Practical Groovy
 Domain-Specific
    Languages

Guillaume Laforge
Groovy Project Manager
Guillaume Laforge
• Groovy Project Manager
• JSR-241 Spec Lead
• Head of Groovy Development
    at SpringSource
•   Initia...
Got a question?
• I always have an answer :-)



              42
A few words about Groovy
• Groovy is a dynamic language for the JVM
 • with a Meta Object Protocol
 • compiles directly to...
a
                                        d
                                      n
                                   ge
...
The context
Subject Matter Experts,
  Business analysts...
Developer producing LOLCODE




HAI
CAN HAS STDIO?
I HAS A VAR
IM IN YR LOOP
   UP VAR!!1
   VISIBLE VAR
   IZ VAR BIGGER ...
Lots of languages...
And in the end...
...nobody understands each other
Expressing requirements...
DSL: a potential solution?
• Use a more expressive language than a general purpose one

• Share a common metaphore of unde...
Towards more readibility (1)
Towards more readibility (1)
Towards more readibility (1)



                          20%
Towards more readibility (2)
Towards more readibility (2)
Towards more readibility (2)




                       80%
a
                                        d
                                      n
                                   ge
...
A collection of DSLs

• In our everyday life, we’re surrounded by DSLs
  • Technical dialects
  • Notations
  • Business l...
Technical dialects
SQL
^[w-.]+@([w-]){2,4}$
Notations
1. e4 e5
2. Nf3 Nc6
3. Bb5 a6
L2 U F-1 B L2 F B -1 U L2
Visual!
Business languages
Real-life Groovy examples
• Anti-malaria drug resistance simulation
• Human Resources employee skills representation
• Ins...
a
                                        d
                                      n
                                   ge
...
A flexible & malleable syntax
• No need to write full-blown classes, use scripts
• Optional typing (def)
 • in scripts, you...
Scripts vs classes
• Hide all the boilerplate technical code
 • an end-user doesn’t need to know about classes
  •   publi...
Optional typing
• No need to bother with types or even generics
 • unless you want to!
• Imagine an interest rate lookup t...
Native syntax constructs
• Lists
 • [Monday, Tuesday, Wednesday]
• Maps
 • [CA: ‘California’, TX: ‘Texas’]
• Ranges
 • def...
Optional parens & semis
• Make statements and expressions
  look more like natural languages



 •   move(left);

 •move  ...
Named arguments
• In Groovy you can mix named and unnamed
  arguments for method parameters
 •  named params are actually ...
BigDecimal by default
• Main reason why financial institutions often decide to
    use Groovy for their business rules!
   ...
BigDecimal by default
• Main reason why financial institutions often decide to
    use Groovy for their business rules!
   ...
Custom control structures,
thanks to closures
• When closures are last, they can be put “out” of the
    parentheses surro...
Operator overloading
                           • Currency amounts
 a+b          a.plus(b)

                            •
...
Groovy’s dynamic heart:

The MOP!
MetaObject Protocol
Groovy’s MOP
• All the accesses to methods, properties, constructors,
    operators, etc. can be intercepted thanks to the...
GroovyObject
• All instances of classes created in Groovy implement
  the GroovyObject interface:
  •   getProperty(String...
MetaClass
• The core of Groovy’s MOP system
  •   invokeConstructor()
  •   invokeMethod() and invokeStaticMethod()
  •   ...
ExpandoMetaClass
• A DSL for MetaClasses!
• MoneyAmount.metaClass.constructor   = { ... }
    Number.metaClass.getDollars ...
The Builder pattern
A builder for HR
• softskills   {
      ideas {
          capture 2
          formulate 3
      }
      ...
  }
  knowhow ...
A builder for HR
• softskills   {
      ideas {
          capture 2
          formulate 3
      }
      ...
  }
  knowhow ...
Builders
• Builders are...
 • a mechanism for creating any tree-structered graph
 • the realization of the GoF builder pat...
The clever trick
• GroovyObject#invokeMethod() is used to catch all
    non-existing method calls in the context of the
  ...
Adding properties to numbers
• Three possible approaches
 • create a Category
   • a category is a kind of decorator for d...
With a Category
•   class DistanceCategory {
        static Distance getMeters(Integer self) {
            new Distance(se...
With an ExpandoMetaClass
•   Number.metaClass.getMeters = {->
        new Distance(delegate, Unit.METERS)
    }

