Moritz Eysholdt, itemis AG

Serializing EMF models with Xtext

Xtextcon 2014!Kiel, Germany

Agenda❖ Parsing vs. Serializing!

❖ Use Cases!

❖ Generating vs. Serializing!

❖ Challenges!

❖ The Contract!

❖ Architecture!

❖ Hooks!

❖ Advice

The New and the Old Serializer

org.eclipse.xtext.serializer• The New Serializer. This presentation is about it.

org.eclipse.xtext.parsetree.reconstr• The Old Serializer. Don’t used it.!• Cryptic Error Messages!• Bad Performance for Large Models!• marked @Deprecated

Parser vs. SerializerXtextResource

ModelModelTextual!Model

Parser

Serializer

load()

save()

Parser vs. SerializerXtextResource

ModelModelTextual!Model

AST

Parser

Serializer

load()

save()

Use CasesQuickfix

Refactoring

Non-Textual !Editors

Non-Textual!Persistence

Generators

Use CasesQuickfix

Refactoring

Non-Textual !Editors

Non-Textual!Persistence

Generators

Use CasesQuickfix

Non-Textual !Editors

Non-Textual!Persistence

Generators

Refactoring

Use CasesQuickfix

Non-Textual!Persistence

Generators

Refactoring

Non-Textual !Editors

EMF Store

<?XML?><?XMI?>

Use CasesQuickfix

Non-Textual!Persistence

Generators

Refactoring

Non-Textual !Editors

Read

Transform

Serialize

Generate, Transpile, Compile, Migrate…

Why Serialize?

❖ object model usually easier to modify than its textual representation!

❖ guaranteed syntactical correctness!

❖ automatic handling of comments!

❖ automatic handling of whitespace

…when I have Xtend, THE language for generators

When not to Serialize

❖ Avoid serializing models that are broken due to parse errors.!

❖ Template languages are simpler when writing out large text chunks that never change.

The hard Contract

Parsing a serialized Model must result !in a model equal to the original.

Model original = createModel()String serialized = serialize(original)Model parsed = parse(serialized)assertEqual(original, parsed)

…enables textual models as persistence format

The soft Contract

Serializing a model that has been modified after parsing!should only change the smallest number of characters necessary.

…keep diffs small

String originalDoc = loadDocument()Model parsed = parse(originalDoc)Model modified = applyModifications(parsed)String newDoc = serialize(modified)int numberOfChars = diff(originalDoc, newDoc).size()

1. Keep textual diffs small.!2. Strictly comply with the semantic model!3. Loosely comply with the node model

Challenges

❖ unassigned elements!

❖ ambiguous mapping from EStructuralFeatures to Assignments!

❖ ambiguous mapping from EClasses to ParserRules and Actions

…because the AST is actually abstract, as in “lack of information”

API Frontends #1

package org.eclipse.xtext.resource;!public class XtextResource extends ResourceImpl {! (…)! public void save(Map<?, ?> options) throws IOException {}! public final void save(OutputStream outputStream, Map<?, ?> options) throws IOException {}}

API Frontends #2package org.eclipse.xtext.serializer;!@ImplementedBy(Serializer.class)public interface ISerializer { public String serialize(EObject obj); public String serialize(EObject obj, SaveOptions options); public void serialize(EObject obj, Writer writer, SaveOptions options) throws IOException; public ReplaceRegion serializeReplacement(EObject obj, SaveOptions options);}

Options:!- format: !

- true: format all!- false: format regions without whitespace information!

- validate: don’t use, it’s an old algorithm.

