Acceleo Query Language

Query and navigate in EMF models

The Acceleo Query Language (AQL) is a language used to navigate and query an EMF model. In this document, you will find the description of all the services of the standard library of AQL.

Introduction

The Acceleo Query Language (AQL) is a language used to navigate and query an EMF model. In this document, you will find the description of the syntax, all the services and the standard library of AQL.
AQL as a query engine is small, simple, fast, extensible and it brings a richer validation than the MTL interpreter.

For those looking for a simple and fast interpreters for your EMF models, AQL can provide you with a lot of features, including:

The AQL interpreter is used in Sirius with the prefix «aql:».

Syntax

Basics

The syntax is very similar to the OCL syntax. An expression always starts with a variable
aVariable

The variable named self represent the current object (think of it as the this in Java).

Let’s consider the following metamodel :

From a variable one can access field or reference values using the . separator.
With self being an instance of Person, self.name returns the value of the attribute name and self.father return the father of the person.

If the attribute or the reference is multi-valued, then self.parents will return a collection.

Calls can be chained, as such self.parents.name will return a collection containing the names of the parents.

If one want to access the collection itself, then the separator -> must be used, as such self.parents.name->size() will return the number of elements in the collection whereas self.parents.name.size() will return a collection containing
the sizes of each name.

AQL can also call methods modeled as EOperations or defined through Java services. The syntax denoting such a call is @. @self.someCall() will call the someCall method and return the result.

Working with collections

Filtering a collection is generaly done using either ->filter(..) to keep elements of a given type or ->select(..) to keep elements which are validating a given condition.

With self being an instance of Family, self.members->filter(family::Man) will return all the members of the family which are mens and self.members->select( p | p.name.startsWith('A')) will return all the members of the family which have a name starting by the letter ‹A›.

To access an element at a particular index you can use the operation ->at(..); self.members->at(1) will return the first person which is a member of the family (in that specific case it is probably better to use self.members->first()

AQL has two kinds of collections, a Sequence which is a list, or an OrderedSet which does not allow doubles. You can convert a Sequence to an OrderedSet by as such : self.members->asSet()

You can also define a collection by extension using the following syntax:

AQL provides operations out of the box to browse the model. Most notably :

Conditions

AQL provides an If but it has to be an expression and not a statement. As such one has to define the else. Here is the syntax

if self.name.startsWith('a') then self else self.eContainer() endif

References

variable_name a reference to a variable myVariable
expression . feature_name implicit collect eClass.name
expression . service_name ( ( expression ( , expression ) * ) ? ) implicit collect myVariable.toString()
expression -> service_name ( ( expression ( , expression ) * ) ? ) call on the collection itself if the expression is not a collection it will be wrapped into an ordered set mySequence->sep(‹,›)

Operators

not expression call the not service not eClass.interface
- expression call the unaryMin service -3
expression + expression call the add service 2 + 2
expression - expression call the sub service 2 – 2
expression * expression call the mult service 2 * 2
expression / expression call the divOp service 2 / 2
expression <= expression call the lessThanEqual service 2 <= 2
expression >= expression call the greaterThanEqual service 2 >= 2
expression < expression call the lessThan service 1 < 2
expression > expression call the greaterThan service 2 > 1
expression <> expression call the differs service 1 <> 2
expression != expression call the differs service 1 != 2
expression = expression call the equals service 1 = 1
expression and expression call the and service eClass.interface and eClass.abstact
expression or expression call the or service eClass.interface or eClass.abstact
expression xor expression call the xor service eClass.interface xor eClass.abstact
expression implies expression call implies service eClass.interface implies eClass.abstact

Structures

( expression ) parenthesis are used to change priority during evaluation (2 + 2 ) * 3
if expression then expression else expression endif conditional expression if eClass.abstract then ‹blue› else ‹red› endif
let new_variable_name ( : type_literal)? ( , new_variable_name ( : type_literal)?)* in expression let allows to define variables in order to factorise expression let container = self.eContainer() in container.eAllContents()

Literals

' escaped_string ' you can use java style escape sequence \u0000 \x00 \\ \' \b \t \n ... 'TODO list:\n\t- walk the dog\n\t- make diner'
[ 0 - 9]+ an integer 100
[ 0 - 9]+ . [ 0 - 9]+ a real 3.14
true the boolean value true true
false the boolean value false false
null the null value null
Sequence{ ( expression ( , expression) * ) ? } a sequence defined in extension Sequence{1, 2, 3, 3}
OrderedSet{ ( expression ( , expression) * ) ? } an ordered set defined in extension OrderedSet{1, 2, 3}
epackage_name :: eenum_name :: eenum_literal_name an EEnumLiteral art::Color::blue

Type literals

String the string type String
Integer the integer type Integer
Real the real type Real
Boolean the string type Boolean
Sequence( type_litral ) a sequence type Sequence(String)
OrderedSet( type_litral ) an ordered set type OrderedSet(String)
epackage_name :: eclassifier_name an eclassifier type ecore::EPackage
{ epackage_name :: eclassifier_name (* * epackage_name :: eclassifier_name) * } a set of eclassifiers {ecore::EPackage | ecore::EClass}

Language Reference

These sections are listing all the services of the standard library of AQL.

