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+# Intro
+
+The Variable interface provides an uniform access model to data in Simantics. It includes
+
+* Tree-structured address space for accessing model **structure** and **properties**
+* Uniform access to model **configuration** and **state**
+
+Key use cases include
+
+* Browsing of the **model configuration and states** (see [[Model Browser]])
+* **Manipulation of objects** (see [[Selection View]])
+* Representation of **tabular data** (see [[Spreadsheets]])
+
+Main functional requirements include representation of
+
+* **Structural models** with **procedural** features (see [[Structural]])
+* **Runtime** data of solvers (see [[Experiments]])
+* **Historical** data from experiments
+* **Ontological** data
+
+# Solution
+
+**Variable** is a tree-structured view into the Simantics data model. Each variable is either a **child** or a **property** and can further contain own children and properties. The difference between a child and a property is that a property contains a **value**.
+
+The variable space browsing methods are used to obtain
+
+* all children
+* children by name
+* all properties
+* properties by name
+* variable by **path**
+* **parent** variable
+
+Other services are
+
+* accessing (get/set) the value of a property variable
+* querying adapter interfaces
+
+A set of built-in properties is required for all variables. These properties have also dedicated interface methods.
+
+* **URI**, which is an unique string identifier and locator of the variable in the tree structure
+* **Parent**, which is the tree parent of the variable
+* **HasName**, which is a local identifier for the variable within its parent. Names are also used to create URIs.
+* **HasLabel**, which is a short textual representation of the variable
+* **hasStandardResource**, which returns the context resource in a **standard graph-based child variable**
+* **Represents**, which is a resource representing the variable **TODO**
+* **Type**, which returns a single type resource classifying the variable
+* **Role**, which tells whether the variable is a **child** or a **property** (TODO: could be deprecated)
+* **DATATYPE**, which returns the data type of a property variable. (TODO: should be HasDatatype)
+
+Other properties and the structure of the variable space is configured in the semantic graph or contributed by custom variable implementations.
+
+Variables can be located using an **URI**, which
+
+* Represents the path from **root variable** (Variables.getRootVariable) into the variable such that
+** **var/xx** represents a getChild(unescaped(xx)) query from var
+** **var#yy** represents a getProperty(unescaped(yy)) query from var
+** the escape function is bidirectional (URIStringUtils.escape and URIStringUtils.unescape)
+* Is an **identifier** (two variables with the same URI are the same in the sense of Java Object.equals)
+* Is a random access identifier (by Variables.getVariable())
+* Examples:  
+** http://www.acme.org/Projects/MyProject/MyModel/Configuration/DiagramN/PI_X#PI_MASS_FLOW
+** http://www.acme.org/Projects/MyProject/MyModel/ExperimentConfiguration/RunName/DiagramN/PI_X#PI_MASS_FLOW#DATATYPE
+
+A common way of identifying a variable is by supplying a **base variable** and a **Relative Variable Identifier (RVI)**.
+
+* RVI represents the path from **base variable** into another variable
+* In textual RVI notation (Variable.browse())
+** **.** represents a getParent() query
+** **/xx** represents a getChild(unescaped(xx)) query
+** **#yy** represents a getProperty(unescaped(yy)) query
+* A literal RVI (Variable.getRVI(), RVI.resolve())
+** Does not need to depend on the names visible in the URI
+** Is based on e.g. resource ids
+** Survives export/import
+
+A **model** variable represents the root of a Simantics model
+* Model variables correspond directly to instances of SIMU.Model in the database
+** Variable and resource URIs are the same
+* For all variables under a model, the model variable can be obtained using Variables.getModel()
+
+A **context** variable under a model provides a view into a **state** of the model
+* The **Type** property of a context variable is inheried from L0.RVIContext
+* A RVI obtained from e.g. model configuration can be used to access similarly identified data from different model states
+** E.g. /DiagramX/ComponentY#PropertyZ can have different values in different contexts
+* The **configuration** context can be used to browse the structure and configuration values of the model
+* **Experiment run** contexts are used to monitor values from simulations or history
+
+The variable interface is bound to Simantics database **transactions**, but is not in any other way bound to the semantic data model, which allows variable implementations to represent arbitrary data models somehow related to Simantics models. All variable-based requests can be listened using standard Simantics database listening.