Before SerializationCreate one State Machine per Context per EClass

Root: "optional"? ID children+=(List | Path);!List returns Child: "list" item+=ID ("," item+=ID)*;!Path returns Child: "path" seg=ID ({Segment.parent=current} seg=ID)+;

Context EClassRoot RootList ChildPath SegmentPath_Segment_2_0 ChildPath_Segment_2_0 Segment

During Serialization #1Find right state machines and !find right path through them

Root: "optional"? ID children+=(List | Path);!List returns Child: "list" item+=ID ("," item+=ID)*;

Context EClassRoot Root

Context EClassList Child

optional Foo list a, b, cOutput:

Architecture #3

Context EClassRoot Root

Context EClassList Child

Context EClassRoot Root

Context EClassList Child

Semantic!assigned grammar elements

Syntactic!unassigned grammar elements

optional Foo list a, b, cOutput:

Architecture #4: Observer

Semantic!Sequencer

Syntactic!Sequencer

HiddenToken!Sequencer Formatter Writer

Events

Listens To

assigend!* RuleCalls!* Terminals!* DataTypes!* Keywords!* CrossRefs

unassigned!* RuleCalls!* Terminals!* DataTypes!* Keywords

* whitespace!* comments

modifies!whitespace

writes to !stream

Output: /*X*/ Foo list a

Serializer HiddenTokenSequencerSyntacticSequencerSemanticSequencer

createSequence(Root, Root)

enterAssignedParserRuleCall(children=List)

enterAssignedParserRuleCall(children=List)

createSequence(List, Children)

acceptUnassignedRuleCall(ID)

acceptAssignedTerminalRuleCall(itemi=ID)

acceptKeyword("list")

acceptAssignedTerminalRuleCall(itemi=ID)

leaveAssignedParserRuleCall(children=List)

leaveAssignedParserRuleCall(children=List)

Formatter Writer

enterAssignedParserRuleCall()

acceptWhitespace()

acceptWhitespace()

acceptComment(/*X*/)

acceptUnassignedRuleCall()

acceptKeyword()

acceptWhitespace()

acceptAssignedTerminalRuleCall()

acceptWhitespace()

enterAssignedParserRuleCall()

Root: "optional"? ID children+=(List | Path);

List returns Child: "list" item+=ID ("," item+=ID)*;

SerializerFragment

fragment = serializer.SerializerFragment auto-inject { generateStub = true // generateDebugData = true}

• SerializerFragment not required to use serializer!!• Only purpose of SerializerFragment is to generate convenience API!• generateDebugData to generate pretty state machine diagrams (graphviz dot)

Hooks #1: ITransientValueService• selectively exclude model-objects and -values from serialization!• transient == NOT serialized!• Default setting from Ecore model: EStructuralFeature.isTransient()

public interface ITransientValueService {! enum ListTransient { NO, SOME, YES }! enum ValueTransient { NO, PREFERABLY, YES }! public ListTransient isListTransient(EObject semanticObject, EStructuralFeature feature);! public boolean isValueInListTransient(EObject semanticObject, int index, EStructuralFeature feature);! public ValueTransient isValueTransient(EObject semanticObject, EStructuralFeature feature);}

Hooks #2: Token Serializationpublic interface ICrossReferenceSerializer { boolean isValid(EObject context, CrossReference crossref, EObject target, INode node); String serializeCrossRef(EObject context, CrossReference crossref, EObject target, INode node);}

public interface IKeywordSerializer { boolean isValid(EObject context, Keyword keyword, Object value); String serializeAssignedKeyword(EObject context, Keyword keyword, Object value, INode node);}

public interface IValueSerializer { boolean isValid(EObject context, RuleCall ruleCall, Object value); String serializeAssignedValue(EObject context, RuleCall ruleCall, Object value, INode node);}

public interface IEnumLiteralSerializer { boolean isValid(EObject context, RuleCall ruleCall, Object value); String serializeAssignedEnumLiteral(EObject context, RuleCall ruleCall, Object value, INode node);}

Hooks #3: SemanticSequencerclass SerSipSemanticSequencer extends AbstractSerSipSemanticSequencer {! @Inject SerSipGrammarAccess grammarAccess;! /** * Constraint: * (children+=Path | children+=List) */ override protected sequence_Root(EObject context, Root semanticObject) { val feeder = createSequencerFeeder(semanticObject) for (child : semanticObject.children) { if (child.seg != null) { feeder.accept(grammarAccess.rootAccess.childrenPathParserRuleCall_2_0_0, child) } else { feeder.accept(grammarAccess.rootAccess.childrenListParserRuleCall_2_0_1, child) } } feeder.finish() }}