    100.m...
Compile-time metaprogramming
• Groovy 1.6 introduced AST Transformations
• Compile-time == No runtime performance penalty!...
AST Transformations
• Two kinds of transformations
 • Global transformations
   • applicable to all compilation units
  • ...
Global transformations
• Implement ASTTransformation
• Annotate the transfo specifying a compilation phase
•   @GroovyASTT...
Local transformations
• Same approach as Globale transformations
• But you don’t need the META-INF file
• Instead create an...
Example: the Spock framework
• Changing the semantics of the original code
• But keeping a valid Groovy syntax
• @Speck
  ...
a
                                        d
                                      n
                                   ge
...
Various integration mechanisms
• Java 6’s javax.script.* APIs (aka JSR-223)
• Spring’s language namespace
• Groovy’s own m...
Java 6’s javax.script.* API
• Groovy 1.6 provides its own implementation of the
  javax.script.* API

• ScriptEngineManage...
Spring’s lang namespace
• POGOs (Plain Old Groovy Objects) can be pre-
    compiled as any POJO and used interchangeably w...
Groovy’s own mechanisms
• Eval
 • for evaluating simple expressions
• GroovyShell
 • for more complex scripts and DSLs
• G...
Eval
• Simple mechanism to evaluate math-like formulas
•Eval.me  (                ‘3*4’)
  Eval.x (1,               ‘3*x +...
GroovyShell
• A Binding provides a context of execution
 • can implement lazy evaluation if needed
• A base script class c...
GroovyClassLoader
• Most powerful mechanism
 • could also visit or change the AST
 • scripts & classes can be loaded from ...
Externalize business rules
• Although Groovy DSLs can be embedded in normal
    Groovy classes, you should externalize the...
a
                                        d
                                      n
                                   ge
...
Start small, with key concepts
Beware overengineering!
Grow your language progressively
Get your hands dirty
Play with the end-users
Let your DSL fly,
it’s not yours,
it’s theirs!
Tight feedback loop
Iterative process
Stay humble.
You can’t get it right the first time.
Don’t design alone at your desk
Involve the end users from the start
Playing it safe
in a sandbox
Various levels
of sandboxing

• Groovy supports the usual Java Security Managers
• Use metaprogramming tricks to prevent c...
Test, test, test!
• Don’t just test for nominal cases
 • Explicitely test for errors!
• Ensure end-users get meaninful err...
a
                              d
                            n
                         ge
                        A
• Su...
Summary
• Groovy’s a great fit for Domain-Specific Languages
 • Malleable & flexible syntax
 • Full object-orientation
• Meta...
?
I kan haz my cheezburgr naw?
 Or do ya reely haz keshtionz?
Appendix
•   http://www.flickr.com/photos/wheatfields/420088151/sizes/l/
•   http://www.flickr.com/photos/therefromhere/518053737/size...
•   http://www.flickr.com/photos/forezt/192554677/sizes/o/
•   http://keremkosaner.files.wordpress.com/2008/04/softwaredevel...
Practical Domain-Specific Languages in Groovy
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Practical Domain-Specific Languages in Groovy