+
+Procedural children and variables are used with large data sets. E.g. query-based views can be exposed. Procedural properties also enable efficient slicing of arrays e.g. URI#Array_Property/0-99
+
+= General assertions in the Variable model =
+
+* All variables except the **root** have a parent
+* Let p be the parent of v. Then v#URI equals p#URI + '/'|'#' + escape(v#HasName)
+* Iff v1#URI equals v2#URI, then v1 and v2 are equal in Java Object.equals sense
+** Other identifications can be established by property values
+* A variable v2 equaling variable v can always be obtained by calling Variables.getVariable(v#URI)
+** The obtained variable **need not be the same object** but **can be**
+** Variables.getVariable can return also variables, which are not reachable by browsing (**TODO**)
+* All property variables have a value
+** No child variable has a value
+** The value of a property variable may be **null**
+** The value of **DATATYPE** property can be null for property variables i.e. property values can be arbitrary Java objects
+* Variable.getProperty returns all the variables returned from Variable.browseProperties
+** Variable.getProperty can return variables not returned by Variable.browseProperties
+* Variable.getChild returns all the variables returned from Variable.browseChildren
+** Variable.getChild can return variables not returned by Variable.browseChildren
+* A variable can be part of at most one **model**
+* A variable can be part of at most one **context**
+* All values can be accessed using either Variable.getValue or Variable.getInterface(Accessor.class)
+* All properties retrieved using Variable.browseProperties shall be available using Variable.getProperty
+* No variable can have a name that begins with one or more dots ('.') due to '.' being a reserved character for browsing the variable address space towards the parent. In other words names matching the pattern **"^\.+.*$"** are forbidden.
+
+# Standard properties
+
+## Connections
+
+* Connection point properties are classified with **http://www.simantics.org/Structural-1.0/ConnectionRelation**
+* The value of a connection point property is an object of class **org.simantics.structural2.variables.Connection**
+
+~~~
+
+public interface Connection {
+
+  Collection<Variable> getConnectionPoints(ReadGraph graph) throws DatabaseException;
+
+}
+
+~~~
+
+It is assumed that instances of **org.simantics.structural2.variables.Connection** have proper identities (equals/hashCode) based on flattened connections.
+
+The value of connection point properties can be **null**. This means that the connection point is not connected.
+
+## String editing operations
+
+* **HasDisplayValue** is a String-valued property, which is a formatted and unit-converted representation of the property value.
+* **expression** is a String-valued property, which is an SCL-formula used to compute the value of the property. If the property can be computed using an expression, this property is always available and returns **null** if an expression has not been defined.
+* **validator** is a **org.simantics.utils.strings.StringInputValidator**-valued property. The validator is used for checking values to be written into a property.
+
+~~~
+
+public interface StringInputProblem {
+
+  enum Severity {
+    Error, Warning
+  }
+
+  String getDescription();
+  int getBegin();
+  int getEnd();
+  Severity getSeverity();
+
+}
+
+public interface StringInputValidator {
+
+  Collection<StringInputProblem> validate(String input);
+
+}
+
+~~~
+
+* **valid** is a Boolean-valued property, which indicates whether the property contains a valid value.
+
+## Property properties
+
+* **required** is a Boolean-valued property, which indicates that the property should contain a valid value
+* **default** is a Boolean-valued property, which indicates that the property value is a default value
+* **readOnly** is a Boolean-valued property, which indicates that the property value can not be written
+
+## Complex datatypes
+
+* Record
+  * All named fields are '/name'
+  * Tuples are named after position e.g. '/11'
+* Union
+  * Union does not show in URI
+* Array
+  * Elements are named after position e.g. '/i-11'
+* Map
+  * Items are named after key
+
+# Standard graph based variable implementation
+
+The standard child and property variables are
+
+* org.simantics.db.layer0.variable.StandardGraphChildVariable
+* org.simantics.db.layer0.variable.StandardGraphPropertyVariable
+
+Their implementation is based on the following interfaces
+
+~~~
+
+package org.simantics.db.layer0.variable;
+
+public interface VariableMap {
+    Variable getVariable(ReadGraph graph, Variable context, String name) throws DatabaseException;
+    // Must not modify collection in any way not possible with put-method.
