
+	auto& pinA = mInputs[a];
+	auto& pinB = mInputs[b];
+
+	if (mWorkflow) {
+		if (pinA.IsConnected() && !pinA.IsConstantConnection()) {
+			auto& conn = *mWorkflow->GetConnectionById(pinA.Connection);
+			conn.DestinationPin = b;
+		}
+		if (pinB.IsConnected() && !pinB.IsConstantConnection()) {
+			auto& conn = *mWorkflow->GetConnectionById(pinB.Connection);
+			conn.DestinationPin = a;
+		}
+	}
+
+	std::swap(pinA, pinB);
+}
+
+WorkflowNode::OutputPin& WorkflowNode::InsertOutputPin(int atIdx) {
+	assert(atIdx >= 0 && atIdx < mOutputs.size());
+
+	mOutputs.push_back(OutputPin{});
+	for (int i = (int)mOutputs.size() - 1, end = atIdx + 1; i >= end; --i) {
+		SwapOutputPin(i, i + 1);
+	}
+
+	return mOutputs[atIdx];
+}
+
+void WorkflowNode::RemoveOutputPin(int pin) {
+	DisconnectOutput(pin);
+	for (int i = 0, end = (int)mOutputs.size() - 1; i < end; ++i) {
+		SwapInputPin(i, i + 1);
+	}
+	mOutputs.resize(mOutputs.size() - 1);
+}
+
+void WorkflowNode::SwapOutputPin(int a, int b) {
+	auto& pinA = mOutputs[a];
+	auto& pinB = mOutputs[b];
+
+	if (mWorkflow) {
+		if (pinA.IsConnected()) {
+			auto& conn = *mWorkflow->GetConnectionById(pinA.Connection);
+			conn.SourcePin = b;
+		}
+		if (pinB.IsConnected()) {
+			auto& conn = *mWorkflow->GetConnectionById(pinB.Connection);
+			conn.SourcePin = a;
+		}
+	}
+
+	std::swap(pinA, pinB);
+}
+
 WorkflowConnection* Workflow::GetConnectionById(size_t id) {
 	return &mConnections[id];
 }
 
-WorkflowStep* Workflow::GetStepById(size_t id) {
-	return mSteps[id].get();
+WorkflowNode* Workflow::GetStepById(size_t id) {
+	return mNodes[id].get();
+}
+
+BaseValue* Workflow::GetConstantById(size_t id) {
+	return mConstants[id].get();
+}
+
+void Workflow::AddStep(std::unique_ptr<WorkflowNode> step) {
+	auto [storage, id] = AllocWorkflowStep();
+	storage = std::move(step);
+	storage->OnAttach(*this, id);
+	storage->mWorkflow = this;
+	storage->mId = id;
+}
+
+void Workflow::RemoveStep(size_t id) {
+	auto& step = mNodes[id];
+	if (step == nullptr) return;
+
+	step->OnDetach();
+	step->mWorkflow = nullptr;
+	step->mId = WorkflowNode::kInvalidId;
+}
+
+void Workflow::Connect(WorkflowNode& source, int sourceNode, WorkflowNode& destination, int destinationNode) {
+	if (source.mInputs[sourceNode].IsConnected()) {
+		DisconnectBySource(source, sourceNode);
+	}
+
+	auto [conn, id] = AllocWorkflowConnection();
+	conn.Source = source.GetId();
+	conn.SourcePin = sourceNode;
+	conn.Destination = destination.GetId();
+	conn.DestinationPin = destinationNode;
+
+	source.mInputs[sourceNode].Connection = id;
+	destination.mInputs[destinationNode].Connection = id;
+}
+
+void Workflow::DisconnectBySource(WorkflowNode& source, int sourceNode) {
+	auto& sn = source.mOutputs[sourceNode];
+	if (!sn.IsConnected()) return;
+
+	auto& conn = mConnections[sn.Connection];
+	auto& dn = mNodes[conn.Destination]->mInputs[conn.DestinationPin];
+
+	sn.Connection = WorkflowConnection::kInvalidId;
+	dn.Connection = WorkflowConnection::kInvalidId;
+	conn = {};
+}
+
+void Workflow::DisconnectByDestination(WorkflowNode& destination, int destinationNode) {
+	auto& dn = destination.mOutputs[destinationNode];
+	if (!dn.IsConnected()) return;
+
+	auto& conn = mConnections[dn.Connection];
+	auto& sn = mNodes[conn.Source]->mInputs[conn.SourcePin];
+
+	sn.Connection = WorkflowConnection::kInvalidId;
+	dn.Connection = WorkflowConnection::kInvalidId;
+	conn = {};
+}
+
+std::pair<WorkflowConnection&, size_t> Workflow::AllocWorkflowConnection() {
+	for (size_t idx = 0; idx < mConnections.size(); ++idx) {
+		auto& elm = mConnections[idx];
+		if (!elm.IsValid()) {
+			return { elm, idx };
+		}
+	}
+
+	auto id = mConnections.size();
+	return { mConnections.emplace_back(WorkflowConnection{}), id };
+}
+
+std::pair<std::unique_ptr<WorkflowNode>&, size_t> Workflow::AllocWorkflowStep() {
+	for (size_t idx = 0; idx < mNodes.size(); ++idx) {
+		auto& elm = mNodes[idx];
+		if (elm == nullptr) {
+			return { elm, idx };
+		}
+	}
+
+	auto id = mNodes.size();
+	return { mNodes.emplace_back(std::unique_ptr<WorkflowNode>()), id };
 }
 