  1. 1. Practical Groovy Domain-Specific Languages Guillaume Laforge Groovy Project Manager
  2. 2. Guillaume Laforge • Groovy Project Manager • JSR-241 Spec Lead • Head of Groovy Development at SpringSource • Initiator of the Grails framework • Co-author of Groovy in Action • Speaker: JavaOne, QCon, JavaZone, Sun TechDays, Devoxx, The Spring Experience, JAX, Dynamic Language World, IJTC, and more...
  3. 3. Got a question? • I always have an answer :-) 42
  4. 4. A few words about Groovy • Groovy is a dynamic language for the JVM • with a Meta Object Protocol • compiles directly to bytecode, seamless Java interop • Open Source ASL 2 project hosted at Codehaus • Relaxed grammar derived from Java 5 • + borrowed good ideas from Ruby, Python, Smalltalk • Fast... for a dynlang on the JVM • Closures, properties, optional typing, BigDecimal by default, nice wrapper APIs, and more...
  5. 5. a d n ge A • The context and the usual issues we face • Some real-life examples of Domain-Specific Languages • Groovy’s DSL capabilities • Integrating a DSL in your application • Considerations to remember when designing your own DSL
  6. 6. The context
  7. 7. Subject Matter Experts, Business analysts...
  8. 8. Developer producing LOLCODE HAI CAN HAS STDIO? I HAS A VAR IM IN YR LOOP UP VAR!!1 VISIBLE VAR IZ VAR BIGGER THAN 10? KTHXBYE IM OUTTA YR LOOP KTHXBYE
  9. 9. Lots of languages...
  10. 10. And in the end... ...nobody understands each other
  11. 11. Expressing requirements...
  12. 12. DSL: a potential solution? • Use a more expressive language than a general purpose one • Share a common metaphore of understanding between developers and subject matter experts • Have domain experts help with the design of the business logic of an application • Avoid cluttering business code with too much boilerplate technical code • Cleanly separate business logic from application code • Let business rules have their own lifecycle
  13. 13. Towards more readibility (1)
  14. 14. Towards more readibility (1)
  15. 15. Towards more readibility (1) 20%
  16. 16. Towards more readibility (2)
  17. 17. Towards more readibility (2)
  18. 18. Towards more readibility (2) 80%
  19. 19. a d n ge A • The context and the usual issues we face • Some real-life examples of Domain-Specific Languages • Groovy’s DSL capabilities • Integrating a DSL in your application • Considerations to remember when designing your own DSL
  20. 20. A collection of DSLs • In our everyday life, we’re surrounded by DSLs • Technical dialects • Notations • Business languages
  21. 21. Technical dialects
  22. 22. SQL
  23. 23. ^[w-.]+@([w-]){2,4}$
  24. 24. Notations
  25. 25. 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6
  26. 26. L2 U F-1 B L2 F B -1 U L2
  27. 27. Visual!
  28. 28. Business languages
  29. 29. Real-life Groovy examples • Anti-malaria drug resistance simulation • Human Resources employee skills representation • Insurance policies risk calculation engine • Loan acceptance rules engine for a financial platform • Mathematica-like langua for nuclear safety simulations • Market data feeds evolution scenarios • and more...
  30. 30. a d n ge A • The context and the usual issues we face • Some real-life examples of Domain-Specific Languages • Groovy’s DSL capabilities • Integrating a DSL in your application • Considerations to remember when designing your own DSL
  31. 31. A flexible & malleable syntax • No need to write full-blown classes, use scripts • Optional typing (def) • in scripts, you can even omit the def keyword • Native syntax constructs • Parentheses & semi-colons are optional • Named arguments • BigDecimal by default for decimal numbers • Closures for custom control structures • Operator overloading
  32. 32. Scripts vs classes • Hide all the boilerplate technical code • an end-user doesn’t need to know about classes • public class Rule { public static void main(String[] args) { System.out.println(“Hello”); } } • println “Hello”
  33. 33. Optional typing • No need to bother with types or even generics • unless you want to! • Imagine an interest rate lookup table method returning some generified type: • Rate<LoanType, Duration, BigDecimal>[] lookupTable() { ... } def table = lookupTable() • No need to repeat the horrible generics type info!
  34. 34. Native syntax constructs • Lists • [Monday, Tuesday, Wednesday] • Maps • [CA: ‘California’, TX: ‘Texas’] • Ranges • def bizDays = Monday..Friday • def allowedAge = 18..65 • You can create your own custom ranges
