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Reflection and Context

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Lecture: Reflection
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Reflection and Context

  1. 1. Reflection and Context Marcus Denker marcus.denker@inria.fr http://rmod.lille.inria.fr © Marcus Denker
  2. 2. Roadmap > I. Sub-Method Structural Reflection > II. Partial Behavioral Reflection > III. Meta Context © Marcus Denker
  3. 3. Smalltalk > Smalltalk has support for reflection > Structural reflection — Classes / methods are objects — Can be changed at runtime > Behavioral reflection — Current execution reified (thisContext) — #doesNotUnderstand / MethodWrappers © Marcus Denker
  4. 4. Can we do better? > Structural Reflection stops at method level — Bytecode in the CompiledMethod: Numbers — Text: Just a String, needs to be compiled > Behavior hard coded in the Virtual Machine — Message Sending — Variable Access > Both structural and behavioral reflection is limited — We should do better! © Marcus Denker
  5. 5. Structural Reflection > Structure modeled as objects — e.g. Classes, methods — Causally connected > Uses: — Development environments — Language extensions and experiments © Marcus Denker
  6. 6. Methods and Reflection > Method are Objects — e.g in Smalltalk > No high-level model for sub-method elements — Message sends — Assignments — Variable access > Structural reflection stops at the granularity of methods © Marcus Denker
  7. 7. Sub-Method Reflection > Many tools work on sub method level — Profiler, Refactoring Tool, Debugger, Type Checker > Communication between tools needed — Example: Code coverage > All tools use different representations — Tools are harder to build — Communication not possible © Marcus Denker
  8. 8. Existing Method Representations > Existing representations for Methods — Text — Bytecode — AST © Marcus Denker
  9. 9. Requirements > Causal Connection > Abstraction Level > Extensibility > Persistency > Size and Performance © Marcus Denker
  10. 10. Text > Low level abstraction — String of characters > Not causally connected — Need to call compiler © Marcus Denker
  11. 11. Bytecode > Low level abstraction — Array of Integers > Missing extensibility — e.g. for tools > Mix of base- and meta-level code — Problems with synthesized code when changing code — Examples: AOP point-cut residues, reflection hooks © Marcus Denker
  12. 12. Abstract Syntax Tree > Not causally connected — Need to call compiler > Not extensible — Fixed set of codes, no way to store meta data > Not persistent — Generated by compiler from text, never stored © Marcus Denker
  13. 13. Solution: Reflective Methods > Annotated, persistent AST > Bytecode generated on demand and cached :ReflectiveMethod :CompiledMethod annotation compiledMethod #(12 13 45 38 98 128 annotation reflectiveMethod 84 72 42 77 22 28 59 32 7 49 51 87 64) Tools VM © Marcus Denker
  14. 14. Persephone > Implementation of Reflective Methods for Squeak 3.9 > Smalltalk compiler generates Reflective Methods — Translated to bytecode on demand > Open Compiler: Plugins — Called before code generation — Transform a copy of the AST © Marcus Denker
  15. 15. Requirements revisited > Abstraction Level OK > Causal Connection OK > Extensibility OK > Persistency OK > Size and Performance OK © Marcus Denker
  16. 16. Annotations > Source visible annotations — extended Smalltalk syntax (9 raisedTo: 10000) <:evaluateAtCompiletime:> > Source invisible annotations — Reflective API — Can reference any object > Every node can be annotated > Semantics: Compiler Plugins © Marcus Denker
  17. 17. Example: Pluggable Type-System > Example for textual annotations bitFromBoolean: aBoolean <:type: Boolean :> ^ (aBoolean ifTrue: [1] ifFalse: [0]) <:type: Integer :> > Optional, pluggable type-system > Types stored as annotations in the Reflective Methods © Marcus Denker
  18. 18. Memory number of classes memory Squeak 3.9 2040 15.7 MB Persephone 2224 20 MB no reflective methods Persephone 2224 123 MB reflective methods © Marcus Denker
  19. 19. Roadmap > I. Sub-Method Structural Reflection > II. Partial Behavioral Reflection > III. Meta Context © Marcus Denker
  20. 20. Behavioral Reflection > Reflect on the execution — method execution — message sending, variable access > In Smalltalk — No model of execution below method body — message sending / variable access hard coded by VM — #doesNotUnderstand / MethodWrappers > Reflective capabilities of Smalltalk should be improved! © Marcus Denker
  21. 21. MetaclassTalk > Extends the Smalltalk metaclass model — Similar to CLOS MOP > Metaclass defines — message lookup — access to instance variables > Problems: — Reflection only controllable at class boundaries — No fine-grained selection (e.g. single operations) — Protocol between base and meta level is fixed © Marcus Denker
  22. 22. Reflex: Partial Behavioral Reflection > Hooksets: collection of operation occurrences > Links — Bind hooksets to meta-objects — Define protocol between base and meta metaobject > Goals — Highly selective reification links activation condition — Flexible meta-level engineering – Protocol specification hookset – Cross-cutting hooksets Tanter, OOPSLA03 © Marcus Denker
  23. 23. Example: Profiler > Operation: — Method execution (around) > Hookset: — All execution operations in a package metaobject > Meta-object: — A profiling tool links activation condition hookset © Marcus Denker