-void WorkflowStep::OnIOAboutToChange() {
-	// TODO more robust solution that handles changes incrementally
-	for (size_t i = 0; i < mInputs.size(); ++i) {
-		DisconnectInput(i);
+struct WorkflowEvaluationContext::RuntimeNode {
+	WorkflowNode* Node;
+};
+
+struct WorkflowEvaluationContext::RuntimeConnection {
+	WorkflowConnection* Connection;
+	std::unique_ptr<BaseValue> Value;
+};
+
+WorkflowEvaluationContext::WorkflowEvaluationContext(Workflow& workflow)
+	: mWorkflow{ &workflow } {
+	mNodes.resize(workflow.mNodes.size());
+	for (size_t i = 0; i < workflow.mNodes.size(); ++i) {
+		mNodes[i].Node = workflow.mNodes[i].get();
 	}
-	for (size_t i = 0; i < mOutputs.size(); ++i) {
-		DisconnectOutput(i);
+
+	mConnections.resize(workflow.mConnections.size());
+	for (size_t i = 0; i < workflow.mConnections.size(); ++i) {
+		mConnections[i].Connection = &workflow.mConnections[i];
 	}
 }
 
-void WorkflowStep::OnIOChanged() {
+BaseValue* WorkflowEvaluationContext::GetConnectionValue(size_t id, bool constant) {
+	if (constant) {
+		return mWorkflow->GetConstantById(id);
+	} else {
+		return mConnections[id].Value.get();
+	}
 }
 
-void Workflow::Connect(size_t source, int sourceNode, size_t destination, int destinationNode) {
-	auto& src = *mSteps[source];
-	auto& o = src.mInputs[sourceNode];
-	if (o.IsConnected()) {
-		DisconnectBySource(source, sourceNode);
+BaseValue* WorkflowEvaluationContext::GetConnectionValue(const WorkflowNode::InputPin& inputPin) {
+	if (inputPin.IsConnected()) {
+		return GetConnectionValue(inputPin.Connection, inputPin.IsConstantConnection());
+	} else {
+		return nullptr;
 	}
+}
+
+void WorkflowEvaluationContext::SetConnectionValue(size_t id, std::unique_ptr<BaseValue> value) {
+	mConnections[id].Value = std::move(value);
+}
 
-	o.ConnectedStep = destination;
-	o.ConnectedNodeIndex = destinationNode;
+void WorkflowEvaluationContext::SetConnectionValue(const WorkflowNode::OutputPin& outputPin, std::unique_ptr<BaseValue> value) {
+	if (outputPin.IsConnected()) {
+		SetConnectionValue(outputPin.Connection, std::move(value));
+	}
+}
 
-	auto& dst = *mSteps[destination];
-	auto& i = dst.mInputs[destinationNode];
-	i.ConnectedStep = source;
-	i.ConnectedNodeIndex = sourceNode;
+void WorkflowEvaluationContext::Run() {
+	// TODO
 }
 
-void Workflow::DisconnectBySource(size_t source, int sourceNode) {
-	auto& src = *mSteps[source];
-	auto& o = src.mOutputs[sourceNode];
-	if (!o.IsConnected()) return;
+void WorkflowEvaluationContext::ReportError(std::string message, const WorkflowNode& node, int pinId, bool inputPin) {
+}
 
-	auto& i = mSteps[o.ConnectedStep]->mInputs[o.ConnectedNodeIndex];
-	i.ConnectedStep = WorkflowStep::kInvalidId;
-	i.ConnectedNodeIndex = -1;
-	o.ConnectedStep = WorkflowStep::kInvalidId;
-	o.ConnectedNodeIndex = -1;
+void WorkflowEvaluationContext::ReportError(std::string message, const WorkflowNode& node) {
 }
 
-void Workflow::DisconnectByDestination(size_t destination, int destinationNode) {
-	auto& dst = *mSteps[destination];
-	auto& i = dst.mInputs[destinationNode];
-	if (!i.IsConnected()) return;
+void WorkflowEvaluationContext::ReportWarning(std::string message, const WorkflowNode& node, int pinId, bool inputPin) {
+}
 