  35. 35. Optional parens & semis • Make statements and expressions look more like natural languages • move(left); •move left
  36. 36. Named arguments • In Groovy you can mix named and unnamed arguments for method parameters • named params are actually put in a map parameter • plus optional parens & semis • take 1.pill, of: Chloroquinine, after: 6.hours • Corresponds to a method signature like: • def take(Map m, MedicineQuantity mq)
  37. 37. BigDecimal by default • Main reason why financial institutions often decide to use Groovy for their business rules! • Although these days rounding issues are overrated! • Java vs Groovy for a simple interpolation equation • BigDecimal uMinusv = c.subtract(a); BigDecimal vMinusl = b.subtract(c); BigDecimal uMinusl = a.subtract(b); return e.multiply(uMinusv) .add(d.multiply(vMinusl)) .divide(uMinusl, 10, BigDecimal.ROUND_HALF_UP); • (d * (b - c) + e * (c - a)) / (a - b)
  38. 38. BigDecimal by default • Main reason why financial institutions often decide to use Groovy for their business rules! • Although these days rounding issues are overrated! • Java vs Groovy for a simple interpolation equation • BigDecimal uMinusv = c.subtract(a); BigDecimal vMinusl = b.subtract(c); BigDecimal uMinusl = a.subtract(b); return e.multiply(uMinusv) .add(d.multiply(vMinusl)) .divide(uMinusl, 10, BigDecimal.ROUND_HALF_UP); • (d * (b - c) + e * (c - a)) / (a - b)
  39. 39. Custom control structures, thanks to closures • When closures are last, they can be put “out” of the parentheses surrounding parameters • unless (account.balance > 100.euros, { account.debit 100.euros }) • unless (account.balance > 100.euros) { account.debit 100.euros } • Signature def unless(boolean b, Closure c)
  40. 40. Operator overloading • Currency amounts a+b a.plus(b) • a-b a.minus(b) 15.euros + 10.dollars a*b a.multiply(b) • Distance handling a/b a.divide(b) a%b a.modulo(b) • 10.kilometers - 10.meters a ** b a.power(b) a|b a.or(b) • Workflow, concurrency a&b a.and(b) • a^b a.xor(b) taskA | taskB & taskC a[b] a.getAt(b) • Credit an account a << b a.leftShift(b) a >> b a.rightShift(b) • account << 10.dollars +a a.positive() account += 10.dollars -a a.negative() account.credit 10.dollars ~a a.bitwiseNegate()
  41. 41. Groovy’s dynamic heart: The MOP! MetaObject Protocol
  42. 42. Groovy’s MOP • All the accesses to methods, properties, constructors, operators, etc. can be intercepted thanks to the MOP • While Java’s behavior is hard-wired at compile- time in the class • Groovy’s runtime behavior is adaptable at runtime through the metaclass. • Different hooks for changing the runtime behavior • GroovyObject, custom MetaClass implementation, categories, ExpandoMetaClass
  43. 43. GroovyObject • All instances of classes created in Groovy implement the GroovyObject interface: • getProperty(String name) • setProperty(String name, Object value) • invokeMethod(String name, Object[] params) • getMetaClass() • setMetaClass(MetaClass mc) • A GO can have “pretended” methods and properties
  44. 44. MetaClass • The core of Groovy’s MOP system • invokeConstructor() • invokeMethod() and invokeStaticMethod() • invokeMissingMethod() • getProperty() and setProperty() • getAttribute() and setAttribute() • respondsTo() and hasProperty() • MetaClasses can change the behavior of existing third- party classes — even from the JDK
  45. 45. ExpandoMetaClass • A DSL for MetaClasses! • MoneyAmount.metaClass.constructor = { ... } Number.metaClass.getDollars = { ... } Distance.metaClass.toMeters = { ... } Distance.metaClass.static.create = { ... } • To avoid repetition of Type.metaClass, you can pass a closure to metaClass { ... } • The delegate variable in closure represents the current instance, and it the default parameter
  46. 46. The Builder pattern
  47. 47. A builder for HR • softskills { ideas { capture 2 formulate 3 } ... } knowhow { languages { java 4 groovy 5 } ... }
  48. 48. A builder for HR • softskills { ideas { capture 2 formulate 3 } ... } knowhow { languages { java 4 groovy 5 } ... }
  49. 49. Builders • Builders are... • a mechanism for creating any tree-structered graph • the realization of the GoF builder pattern at the syntax level in Groovy • simply a clever use of chained method invocation, closures, parentheses omission, and use of the GroovyObject methods • Existing builders • XML, Object graph, Swing, Ant, JMX, and more...