Root: "optional"? ID children+=(Path | List);!List returns Child: "list" item+=ID ("," item+=ID)*;!Path returns Child: "path" seg=ID ({Segment.parent=current} seg=ID)+;

Hooks #4: SyntacticSequencerclass SerSipSyntacticSequencer extends AbstractSerSipSyntacticSequencer {! /** * terminal ID : '^'?('a'..'z'|'A'..'Z'|'_') ('a'..'z'|'A'..'Z'|'_'|'0'..'9')*; */ override getIDToken(EObject semanticObject, RuleCall ruleCall, INode node) { if (node != null) return getTokenText(node); return ""; }! /** * Syntax: * 'optional'? */ override emit_Root_OptionalKeyword_0_q(EObject semanticObject, ISynNavigable transition, List<INode> nodes) { acceptNodes(transition, nodes); }}

Root: "optional"? ID children+=(Path | List);

Understanding Ambiguities• A grammar is ambiguous for the serializer if there can be more

than one textual syntax for a given model.!!

• A grammar is ambiguous for the parser if there can be more than one model for a given textual syntax1.

R1: foo=ID | bar=ID;

R2: (“foo” val=ID) | (“bar” val=ID);

Does “a” parse

to or ?foo=a bar=b

Does serialize to

“foo b” or “bar b”?

val=b

pars

ese

rializ

e

1. that is not completely correct. A grammar is ambiguous when there is more than one path to consume a given input. That, however, usually leads to different models.

Understanding Ambiguities

Root: (foos=Foo | bars=Bar)*;!Foo: “foo” foo=ID;!Bar: “bar” bar=ID;

(1) .

(2) bar b foo a(3) foo a bar b

Rootfoo=a bar=b

Does (1) serialize to (2) or (3)?

Avoiding AmbiguitiesAnalysis: Grammars can be ambiguous for the serializer.!!Example:

Root: (foos=Foo | bars=Bar)*;!Foo: “foo” foo=ID;!Bar: “bar” bar=ID;

Root: members+=Member;!Member: Foo | Bar;!Foo: “foo” foo=ID;!Bar: “bar” bar=ID;

• Single “member” list to maintain order!• “Foo” and “Bar” extend “Member”!• implement getFoo() and getBar() as filters on getMembers()

Avoid Unserializeable ModelsThe Xtext grammar can imply constraints on your model. !!Serialization is only possible if the model complies with theses constraints because the grammar does not define syntax for models that don’t comply. Examples:

GRAMMAR CONSTRAINT

R1: name=ID; name != null

R2: (name=ID | title=STING); (name != null) ^ (title != null)

R3: items+=ID+; items.size() >= 1

R4: (a+=ID b+=ID)*; a.size() == b.size()

Avoid Unserializeable ModelsSolution:!!a) Ensure your grammar does not imply constraints. !b) Ensure your TransientValueService prevents constraint violations.!Implement Xtext Validation to enforce constraints.!!This will also improve your error messages but may make content assist too chatty.

RESTRICTIVE SAFE

R1: name=ID; R1: name=ID?;R1: (name=ID | “?”);

R2: (name=ID | title=STING); R2: name=ID? title=STING?;R3: items+=ID+; R3: items+=ID*;

R4: (a+=ID b+=ID)*;R4: (a+=ID b+=ID)* (“spareA={“ a+=ID+ “}”)? (“spareB={“ b+=ID+ “}”)?;

Configure Your Scope RightYou can (and need) to configure global scoping for a ResourceSet!

XtextLiveScopeResourceSetProviderXtextResourceSetProvider

ResourceSet

Dirty Editor State

Index

(default) (what you need)

<shadows>

<shadows>

Dirty Editor State

Index

<shadows>

Happy Serializing!