-	auto& o = mSteps[i.ConnectedStep]->mOutputs[i.ConnectedNodeIndex];
-	i.ConnectedStep = WorkflowStep::kInvalidId;
-	i.ConnectedNodeIndex = -1;
-	o.ConnectedStep = WorkflowStep::kInvalidId;
-	o.ConnectedNodeIndex = -1;
+void WorkflowEvaluationContext::ReportWarning(std::string message, const WorkflowNode& node) {
 }
diff --git a/core/src/Model/Workflow.hpp b/core/src/Model/Workflow.hpp
index a90ef21..2c83816 100644
--- a/core/src/Model/Workflow.hpp
+++ b/core/src/Model/Workflow.hpp
@@ -1,6 +1,7 @@
 #pragma once
 
-#include "EvaluatedValue.hpp"
+#include "Model/EvaluatedValue.hpp"
+#include "cplt_fwd.hpp"
 
 #include <cstddef>
 #include <cstdint>
@@ -10,92 +11,180 @@
 
 class WorkflowConnection {
 public:
+	static constexpr auto kInvalidId = std::numeric_limits<size_t>::max();
+
 	size_t Source;
 	size_t Destination;
+	int SourcePin;
+	int DestinationPin;
+
+public:
+	WorkflowConnection();
+
+	bool IsValid() const;
 };
 
-class WorkflowStep {
+class WorkflowNode {
 public:
 	static constexpr auto kInvalidId = std::numeric_limits<size_t>::max();
 
-	// TODO do we even need this?
-	//	enum Type {
-	//		NumericAdditionType,
-	//		NumericSubtractionType,
-	//		NumericMultiplicationType,
-	//		NumericDivisionType,
-	//		NumericExpressionType,
-	//		TextFormattingType,
-	//		FormInputType,
-	//		DatabaseRowsInputType,
-	//		DocumentTemplateExpansionType,
-	//
-	//		InvalidType,
-	//		TypeCount = InvalidType,
-	//	};
+	enum Type {
+		InputType,
+		TransformType,
+		OutputType,
+	};
+
+	enum Kind {
+		KD_NumericAddition,
+		KD_NumericSubtraction,
+		KD_NumericMultiplication,
+		KD_NumericDivision,
+		KD_NumericExpression,
+		KD_TextFormatting,
+		KD_DocumentTemplateExpansion,
+		KD_FormInput,
+		KD_DatabaseRowsInput,
+
+		InvalidKind,
+		KindCount = InvalidKind,
+	};
 
 protected:
-	struct InputNode {
-		size_t ConnectedStep = kInvalidId;
-		int ConnectedNodeIndex = -1;
-		BaseValue::Type MatchingType = BaseValue::InvalidType;
+	struct InputPin {
+		size_t Connection = WorkflowConnection::kInvalidId;
+		BaseValue::Kind MatchingType = BaseValue::KindInvalid;
+		bool ConnectionToConst = false;
 
+		/// A constant connection connects from a user-specified constant value, feeding to a valid \c Destination and \c DestinationPin (i.e. input pins).
+		bool IsConstantConnection() const;
 		bool IsConnected() const;
 	};
 
-	struct OutputNode {
-		size_t ConnectedStep = kInvalidId;
-		int ConnectedNodeIndex = -1;
-		BaseValue::Type MatchingType = BaseValue::InvalidType;
+	struct OutputPin {
+		size_t Connection = WorkflowConnection::kInvalidId;
+		BaseValue::Kind MatchingType = BaseValue::KindInvalid;
 
 		bool IsConnected() const;
 	};
 
 	friend class Workflow;
+	friend class WorkflowEvaluationError;
+	friend class WorkflowEvaluationContext;
+
 	Workflow* mWorkflow;
 	size_t mId;
-	std::vector<InputNode> mInputs;
-	std::vector<OutputNode> mOutputs;
+	std::vector<InputPin> mInputs;
+	std::vector<OutputPin> mOutputs;
+	Type mType;
+	Kind mKind;
 
 public:
-	virtual ~WorkflowStep() = default;
+	WorkflowNode(Type type, Kind kind);
+	virtual ~WorkflowNode() = default;
 
-	WorkflowStep(const WorkflowStep&) = delete;
-	WorkflowStep& operator=(const WorkflowStep&) = delete;
-	WorkflowStep(WorkflowStep&&) = default;
-	WorkflowStep& operator=(WorkflowStep&&) = default;
+	WorkflowNode(const WorkflowNode&) = delete;
+	WorkflowNode& operator=(const WorkflowNode&) = delete;
+	WorkflowNode(WorkflowNode&&) = default;
+	WorkflowNode& operator=(WorkflowNode&&) = default;
 
 	size_t GetId() const;
+	Type GetType() const;
+	Kind GetKind() const;
 
-	void ConnectInput(int nodeId, size_t outputId, int outputNodeId);
+	void ConnectInput(int nodeId, WorkflowNode& output, int outputNodeId);
 	void DisconnectInput(int nodeId);
 	bool IsInputConnected(int nodeId) const;
 