+    void getVariables(ReadGraph graph, Variable context, Map<String, Variable> map) throws DatabaseException;
+}
+
+~~~
+
+~~~
+
+package org.simantics.db.layer0.variable;
+
+public interface ValueAccessor {
+    Object getValue(ReadGraph graph, Variable context) throws DatabaseException;
+    Object getValue(ReadGraph graph, Variable context, Binding binding) throws DatabaseException;
+    void setValue(WriteGraph graph, Variable context, Object value) throws DatabaseException;
+    void setValue(WriteGraph graph, Variable context, Object value, Binding binding) throws DatabaseException;
+}
+
+~~~
+
+Implementations of the above interfaces can be bound to instances and types in the database via the following standard properties
+
+~~~
+
+L0.Entity
+  >-- L0.hasStandardResource ==> "Resource" <R L0.HasProperty : L0.FunctionalRelation
+    L0.HasDescription "The backend resource for standard graph-based Variables."
+  >-- L0.domainProperties ==> "VariableMap" <R L0.HasProperty : L0.FunctionalRelation
+    L0.HasDescription "Retruns a map of all domain properties of the entity."
+  >-- L0.domainChildren ==> "VariableMap" <R L0.HasProperty : L0.FunctionalRelation
+    L0.HasDescription "Retruns a map of all domain children of the entity."
+  >-- L0.valueAccessor ==> "ValueAccessor" <R L0.HasProperty : L0.FunctionalRelation
+    L0.HasDescription "Returns an interface for accessing the value."
+  @L0.assert L0.hasStandardResource
+    L0.Functions.hasStandardResource : L0.Function
+  @L0.assert L0.domainProperties L0.Functions.standardChildDomainProperties
+  @L0.assert L0.domainChildren L0.Functions.standardChildDomainChildren
+
+L0.Value
+  @L0.assert L0.domainProperties L0.Functions.standardPropertyDomainProperties
+  @L0.assert L0.domainChildren L0.Functions.standardPropertyDomainChildren
+  @L0.assert L0.valueAccessor L0.Functions.standardValueAccessor
+
+~~~
+
+The standard implementation is
+
+~~~
+
+@SCLValue(type = "ValueAccessor")
+public static ValueAccessor standardValueAccessor = new ValueAccessor() {
+
+	@Override
+	public Object getValue(ReadGraph graph, Variable context) throws DatabaseException {
+	    	StandardGraphPropertyVariable variable = (StandardGraphPropertyVariable)context;
+		try {
+			ValueAccessor propertyAccessor = getPossiblePropertyValueAccessor(graph, variable);
+			if(propertyAccessor != null) return propertyAccessor.getValue(graph, context);
+			if(graph.sync(new IsEnumeratedValue(variable.getRepresents(graph)))) {
+				Layer0 L0 = Layer0.getInstance(graph);
+				return graph.getRelatedValue2(variable.getRepresents(graph), L0.HasLabel);
+			}
+			if(variable.adapterClass != null) {
+				return graph.adaptRelated(variable.parentResource, variable.property, variable.adapterClass);
+			} else {
+				return graph.getRelatedValue2(variable.parentResource, variable.property, variable);
+			}
+		} catch (NoSingleResultException e) {
+			throw new MissingVariableValueException(variable.getPossibleURI(graph), e);
+		} catch (DoesNotContainValueException e) {
+			throw new MissingVariableValueException(variable.getPossibleURI(graph), e);
+		} catch (DatabaseException e) {
+			throw new MissingVariableValueException(variable.getPossibleURI(graph), e);
+		}
+	}
+	@Override
+	public Object getValue(ReadGraph graph, Variable context, Binding binding) throws DatabaseException {
+	    	StandardGraphPropertyVariable variable = (StandardGraphPropertyVariable)context;
+		try {
+			ValueAccessor propertyAccessor = getPossiblePropertyValueAccessor(graph, variable);
+			if(propertyAccessor != null) return propertyAccessor.getValue(graph, context, binding);
+			if(graph.sync(new IsEnumeratedValue(variable.getRepresents(graph)))) {
+				Layer0 L0 = Layer0.getInstance(graph);
+				return graph.getRelatedValue2(variable.getRepresents(graph), L0.HasLabel, binding);
+			}
+			if(variable.adapterClass != null) {
+				return graph.adaptRelated(variable.parentResource, variable.property, variable.adapterClass);
+			} else {
+				return graph.getRelatedValue2(variable.parentResource, variable.property, variable, binding);
+			}
+		} catch (NoSingleResultException e) {