  50. 50. The clever trick • GroovyObject#invokeMethod() is used to catch all non-existing method calls in the context of the builder • The nesting of closures visually shows the level of nesting / depth in the tree • builder.m1(attr1:1, attr2:2, { builder.m2(..., {...}) } becomes equivalent to builder.m1(attr1:1, attr2:2) { m2(...) {...} } thanks to parens omission
  51. 51. Adding properties to numbers • Three possible approaches • create a Category • a category is a kind of decorator for default MCs • create a custom MetaClass • a full-blown MC class to implement and to set on the POGO instance • use ExpandoMetaClass • friendlier DSL approach but with a catch
  52. 52. With a Category • class DistanceCategory { static Distance getMeters(Integer self) { new Distance(self, Unit.METERS) } } use(DistanceCategory) { 100.meters } • Interesting scope: thread-bound & lexical • But doesn’t work across the hierarchy of classes • ie. subclasses won’t benefit from the new property
  53. 53. With an ExpandoMetaClass • Number.metaClass.getMeters = {-> new Distance(delegate, Unit.METERS) } 100.meters • Works for the class hierarchy for POJOs, and a flag exists to make it work for POGOs too • But the catch is it’s really a global change, so beware EMC enhancements collisions
  54. 54. Compile-time metaprogramming • Groovy 1.6 introduced AST Transformations • Compile-time == No runtime performance penalty! Transformation
  55. 55. AST Transformations • Two kinds of transformations • Global transformations • applicable to all compilation units • Local transformations • applicable to marked program elements • using specific marker annotations
  56. 56. Global transformations • Implement ASTTransformation • Annotate the transfo specifying a compilation phase • @GroovyASTTransformation(phase=CompilePhase.CONVERSION) public class MyTransformation implements ASTTransformation { public void visit(ASTNode[] nodes, SourceUnit unit) { ... } } • For discovery, create the file META-INF/services/ org.codehaus.groovy.transform.ASTTransformation • Add the fully qualified name of the class in that file
  57. 57. Local transformations • Same approach as Globale transformations • But you don’t need the META-INF file • Instead create an annotation to specify on which element the transformation should apply • @Retention(RetentionPolicy.SOURCE) @Target([ElementType.METHOD]) @GroovyASTTransformationClass( [quot;fqn.MyTransformationquot;]) public @interface WithLogging {...}
  58. 58. Example: the Spock framework • Changing the semantics of the original code • But keeping a valid Groovy syntax • @Speck class HelloSpock { def quot;can you figure out what I'm up to?quot;() { expect: name.size() == size where: name << [quot;Kirkquot;, quot;Spockquot;, quot;Scottyquot;] size << [4, 5, 6] } }
  59. 59. a d n ge A • The context and the usual issues we face • Some real-life examples of Domain-Specific Languages • Groovy’s DSL capabilities • Integrating a DSL in your application • Considerations to remember when designing your own DSL
  60. 60. Various integration mechanisms • Java 6’s javax.script.* APIs (aka JSR-223) • Spring’s language namespace • Groovy’s own mechanisms • But a key idea is to externalize those DSL programs • DSL programs can have their own lifecycle • no need to redeploy an application because of a rule change • business people won’t see the technical code
  61. 61. Java 6’s javax.script.* API • Groovy 1.6 provides its own implementation of the javax.script.* API • ScriptEngineManager mgr = new ScriptEngineManager(); ScriptEngine engine = mgr.getEngineByName(“Groovy”); String result = (String)engine.eval(“2+3”);
  62. 62. Spring’s lang namespace • POGOs (Plain Old Groovy Objects) can be pre- compiled as any POJO and used interchangeably with POJOs in a Spring application • But Groovy scripts & classes can be loaded at runtime through the <lang:groovy/> namespace and tag • Reloadable on change • Customizable through a custom MetaClass • <lang:groovy id=quot;eventsquot; script-source=quot;classpath:dsl/eventsChart.groovyquot; customizer-ref=quot;eventsMetaClassquot; />
  63. 63. Groovy’s own mechanisms • Eval • for evaluating simple expressions • GroovyShell • for more complex scripts and DSLs • GroovyClassLoader • the most powerful mechanism