-	void ConnectOutput(int nodeId, size_t inputId, int inputNodeId);
+	void ConnectOutput(int nodeId, WorkflowNode& input, int inputNodeId);
 	void DisconnectOutput(int nodeId);
 	bool IsOutputConnected(int nodeId) const;
 
+	virtual void Evaluate(WorkflowEvaluationContext& ctx) = 0;
+
 protected:
-	/// Child classes should call this whenever \c mInputs and \c mOutputs are about to be modified (append or remove)
-	/// after invocation of the constructor.
-	void OnIOAboutToChange();
-	void OnIOChanged();
+	InputPin& InsertInputPin(int atIdx);
+	void RemoveInputPin(int pin);
+	void SwapInputPin(int a, int b);
+	OutputPin& InsertOutputPin(int atIdx);
+	void RemoveOutputPin(int pin);
+	void SwapOutputPin(int a, int b);
+
+	/* For \c Workflow to invoke, override by implementations */
+
+	void OnAttach(Workflow& workflow, size_t newId) {}
+	void OnDetach() {}
 };
 
 class Workflow {
 private:
+	friend class WorkflowEvaluationContext;
+
 	std::vector<WorkflowConnection> mConnections;
-	std::vector<std::unique_ptr<WorkflowStep>> mSteps;
+	std::vector<std::unique_ptr<WorkflowNode>> mNodes;
+	std::vector<std::unique_ptr<BaseValue>> mConstants;
 
 public:
 	WorkflowConnection* GetConnectionById(size_t id);
-	WorkflowStep* GetStepById(size_t id);
+	WorkflowNode* GetStepById(size_t id);
+	BaseValue* GetConstantById(size_t id);
+
+	void AddStep(std::unique_ptr<WorkflowNode> step);
+	void RemoveStep(size_t id);
+
+	void Connect(WorkflowNode& source, int sourceNode, WorkflowNode& destination, int destinationNode);
+	void DisconnectBySource(WorkflowNode& source, int sourceNode);
+	void DisconnectByDestination(WorkflowNode& destination, int destinationNode);
 
-	void Connect(size_t source, int sourceNode, size_t destination, int destinationNode);
-	void DisconnectBySource(size_t source, int sourceNode);
-	void DisconnectByDestination(size_t destination, int destinationNode);
+private:
+	std::pair<WorkflowConnection&, size_t> AllocWorkflowConnection();
+	std::pair<std::unique_ptr<WorkflowNode>&, size_t> AllocWorkflowStep();
+};
+
+class WorkflowEvaluationError {
+public:
+	enum MessageType : int16_t {
+		Error,
+		Warning,
+	};
+
+	enum PinType : int16_t {
+		NoPin,
+		InputPin,
+		OutputPin,
+	};
+
+public:
+	std::string Message;
+	size_t NodeId;
+	int PinId;
+	PinType PinType;
+	MessageType Type;
 };
 
 class WorkflowEvaluationContext {
+private:
+	struct RuntimeNode;
+	struct RuntimeConnection;
+
+	Workflow* mWorkflow;
+	std::vector<RuntimeNode> mNodes;
+	std::vector<RuntimeConnection> mConnections;
+	std::vector<WorkflowEvaluationError> mErrors;
+	std::vector<WorkflowEvaluationError> mWarnings;
+
+public:
+	WorkflowEvaluationContext(Workflow& workflow);
+
+	BaseValue* GetConnectionValue(size_t id, bool constant);
+	BaseValue* GetConnectionValue(const WorkflowNode::InputPin& inputPin);
+	void SetConnectionValue(size_t id, std::unique_ptr<BaseValue> value);
+	void SetConnectionValue(const WorkflowNode::OutputPin& outputPin, std::unique_ptr<BaseValue> value);
+
+	void ReportError(std::string message, const WorkflowNode& node, int pinId, bool inputPin);
+	void ReportError(std::string message, const WorkflowNode& node);
+	void ReportWarning(std::string message, const WorkflowNode& node, int pinId, bool inputPin);
+	void ReportWarning(std::string message, const WorkflowNode& node);
 