+			throw new MissingVariableValueException(variable.getPossibleURI(graph));
+		} catch (DoesNotContainValueException e) {
+			throw new MissingVariableValueException(variable.getPossibleURI(graph));
+		} catch (DatabaseException e) {
+			throw new MissingVariableValueException(variable.getPossibleURI(graph));
+		}
+	}
+	@Override
+	public void setValue(WriteGraph graph, Variable context, Object value) throws DatabaseException {
+	    	StandardGraphPropertyVariable variable = (StandardGraphPropertyVariable)context;
+		ValueAccessor propertyAccessor = getPossiblePropertyValueAccessor(graph, variable);
+		if(propertyAccessor != null) {
+			propertyAccessor.setValue(graph, context, value);
+			return;
+		}
+		Function4<WriteGraph, Variable, Object, Object, String> modifier = context.getPossiblePropertyValue(graph, Variables.INPUT_MODIFIER);
+		if(modifier == null) modifier = VariableUtils.defaultInputModifier;
+		try {
+			modifier.apply(graph, context, value, Bindings.getBinding(value.getClass()));
+		} catch (BindingConstructionException e) {
+			throw new DatabaseException(e);
+		}
+	}
+	@Override
+	public void setValue(WriteGraph graph, Variable context, Object value, Binding binding) throws DatabaseException {
+	    	StandardGraphPropertyVariable variable = (StandardGraphPropertyVariable)context;
+		ValueAccessor propertyAccessor = getPossiblePropertyValueAccessor(graph, variable);
+		if(propertyAccessor != null) {
+			propertyAccessor.setValue(graph, context, value, binding);
+			return;
+		}
+		Function4<WriteGraph, Variable, Object, Object, String> modifier = context.getPossiblePropertyValue(graph, Variables.INPUT_MODIFIER);
+		if(modifier == null) modifier = VariableUtils.defaultInputModifier;
+		modifier.apply(graph, context, value, binding);
+	}
+};
+
+@SCLValue(type = "VariableMap")
+public static VariableMap standardChildDomainProperties = new VariableMap() {
+
+	@Override
+	public Variable getVariable(ReadGraph graph, Variable context, String name) throws DatabaseException {
+	       	final StandardGraphChildVariable variable = (StandardGraphChildVariable)context;
+	       	return getPossiblePropertyFromContext(graph, variable, variable.resource, name);
+	}
+	@Override
+	public void getVariables(ReadGraph graph, Variable context, Map<String, Variable> map) throws DatabaseException {
+    		StandardGraphChildVariable variable = (StandardGraphChildVariable)context;
+    		collectPropertiesFromContext(graph, variable, variable.resource, map);
+	}
+
+};
+
+@SCLValue(type = "VariableMap")
+public static VariableMap standardPropertyDomainProperties = new VariableMap() {
+
+	@Override
+	public Variable getVariable(ReadGraph graph, Variable context, String name) throws DatabaseException {
+        	StandardGraphPropertyVariable variable = (StandardGraphPropertyVariable)context;
+        	Resource literal = graph.getPossibleObject(variable.parentResource, variable.property);
+        	if(literal != null) {
+        		Variable result = getPossiblePropertyFromContext(graph, variable, literal, name);
+        		if(result != null) return result;
+        	}
+        	return getPossiblePropertyFromContext(graph, variable, variable.property, name);
+	}
+	@Override
+	public void getVariables(ReadGraph graph, Variable context, Map<String, Variable> map) throws DatabaseException {
+		StandardGraphPropertyVariable variable = (StandardGraphPropertyVariable)context;
+    		collectPropertiesFromContext(graph, variable, variable.property, map);
+        	Resource literal = graph.getPossibleObject(variable.parentResource, variable.property);
+        	if(literal != null) collectPropertiesFromContext(graph, variable, literal, map);
+	}
+
+};
+
+@SCLValue(type = "VariableMap")
+public static VariableMap standardChildDomainChildren = new VariableMap() {
+
+	@Override
+	public Variable getVariable(ReadGraph graph, Variable context, String name) throws DatabaseException {
+		StandardGraphChildVariable variable = (StandardGraphChildVariable)context;
+		Map<String, Resource> children = graph.syncRequest(new UnescapedChildMapOfResource(variable.resource));
+		Resource child = children.get(name);
+		if(child == null) return null;