Migrating from MTL queries

As languages, AQL and MTL are very close yet there are some notable differences:

Implicit variable references

There is no implicit variable reference. With this change, you can easily find out if you are using a feature of an object or a string representation of said object. As a result, instead of using @, you must use @self.something if you want to access the feature named «something» of the current object or «something» if you want to retrieve the object named something.

In a lambda expression, you must now define the name of the variable used for the iteration in order to easily identify which variable is used by an expression. In Acceleo MTL, you can write Sequence{self}->collect(eAllContents(uml::Property)) and Acceleo will use the implicit iterator as a source of the operation eAllContents.

The problem comes when using a lambda like Sequence{self}->collect(something), we can’t know if «something» is a feature of «self» or if it is another variable.

Using AQL, you will now have to write either collect(m | m.eAllContents(uml::Property)) or collect(m: uml::Model | eAllContents(uml::Property)).

Collect and flatten

When a call or a feature acces is done on a collection the result is flattened for the first level. For instance a service returning a collection called on a collection will return a collection of elements and not a collection of collection of elements.

Type literals & children EPackages

Type literals can’t be in the form someEPackage::someSubEPackage::SomeEClass but instead someSubEPackage::SomeEClass should be directly used. Note that the name of the EPackage is mandatory. Type literals are handled just like any other type.

Calls like self.eAllContents(self.eClass()) are possible and will return all the children of type compatible with “self”.

Furthermore if you need a type literal as a parameter in your own service, you just have to have a first parameter with the type : Set<EClass>. Yes, that’s an important point, any type in AQL is possibly a union of several existing types, hence the collection here. As such the syntax for creating Sets or collections can be used as a substitute for type literals.

Enumeration literals & children EPackages

Enumeration literal should be prefixed with the name of the containing EPacakge for instance «myPackage::myEnum::value».

Collections

You can only have Sequences or OrderedSets as collections and as such the order of their elements is always deterministic. In Acceleo MTL, you had access to Sets, which are now OrderedSets and Bags, which are now Sequences. Those four kinds of collections were motivated by the fact that Sequence and OrderedSet were ordered contrary to Sets and Bags. On another side, OrderedSets and Sets did not accept any duplicate contrary to Bags and Sequences.

By careful reviewing the use of those collections in various Acceleo generators and Sirius Designers we have quickly found out that the lack of determinism in the order of the collections Sets and Bags was a major issue for our users. As a result, only two collections remain, the Sequence which can contain any kind of element and the OrderedSet which has a similar behavior except that it does not accept duplicates.

Previously in Acceleo MTL, you could transform a literal into a collection by using the operator -> on the literal directly. In Acceleo MTL, the collection created was a Bag which is not available anymore. It is recommended to use the extension notation like Sequence{self} or OrderedSet{self}. By default in AQL the created collection is an OrderedSet.

Renamed operations

Some operations have been renamed. As such «addAll» and «removeAll» have been renamed «add» and «sub» because those two names are used by AQL in order to provide access to the operator «+» and «-». As a result we can now write in AQL «firstSequence + secondSequence» or «firstSet - secondSet».

Typing

AQL is way smarter than MTL regarding to the types of your expressions. As a result, you can combine expressions using multiple types quite easily. For example, this is a valid AQL expression self.eContents(uml::Class).add(self.eContents(ecore::EClass)).name. In Acceleo MTL, we could not use this behavior because Acceleo MTL had to fall back to the concept EObject which does not have a feature «name» while AQL knows that the collection contains objects that are either «uml::Class» or «ecore::EClass» and both of those types have a feature named «name».

null handling

AQL handles null (OclVoid) differently from ocl, a null value will not cause a failure but will be silently handled.
For example, null.oclIsKindOf(ecore::EClass) would have returned true for MTL/OCL, forcing users to use not self.oclIsUndefined() and self.oclIsKindOf(ecore::EClass) instead. This is no longer true in AQL, where «null» doesn’t conform to any type, so null.oclIsKindOf(ecore::EClass) will return false. Note that it’s still possible to «cast» null in any given classifier. null.oclAsType(ecore::EClass) will not fail at runtime.

Furthermore oclIsUndefined() does not exist in AQL and should be replaced by a ... <> null expression.

Migrating from Acceleo2 queries

EClassifier references

All operations referencing a type are now using a type literal with the name of the EPackage and the name of the type instead of a string with the name of the type. As a result, eObject.eAllContents('EClass') would be translated using eObject.eAllContents('ecore::EClass'). This allows AQL to now in which EPackage to look for the type and as such, it improves the quality of the validation.