+	/// Run until all possible paths have been evaluated.
+	void Run();
 };
diff --git a/core/src/Model/WorkflowNodes.cpp b/core/src/Model/WorkflowNodes.cpp
new file mode 100644
index 0000000..8c315b6
--- /dev/null
+++ b/core/src/Model/WorkflowNodes.cpp
@@ -0,0 +1,309 @@
+#include "WorkflowNodes.hpp"
+
+#include "Utils/Macros.hpp"
+#include "Utils/RTTI.hpp"
+#include "Utils/Variant.hpp"
+
+#include <cassert>
+#include <variant>
+
+WorkflowNode::Kind NumericOperationNode::OperationTypeToNodeKind(OperationType type) {
+	switch (type) {
+		case Addition: return KD_NumericAddition;
+		case Subtraction: return KD_NumericSubtraction;
+		case Multiplication: return KD_NumericMultiplication;
+		case Division: return KD_NumericDivision;
+		default: UNREACHABLE;
+	}
+}
+
+NumericOperationNode::OperationType NumericOperationNode::NodeKindToOperationType(Kind kind) {
+	switch (kind) {
+		case KD_NumericAddition: return Addition;
+		case KD_NumericSubtraction: return Subtraction;
+		case KD_NumericMultiplication: return Multiplication;
+		case KD_NumericDivision: return Division;
+		default: UNREACHABLE;
+	}
+}
+
+bool NumericOperationNode::IsInstance(const WorkflowNode* node) {
+	return node->GetKind() >= KD_NumericAddition && node->GetKind() <= KD_NumericDivision;
+}
+
+NumericOperationNode::NumericOperationNode(OperationType type)
+	: WorkflowNode(TransformType, OperationTypeToNodeKind(type))
+	, mType{ type } {
+	mInputs.resize(2);
+	mInputs[0].MatchingType = BaseValue::KD_Numeric;
+	mInputs[1].MatchingType = BaseValue::KD_Numeric;
+
+	mOutputs.resize(1);
+	mOutputs[0].MatchingType = BaseValue::KD_Numeric;
+}
+
+void NumericOperationNode::Evaluate(WorkflowEvaluationContext& ctx) {
+	auto lhsVal = dyn_cast<NumericValue>(ctx.GetConnectionValue(mInputs[0]));
+	if (!lhsVal) return;
+	double lhs = lhsVal->GetValue();
+
+	auto rhsVal = dyn_cast<NumericValue>(ctx.GetConnectionValue(mInputs[1]));
+	if (!rhsVal) return;
+	double rhs = rhsVal->GetValue();
+
+	double res;
+	switch (mType) {
+		case Addition: res = lhs + rhs; break;
+		case Subtraction: res = lhs - rhs; break;
+		case Multiplication: res = lhs * rhs; break;
+		case Division: {
+			if (rhs == 0.0) {
+				// TODO localize
+				ctx.ReportError("Error: division by 0", *this);
+				return;
+			}
+			res = lhs / rhs;
+		} break;
+
+		default: return;
+	}
+
+	ctx.SetConnectionValue(mOutputs[0], res);
+}
+
+bool NumericExpressionNode::IsInstance(const WorkflowNode* node) {
+	return node->GetKind() == KD_NumericExpression;
+}
+
+NumericExpressionNode::NumericExpressionNode()
+	: WorkflowNode(TransformType, KD_NumericExpression) {
+}
+
+struct FormatTextNode::Argument {
+	ArgumentType ArgumentType;
+	int PinIdx;
+
+	template <class TFunction>
+	static void ForArguments(std::vector<Element>::iterator begin, std::vector<Element>::iterator end, const TFunction& func) {
+		for (auto it = begin; it != end; ++it) {
+			auto& elm = *it;
+			if (auto arg = std::get_if<Argument>(&elm)) {
+				func(*arg);
+			}
+		}
+	}
+
+	/// Find the pin index that the \c elmIdx -th element should have, based on the elements coming before it.
+	static int FindPinForElement(const std::vector<Element>& vec, int elmIdx) {
+		for (int i = elmIdx; i >= 0; --i) {
+			auto& elm = vec[i];
+			if (auto arg = std::get_if<Argument>(&elm)) {
+				return arg->PinIdx + 1;
+			}
+		}
+		return 0;
+	}
+};
+
+BaseValue::Kind FormatTextNode::ArgumentTypeToValueKind(FormatTextNode::ArgumentType arg) {
+	switch (arg) {
+		case NumericArgument: return BaseValue::KD_Numeric;
+		case TextArgument: return BaseValue::KD_Text;
+		case DateTimeArgument: return BaseValue::KD_DateTime;
+		default: UNREACHABLE;
+	}
+}
+
+bool FormatTextNode::IsInstance(const WorkflowNode* node) {
+	return node->GetKind() == KD_TextFormatting;
+}
+
+FormatTextNode::FormatTextNode()
+	: WorkflowNode(TransformType, KD_TextFormatting) {
+}
+
+int FormatTextNode::GetElementCount() const {
+	return mElements.size();
+}
+
+const FormatTextNode::Element& FormatTextNode::GetElement(int idx) const {
+	return mElements[idx];
+}
+
+void FormatTextNode::SetElement(int idx, std::string text) {
+	assert(idx >= 0 && idx < mElements.size());
+
+	std::visit(
+		Overloaded{
+			[&](const std::string& original) { mMinOutputChars -= original.size(); },
+			[&](const Argument& original) { PreRemoveElement(idx); },
+		},