  24. 24. Reflex for Squeak > Partial Behavioral Reflection pioneered in Java — Code transformation at load time — Not unanticipated (it’s Java...) > Geppetto: Partial Behavioral Reflection for Smalltalk — For Squeak 3.9 with Bytecode transformation © Marcus Denker
  25. 25. Problems > Annotation performance — Decompile bytecode > Execution performance — Preambles for stack manipulation > Low-level representation — ifTrue:ifFalse: — Blocks — Global variables © Marcus Denker
  26. 26. Links as Annotations > Links can be annotations on the AST Method Meta Link © Marcus Denker
  27. 27. Behavioral Reflection: Flexible meta-object links activation condition source code (AST) > Very Flexible © Marcus Denker
  28. 28. Behavioral Reflection: CLOS meta-object links activation condition source code (AST) > Meta-class MOP (CLOS) © Marcus Denker
  29. 29. Behavioral Reflection: AOP meta-object links activation condition source code (AST) > Aspects © Marcus Denker
  30. 30. Behavioral Reflection: Tracer tracer metaobject link source code (AST) > Tracer © Marcus Denker
  31. 31. Properties > Very fast annotations — No decompile! > On-the-fly code generation — Only code executed gets generated > Generated code is fast — Better then working on bytecode level © Marcus Denker
  32. 32. Demo > Show Bounce Demo © Marcus Denker
  33. 33. Reflectivity > Prototype implementation in Squeak — Sub-Method Structure — Partial Behavioral Reflection > Download: http:/scg.unibe.ch/Research/Reflectivity © Marcus Denker
  34. 34. Reflectivity: Pharo > Not yet.... but soon. — Slowly revisiting all research stuff — Now we can do it for real! – Engineering vs. Research... © Marcus Denker
  35. 35. Roadmap > I. Sub-Method Structural Reflection > II. Partial Behavioral Reflection > III. Meta Context © Marcus Denker
  36. 36. Behavioral Reflection: Flexible meta-object links activation condition source code (AST) > Letʼs use it! © Marcus Denker
  37. 37. Problem: Recursion > Behavioral reflection cannot be applied to the whole system — System classes — Meta-objects © Marcus Denker
  38. 38. Example: Beeper > Call the Beeper from OrderedCollection>>#add beepLink := Link new metaObject: Beeper. beepLink selector: #beep. (OrderedCollection>>#add:) methodNode link: beepLink. © Marcus Denker
  39. 39. Meta-object Call Recursion Base Level Meta Object Meta Object #add: send #beep send #add: send #beep send #add: send Infinite recursion © Marcus Denker
  40. 40. Ad-hoc Solutions > Code duplication > Adding special tests © Marcus Denker
  41. 41. Tower of Interpreters > Smith, 1982 © Marcus Denker
  42. 42. The Real Problem Representing Meta-Level Execution © Marcus Denker
  43. 43. The Meta-Context Base Level Meta Level MetaContext activation > Link enables MetaContext MetaContext deactivation © Marcus Denker
  44. 44. Context-aware Links Base Level Meta Level Stop meta-level call > Disable call when already on the meta-level © Marcus Denker
  45. 45. MetaContext > Recursion problem solved meta-object meta links level 0 operation base © Marcus Denker
  46. 46. MetaContext metameta- object meta-2 link level 1 > Meta-level analysis: meta-object meta — Trace the tracer links level 0 operation base © Marcus Denker
  47. 47. MetaContext > Recursion problem > Missing representation of meta-level execution > Meta-context — Solves the recursion problem — Enables meta-level analysis © Marcus Denker
  48. 48. transaction set-slot insert-slot remove-slot define-method remove-method declare-delegation system set-slot insert-slot remove-slot define-method remove-method declare-delegation © Marcus Denker
  49. 49. non-destructive operations also need to see the objects belonging to the current transaction (empty? inventory ) (empty? inventory’ ) alter-object (with-context @transaction (defmethod send (selector arguments) (let ((log (log (host-context)))) (do-slots (arguments argument index) (setf ($slot-value arguments index) (gethash argument log argument)))) (resend))) case study is not implemented purely on top of the object model - System must make sure that any method invoked in transaction context sees the © Marcus Denker transactional versions of modified objects, rather than the original versions.
  50. 50. Rethinking Reflection > Meta change “shows through” — Introspection shows implementation — Recursion and confusion of meta levels > Reflective change is always global — Any change is visible to the whole system — No way to batch multiple changes into one atomic operation © Marcus Denker
  51. 51. Next steps > Generalize context model: — Beyond context as control flow. > Virtual machine support... to make it practical > What is the next reflective language kernel? © Marcus Denker
  52. 52. A lot of open questions... thats why it is Research... ????? © Marcus Denker
  53. 53. Questions ? © Marcus Denker
  54. 54. License > http://creativecommons.org/licenses/by-sa/2.5/ Attribution-ShareAlike 2.5 You are free: • to copy, distribute, display, and perform the work • to make derivative works • to make commercial use of the work Under the following conditions: Attribution. You must attribute the work in the manner specified by the author or licensor. " Share Alike. If you alter, transform, or build upon this work, you may distribute the resulting work only under a license identical to this one. • For any reuse or distribution, you must make clear to others the license terms of this work. • Any of these conditions can be waived if you get permission from the copyright holder. Your fair use and other rights are in no way affected by the above. © Marcus Denker

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