+		return graph.getPossibleContextualAdapter(child, variable, Variable.class, Variable.class);
+	}
+	@Override
+	public void getVariables(ReadGraph graph, Variable context, Map<String, Variable> map) throws DatabaseException {
+		StandardGraphChildVariable variable = (StandardGraphChildVariable)context;
+		for(Map.Entry<String, Resource> entry : graph.syncRequest(new UnescapedChildMapOfResource(variable.resource)).entrySet()) {
+			String name = entry.getKey();
+			Resource child = entry.getValue();
+			Variable var = graph.getPossibleContextualAdapter(child, variable, Variable.class, Variable.class);
+			if(var != null) {
+				map.put(name, var);
+			} else {
+				System.err.println("No adapter for " + child + " in " + variable.getURI(graph));
+			}
+		}
+	}
+
+};
+
+@SCLValue(type = "VariableMap")
+public static VariableMap standardPropertyDomainChildren = new VariableMap() {
+
+	@Override
+	public Variable getVariable(ReadGraph graph, Variable context, String name) throws DatabaseException {
+        	StandardGraphPropertyVariable variable = (StandardGraphPropertyVariable)context;
+    		Datatype dt = variable.getDatatype(graph);
+    		if (dt instanceof ArrayType) {
+    		    ChildReference ref = getPossibleIndexReference(name);
+    		    if (ref != null)
+    		        return new SubliteralPropertyVariable(variable, ref);
+    		}
+    		return null;
+	}
+	@Override
+	public void getVariables(ReadGraph graph, Variable context, Map<String, Variable> map) throws DatabaseException {
+	}
+
+};
+
+~~~
+
+## Informally
+
+Standard child modelling assumes a context resource res.
+
+* childResource : where (res, L0.ConsistsOf, childResource)
+* childVariable = graph.adaptContextual(childResource, this, Variable.class, Variable.class)
+
+Standard property modelling assumes a context resource res.
+
+* (predicate, object) : where (res, predicate, object) and (predicate <R L0.HasProperty)
+* childProperty = graph.adaptContextual(object, ModelledVariablePropertyDescriptor(this, predicate), Variable.class)
+
+For property variables there are two context resources available, the associated value and the predicate. Properties are found from the value context first and then from the predicate context.
+
+The graph.adaptContextual step is an extension point for defining custom variable implementations contributed using the adapter (adapters.xml) mechanism.
+
+# Frequent cases
+
+1. Given variable v, obtain model (Resource)
+   call Variables.getModel(graph, v)
+2. Given variable v, obtain context (Variable)
+   call Variables.getContext(graph, v)
+3. Given Variable URI, obtain Variable
+   call Variables.getVariable(graph, uri)
+4. Obtain string representation for value of Variable v
+   obtain value of v#HasDisplayValue
+
+to be continued...
+
+# Open issues
+
+## Generic simulator support in Variable ==
+
+Current hypothesis is that we extends standard graph based implementation by a special interface, which serves the simulator (or other) data. The standard implementation then combines data from graph and simulator. The special interface is designed such that is supports custom synchronization and lazy operation and can also be directly utilized in remote simulators.
+
+## Variable syntax in SCL
+
+* Implement variable interface methods as normal functions
+  * Example: <code>browse entrypoint "./Out#sdf"</code>
+* Use . and #-for browsing
+  * How to browse parent? Explicit function
+  * Example: <code>(parent entrypoint).Out#sdf</code>
+  * Special operator for parents (binds stronger than . or #):
+  * Example: <code>!entrypoint.Out#sdf</code>
+* Resolve entrypoints in the context
+  * Example: <code>Out#sdf</code>
+  * Local variable definitions may shadow context
+
+## Experiment modelling
+
+The modelling of experiment run contexts and the value types of properties in states is still underway.
+
+# Refactoring
+
+* <strike>add Collection<Variable> browseProperties(ReadGraph graph, String classification) throws DatabaseException;</strike>
+* add standard property **classifications**, which returns a set of strings
+* <strike>deprecate browseChildren, browseProperties => add get</strike>