+		mElements[idx]);
+
+	mMinOutputChars += text.size();
+	mElements[idx] = std::move(text);
+}
+
+void FormatTextNode::SetElement(int idx, ArgumentType argument) {
+	assert(idx >= 0 && idx < mElements.size());
+
+	std::visit(
+		Overloaded{
+			[&](const std::string& original) {
+				mMinOutputChars -= original.size();
+
+				mElements[idx] = Argument{
+					.ArgumentType = argument,
+					.PinIdx = Argument::FindPinForElement(mElements, idx),
+				};
+				/* `original` is invalid from this point */
+			},
+			[&](const Argument& original) {
+				int pinIdx = original.PinIdx;
+
+				// Create pin
+				auto& pin = mInputs[pinIdx];
+				pin.MatchingType = ArgumentTypeToValueKind(argument);
+
+				// Create element
+				mElements[idx] = Argument{
+					.ArgumentType = argument,
+					.PinIdx = pinIdx,
+				};
+				/* `original` is invalid from this point */
+			},
+		},
+		mElements[idx]);
+}
+
+void FormatTextNode::InsertElement(int idx, std::string text) {
+	assert(idx >= 0);
+	if (idx >= mElements.size()) AppendElement(std::move(text));
+
+	mMinOutputChars += text.size();
+	mElements.insert(mElements.begin() + idx, std::move(text));
+}
+
+void FormatTextNode::InsertElement(int idx, ArgumentType argument) {
+	assert(idx >= 0);
+	if (idx >= mElements.size()) AppendElement(argument);
+
+	int pinIdx = Argument::FindPinForElement(mElements, idx);
+
+	// Create pin
+	auto& pin = InsertInputPin(pinIdx);
+	pin.MatchingType = ArgumentTypeToValueKind(argument);
+
+	// Create element
+	mElements.insert(
+		mElements.begin() + idx,
+		Argument{
+			.ArgumentType = argument,
+			.PinIdx = pinIdx,
+		});
+}
+
+void FormatTextNode::AppendElement(std::string text) {
+	mMinOutputChars += text.size();
+	mElements.push_back(std::move(text));
+}
+
+void FormatTextNode::AppendElement(ArgumentType argument) {
+	int pinIdx = mInputs.size();
+	// Create pin
+	mInputs.push_back(InputPin{});
+	mInputs.back().MatchingType = ArgumentTypeToValueKind(argument);
+	// Creat eelement
+	mElements.push_back(Argument{
+		.ArgumentType = argument,
+		.PinIdx = pinIdx,
+	});
+}
+
+void FormatTextNode::RemoveElement(int idx) {
+	assert(idx >= 0 && idx < mElements.size());
+
+	PreRemoveElement(idx);
+	if (auto arg = std::get_if<Argument>(&mElements[idx])) {
+		RemoveInputPin(arg->PinIdx);
+	}
+	mElements.erase(mElements.begin() + idx);
+}
+
+void FormatTextNode::Evaluate(WorkflowEvaluationContext& ctx) {
+	std::string result;
+	result.reserve((size_t)(mMinOutputChars * 1.5f));
+
+	auto HandleText = [&](const std::string& str) {
+		result += str;
+	};
+	auto HandleArgument = [&](const Argument& arg) {
+		switch (arg.ArgumentType) {
+			case NumericArgument: {
+				if (auto val = dyn_cast<NumericValue>(ctx.GetConnectionValue(mInputs[arg.PinIdx]))) {
+					result += val->GetString();
+				} else {
+					// TODO localize
+					ctx.ReportError("Non-numeric value connected to a numeric text format parameter.", *this);
+				}
+			} break;
+			case TextArgument: {
+				if (auto val = dyn_cast<TextValue>(ctx.GetConnectionValue(mInputs[arg.PinIdx]))) {
+					result += val->GetValue();
+				} else {
+					// TODO localize
+					ctx.ReportError("Non-text value connected to a textual text format parameter.", *this);
+				}
+			} break;
+			case DateTimeArgument: {
+				if (auto val = dyn_cast<DateTimeValue>(ctx.GetConnectionValue(mInputs[arg.PinIdx]))) {
+					result += val->GetString();
+				} else {
+					// TODO localize
+					ctx.ReportError("Non-date/time value connected to a date/time text format parameter.", *this);
+				}
+			} break;
+		}
+	};
+
+	for (auto& elm : mElements) {
+		std::visit(Overloaded{ HandleText, HandleArgument }, elm);
+	}
+}
+
+void FormatTextNode::PreRemoveElement(int idx) {
+	auto& elm = mElements[idx];
+	if (auto arg = std::get_if<Argument>(&elm)) {
+		RemoveInputPin(arg->PinIdx);
+		Argument::ForArguments(
+			mElements.begin() + idx + 1,
+			mElements.end(),
+			[&](Argument& arg) {
+				arg.PinIdx--;
+			});
+	}
+}
+
+bool DocumentTemplateExpansionNode::IsInstance(const WorkflowNode* node) {
+	return node->GetKind() == KD_DocumentTemplateExpansion;
+}
+
+DocumentTemplateExpansionNode::DocumentTemplateExpansionNode()
+	: WorkflowNode(TransformType, KD_DocumentTemplateExpansion) {
+}
+
+bool FormInputNode::IsInstance(const WorkflowNode* node) {
+	return node->GetKind() == KD_FormInput;