Types and cast

In order to test the type of an EObject, a common pattern in Acceleo 2 was to treat the EObject as a collection and filter said collection on the type desired to see if the size of the collection changed. In AQL, you have access to the operations oclIsTypeOf and oclIsKindOf. You can thus test the type of an EObject with the expression «eObject.oclIsKindOf(ecore::EStructuralFeature)» or «eObject.oclIsTypeOf(ecore::EAttribute)». You can use the operation oclIsKindOf to test if an object has the type of the given parameter or one of its subtype. On the other hand, you can use the operation oclIsTypeOf to test if an object has exactly the type of the given parameter.

Casting in AQL is useless, since AQL is very understandable when it comes to types, it will always tries its best to evaluate your expression.

Since AQL is very close to Acceleo MTL, you can find some additional documentation using the Acceleo equivalence documentation in the Acceleo documentation.

eContainer(«TypeName»)

In Acceleo2 self.eContainer("TypeName") actually had the behavior of returning self if it was matching the TypeName. As such, when migrating from an eContainer(..) call you should either make sure that this behavior is not needed or use the
compatibility method provided by AQL : self.eContainerOrSelf(some::Type)

Using AQL programmatically

This section provide information and code snippet. It will help you to integrate AQL in your own tool.

Simple overview of AQL:

Type validation

For each node of the AST we create a set of possible types as follow:

A special type NothingType is used to mark a problem on a given node of the AST. Those NothingTypes are then used to create validation messages. If an AST node has only NothingTypes validation messages will be set as errors for this node, otherwise they are set as warnings.

Completion

The completion rely on the AST production and the type validation.
The identifier fragments preceding (prefix) and following (remaining) the cursor position are removed from the expression to parse. The prefix and remaining are used later to filter the proposals. Many filters can be implemented: filter only on prefix, filter on prefix and remaining, same strategies with support for camel case, ...

Completion on the AST:

Creating and setting the environment

To get a fresh environment you can use one of the following snippet:

IQueryEnvironment queryEnvironment = Query.newEnvironmentWithDefaultServices(null);

To get an environment with predefined services.

or

IQueryEnvironment queryEnvironment = Query.newEnvironment(null);

To get an environment with no predefined services. It can be useful to create your own language primitives.

Note that you can also provide a CrossReferenceProvider to define the scope of cross references in your environment. See CrossReferencerToAQL for more details.

You can register new services Class as follow:

ServiceRegistrationResult registrationResult = queryEnvironment.registerServicePackage(MyServices.class);

The registration result contains information about services overrides.

You can also register your EPackages. Only registered EPackages are used to validate and evaluate AQL expression.

queryEnvironment.registerEPackage(MyEPackage.eINSTANCE);

In some cases you might also want to create custom mappings between an EClass and its Class. A basic case is the use of EMap:

queryEnvironment.registerCustomClassMapping(EcorePackage.eINSTANCE.getEStringToStringMapEntry(), EStringToStringMapEntryImpl.class);

By default the EClass is mapped to Map.Entry which is not an EObject. This prevents using services on EObject.

Building an AQL expression

The first step is building your expression from a String to an AST:

QueryBuilderEngine builder = new QueryBuilderEngine(queryEnvironment);
AstResult astResult = builder.build("self.name");

Evaluating an AQL expression

To evaluate an AQL expression you can use the QueryEvaluationEngine

QueryEvaluationEngine engine = new QueryEvaluationEngine(queryEnvironment);
Map<String, Object> variables = Maps.newHashMap();
variables.put("self", EcorePackage.eINSTANCE);
EvaluationResult evaluationResult = engine.eval(astResult, variables);

Here we only use one variable for demonstration purpose.

Validating an AQL expression (optional)

This step is optional for evaluation. You can evaluate an AQL expression without validating it in the first place.

Map<String, Set<IType>> variableTypes = new LinkedHashMap<String, Set<IType>>();
Set<IType> selfTypes = new LinkedHashSet<IType>();
selfTypes.add(new EClassifierType(queryEnvironment, EcorePackage.eINSTANCE.getEPackage()));
variableTypes.put("self", selfTypes);
AstValidator validator = new AstValidator(queryEnvironment, variableTypes);
IValidationResult validationResult = validator.validate(astResult);

Completing an AQL expression

To do this use the QueryCompletionEngine, it will build the query and validate it for you. It will also compute needed prefix and suffix if any:

Map<String, Set<IType>> variableTypes = new LinkedHashMap<String, Set<IType>>();
Set<IType> selfTypes = new LinkedHashSet<IType>();
selfTypes.add(new EClassifierType(queryEnvironment, EcorePackage.eINSTANCE.getEPackage()));
variableTypes.put("self", selfTypes);
QueryCompletionEngine engine = new QueryCompletionEngine(queryEnvironment);
ICompletionResult completionResult = engine.getCompletion("self.", 5, variableTypes);
List<ICompletionProposal> proposals = completionResult.getProposals(new BasicFilter(completionResult));

Here 5 is the offset where the completion should be computed in the given expression.