  64. 64. Eval • Simple mechanism to evaluate math-like formulas •Eval.me ( ‘3*4’) Eval.x (1, ‘3*x + 4’) Eval.xy (1, 2, ‘x + y’) Eval.xyz(1, 2, 3, ‘x * y - z’)
  65. 65. GroovyShell • A Binding provides a context of execution • can implement lazy evaluation if needed • A base script class can be specified • def binding = new Binding() binding.mass = 22.3 binding.velocity = 10.6 def shell = new GroovyShell(binding) shell.evaluate(“mass * velocity ** 2 / 2”)
  66. 66. GroovyClassLoader • Most powerful mechanism • could also visit or change the AST • scripts & classes can be loaded from elsewhere • more control on compilation • GroovyClassLoader gcl = new GroovyClassLoader(); Class clazz = gcl.parseClass( new File(“f.groovy”)); GroovyObject instance = (GroovyObject)clazz.newInstance(); instance.setMetaClass(customMC);
  67. 67. Externalize business rules • Although Groovy DSLs can be embedded in normal Groovy classes, you should externalize them • Store them elsewhere • in a database, an XML file, etc. • Benefits • Business rules are not entangled in technical application code • Business rules can have their own lifecycle, without requiring application redeployments
  68. 68. a d n ge A • The context and the usual issues we face • Some real-life examples of Domain-Specific Languages • Groovy’s DSL capabilities • Integrating a DSL in your application • Considerations to remember when designing your own DSL
  69. 69. Start small, with key concepts Beware overengineering!
  70. 70. Grow your language progressively
  71. 71. Get your hands dirty Play with the end-users
  72. 72. Let your DSL fly, it’s not yours, it’s theirs!
  73. 73. Tight feedback loop Iterative process
  74. 74. Stay humble. You can’t get it right the first time. Don’t design alone at your desk Involve the end users from the start
  75. 75. Playing it safe in a sandbox
  76. 76. Various levels of sandboxing • Groovy supports the usual Java Security Managers • Use metaprogramming tricks to prevent calling / instanciating certain classes • Create a special GroovyClassLoader AST code visitor to filter only the nodes of the AST you want to keep • ArithmeticShell in Groovy’s samples
  77. 77. Test, test, test! • Don’t just test for nominal cases • Explicitely test for errors! • Ensure end-users get meaninful error messages
  78. 78. a d n ge A • Summary • Questions & Answers
  79. 79. Summary • Groovy’s a great fit for Domain-Specific Languages • Malleable & flexible syntax • Full object-orientation • Metaprogramming capabilities • Runtime metaprogramming • Compile-time metaprogramming • Groovy’s very often used for mission-critical DSLs
  80. 80. ? I kan haz my cheezburgr naw? Or do ya reely haz keshtionz?
  81. 81. Appendix
  82. 82. • http://www.flickr.com/photos/wheatfields/420088151/sizes/l/ • http://www.flickr.com/photos/therefromhere/518053737/sizes/l/ • http://www.flickr.com/photos/romainguy/230416692/sizes/l/ • http://www.flickr.com/photos/addictive_picasso/2874279971/sizes/l/ • http://www.flickr.com/photos/huangjiahui/3127634297/sizes/l/ • http://www.flickr.com/photos/25831000@N08/3064515804/sizes/o/ • http://www.flickr.com/photos/lanier67/3147696168/sizes/l/ • http://www.flickr.com/photos/ktb/4916063/sizes/o/ • http://www.flickr.com/photos/nathonline/918128338/sizes/l/ • http://www.flickr.com/photos/kevinsteele/39300193/sizes/l/ • http://commons.wikimedia.org/wiki/File:Brueghel-tower-of-babel.jpg • http://commons.wikimedia.org/wiki/File:Platypus.jpg • http://www.flickr.com/photos/joaomoura/2317171808/sizes/l/ • http://www.flickr.com/photos/wiccked/132687067/ • http://www.flickr.com/photos/timsamoff/252370986/sizes/l/ • http://www.flickr.com/photos/29738009@N08/2975466425/sizes/l/ • http://www.flickr.com/photos/howie_berlin/180121635/sizes/o/ • http://www.flickr.com/photos/yogi/1281980605/sizes/l/ • http://www.flickr.com/photos/dorseygraphics/1336468896/sizes/l/ • http://www.flickr.com/photos/xcbiker/386876546/sizes/l/ • http://www.flickr.com/photos/pietel/152403711/sizes/o/
  83. 83. • http://www.flickr.com/photos/forezt/192554677/sizes/o/ • http://keremkosaner.files.wordpress.com/2008/04/softwaredevelopment.gif • http://www.jouy.inra.fr • http://www.flickr.com/photos/ejpphoto/408101818/sizes/o/ • http://www.flickr.com/photos/solaro/2127576608/sizes/l/ • http://www.flickr.com/photos/biggreymare/2846899405/sizes/l/ •

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