+}
+
+FormInputNode::FormInputNode()
+	: WorkflowNode(InputType, KD_FormInput) {
+}
+
+bool DatabaseRowsInputNode::IsInstance(const WorkflowNode* node) {
+	return node->GetKind() == KD_DatabaseRowsInput;
+}
+
+DatabaseRowsInputNode::DatabaseRowsInputNode()
+	: WorkflowNode(InputType, KD_DatabaseRowsInput) {
+}
diff --git a/core/src/Model/WorkflowNodes.hpp b/core/src/Model/WorkflowNodes.hpp
new file mode 100644
index 0000000..e7d4bfc
--- /dev/null
+++ b/core/src/Model/WorkflowNodes.hpp
@@ -0,0 +1,101 @@
+#pragma once
+
+#include "Model/Workflow.hpp"
+
+#include <cstddef>
+#include <memory>
+#include <variant>
+#include <vector>
+
+class NumericOperationNode : public WorkflowNode {
+public:
+	enum OperationType {
+		Addition,
+		Subtraction,
+		Multiplication,
+		Division,
+
+		InvalidType,
+		TypeCount = InvalidType,
+	};
+
+private:
+	OperationType mType;
+
+public:
+	static Kind OperationTypeToNodeKind(OperationType type);
+	static OperationType NodeKindToOperationType(Kind kind);
+	static bool IsInstance(const WorkflowNode* node);
+	NumericOperationNode(OperationType type);
+
+	virtual void Evaluate(WorkflowEvaluationContext& ctx) override;
+};
+
+class NumericExpressionNode : public WorkflowNode {
+public:
+	static bool IsInstance(const WorkflowNode* node);
+	NumericExpressionNode();
+
+	// TODO
+};
+
+class FormatTextNode : public WorkflowNode {
+public:
+	enum ArgumentType {
+		NumericArgument,
+		TextArgument,
+		DateTimeArgument,
+	};
+
+private:
+	struct Argument;
+	using Element = std::variant<std::string, Argument>;
+
+	std::vector<Element> mElements;
+	int mMinOutputChars;
+
+public:
+	static BaseValue::Kind ArgumentTypeToValueKind(ArgumentType arg);
+	static bool IsInstance(const WorkflowNode* node);
+	FormatTextNode();
+
+	int GetElementCount() const;
+	const Element& GetElement(int idx) const;
+
+	void SetElement(int idx, std::string text);
+	void SetElement(int idx, ArgumentType argument);
+	void InsertElement(int idx, std::string text);
+	void InsertElement(int idx, ArgumentType argument);
+	void AppendElement(std::string text);
+	void AppendElement(ArgumentType argument);
+	void RemoveElement(int idx);
+
+	virtual void Evaluate(WorkflowEvaluationContext& ctx) override;
+
+private:
+	void PreRemoveElement(int idx);
+};
+
+class DocumentTemplateExpansionNode : public WorkflowNode {
+public:
+	static bool IsInstance(const WorkflowNode* node);
+	DocumentTemplateExpansionNode();
+
+	// TODO
+};
+
+class FormInputNode : public WorkflowNode {
+public:
+	static bool IsInstance(const WorkflowNode* node);
+	FormInputNode();
+
+	// TODO
+};
+
+class DatabaseRowsInputNode : public WorkflowNode {
+public:
+	static bool IsInstance(const WorkflowNode* node);
+	DatabaseRowsInputNode();
+
+	// TODO
+};
diff --git a/core/src/Model/WorkflowSteps.cpp b/core/src/Model/WorkflowSteps.cpp
deleted file mode 100644
index 815c5ca..0000000
--- a/core/src/Model/WorkflowSteps.cpp
+++ /dev/null
@@ -1,11 +0,0 @@
-#include "WorkflowSteps.hpp"
-
-NumericOperationStep::NumericOperationStep(NumericOperationStep::Type type)
-	: mType{ type } {
-	mInputs.resize(2);
-	mInputs[0].MatchingType = BaseValue::NumericType;
-	mInputs[1].MatchingType = BaseValue::NumericType;
-
-	mOutputs.resize(1);
-	mOutputs[0].MatchingType = BaseValue::NumericType;
-}
diff --git a/core/src/Model/WorkflowSteps.hpp b/core/src/Model/WorkflowSteps.hpp
deleted file mode 100644
index 819ca63..0000000
--- a/core/src/Model/WorkflowSteps.hpp
+++ /dev/null
@@ -1,44 +0,0 @@
-#pragma once
-
-#include "Model/Workflow.hpp"
-
-#include <memory>
-
-class NumericOperationStep : public WorkflowStep {
-public:
-	enum Type {
-		Addition,
-		Subtraction,
-		Multiplication,
-		Division,
-
-		InvalidType,
-		TypeCount = InvalidType,
-	};
-
-private:
-	Type mType;
-
-public:
-	NumericOperationStep(Type type);
-};
-
-class NumericExpressionStep : public WorkflowStep {
-public:
-};
-
-class FormatTextStep : public WorkflowStep {
-public:
-};
-
-class FormInputStep : public WorkflowStep {
-public:
-};
-
-class DatabaseRowsInputStep : public WorkflowStep {
-public:
-};
-
-class DocumentTemplateExpansionStep : public WorkflowStep {
-public:
-};
diff --git a/core/src/Model/fwd.hpp b/core/src/Model/fwd.hpp
index a2156aa..a7df889 100644
--- a/core/src/Model/fwd.hpp
+++ b/core/src/Model/fwd.hpp
@@ -32,14 +32,15 @@ class TransactionModel;
 
 // Workflow.hpp
 class WorkflowConnection;
-class WorkflowStep;
+class WorkflowNode;
 class Workflow;
+class WorkflowEvaluationError;
 class WorkflowEvaluationContext;
 
 // WorkflowSteps.hpp
-class NumericOperationStep;
-class NumericExpressionStep;
-class FormatTextStep;
-class FormInputStep;
-class DatabaseRowsInputStep;
-class DocumentTemplateExpansionStep;
+class NumericOperationNode;
+class NumericExpressionNode;
+class FormatTextNode;
+class DocumentTemplateExpansionNode;
+class FormInputNode;
+class DatabaseRowsInputNode;
diff --git a/core/src/Utils/Macros.hpp b/core/src/Utils/Macros.hpp
index cb949b2..ab846f4 100644
--- a/core/src/Utils/Macros.hpp
+++ b/core/src/Utils/Macros.hpp
@@ -1,6 +1,12 @@
-#pragma  once
+#pragma once
 
 #define CONCAT_IMPL(a, b) a##b
 #define CONCAT(a, b) CONCAT_IMPL(a, b)
 
 #define UNIQUE_NAME(prefix) CONCAT(prefix, __LINE__)
+
+#if defined(_MSC_VER)
+#	define UNREACHABLE __assume(false)
+#elif defined(__clang__) || defined(__GNUC__)
+#	define UUNREACHABLE __builtin_unreachable()
+#endif
diff --git a/core/src/Utils/RTTI.hpp b/core/src/Utils/RTTI.hpp
new file mode 100644
index 0000000..bc0d289
--- /dev/null
+++ b/core/src/Utils/RTTI.hpp
@@ -0,0 +1,44 @@
+#pragma once
+
+#include <cassert>
+
+template <class T, class TBase>
+bool is_a(TBase* t) {
+	assert(t != nullptr);
+	return T::IsInstance(t);
+}
+
+template <class T, class TBase>
+bool is_a_nullable(TBase* t) {
+	if (t) {
+		return is_a<T, TBase>(t);
+	} else {
+		return false;
+	}
+}
+
+template <class T, class TBase>
+T* dyn_cast(TBase* t) {
+	assert(t != nullptr);
+	if (T::IsInstance(t)) {
+		return static_cast<T*>(t);
+	} else {
+		return nullptr;
+	}
+}
+
+template <class T, class TBase>
+const T* dyn_cast(const TBase* t) {
+	assert(t != nullptr);
+	if (T::IsInstance(t)) {
+		return static_cast<const T*>(t);
+	} else {
+		return nullptr;
+	}
+}
+
+template <class T, class TBase>
+T* dyn_cast_nullable(TBase* t) {
+	if (!t) return nullptr;
+	return dyn_cast<T, TBase>(t);
+}
diff --git a/core/src/Utils/Variant.hpp b/core/src/Utils/Variant.hpp
new file mode 100644
index 0000000..7fdb2dc
--- /dev/null
+++ b/core/src/Utils/Variant.hpp
@@ -0,0 +1,27 @@
+#pragma once
+
+#include <utility>
+#include <variant>
+
+template <class... Ts>
+struct Overloaded : Ts... { using Ts::operator()...; };
+template <class... Ts>
+Overloaded(Ts...) -> Overloaded<Ts...>;
+
+template <class... Args>
+struct VariantCastProxy {
+	std::variant<Args...> v;
+
+	template <class... ToArgs>
+	operator std::variant<ToArgs...>() const {
+		return std::visit(
+			[](auto&& arg) -> std::variant<ToArgs...> { return arg; },
+			v);
+	}
+};
+
+/// Use snake_case naming to align with `static_cast`, `dynamic_cast`, etc..
+template <class... Args>
+auto variant_cast(std::variant<Args...> v) -> VariantCastProxy<Args...> {
+	return { std::move(v) };
+}