Advanced Reflection in Pharo
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This document discusses advanced reflection capabilities in Pharo. It describes how instance variables are currently represented as strings but could be modeled as objects using layouts and slots. It also explains how method structures are not currently modeled at a fine-grained level and proposes using abstract syntax trees (ASTs) to represent methods as objects with sub-elements like message sends, assignments, and variable access. This would enable capabilities like behavioral reflection through meta-object links on AST nodes. The document advocates "ASTs everywhere" and sub-method reflection with meta-links in future versions of Pharo.
![Typed Slots
Slot subclass: #TypedSlot
layout: PointerLayout
slots: {#x => TypedSlot type: Integer}.
!
!
TypedSlot >> write: aValue to: anInstance
(aValue isNil or: [aValue isKindOf: type])
ifFalse: [ InvalidTypeError signal ].
super write: aValue to: anInstance.](https://image.slidesharecdn.com/lecture-131029053831-phpapp02/85/Advanced-Reflection-in-Pharo-17-320.jpg)
Advanced Reflection in Pharo
- 1. Advanced Reflection in Pharo Marcus Denker http://rmod.lille.inria.fr
- 2. What you know… • Smalltalk is reflective • e.g. Classes and Methods are Objects • Reflective API on all Objects
- 4. 2 instance variables Object subclass: #Point! ! instanceVariableNames: 'x y'! ! classVariableNames: ''! ! poolDictionaries: ''! ! category: 'Kernel-BasicObjects'
- 5. • Ask the class: Point instVarNames • read: 3@4 instVarNamed: #x • write: 3@4 instVarNamed: #x put: 5
- 6. Great!
- 7. But…
- 8. This is just a String! Object subclass: #Point! ! instanceVariableNames: 'x y'! ! classVariableNames: ''! ! poolDictionaries: ''! ! category: 'Kernel-BasicObjects'
- 9. returns an Array of Strings: • ! Point instVarNames
- 10. Why not Objects?
- 11. We can do better!
- 12. • All classes have a Layout ! • Describes the memory layout defined by a class ! • Layout and all the description are Objects
- 13. • Point layout • • Array layout • • a Normal Object an Array of Pointers ByteArray layout • an Array of Bytes
- 14. Point layout allSlots ! ==> an OrderedCollection(x => Slot y => Slot)
- 15. • Slots know how to read values from Objects ! mySlot := Point layout resolveSlot: #x. mySlot read: 3@4.
- 16. Why?
- 17. Typed Slots Slot subclass: #TypedSlot layout: PointerLayout slots: {#x => TypedSlot type: Integer}. ! ! TypedSlot >> write: aValue to: anInstance (aValue isNil or: [aValue isKindOf: type]) ifFalse: [ InvalidTypeError signal ]. super write: aValue to: anInstance.
- 18. Property Slots Object subclass: #PropertyObject layout: PointerLayout slots: { field => Slot property1 => PropertySlot. property2 => PropertySlot. ... propertyN => PropertySlot. }
- 19. Others • BitSlot • BooleanSlot • Alias • Relationships (e.g. one-one, one-many) • …. Your Domain level Slot!
- 20. More in Paper from OOPSLA
- 21. Status • Slots are in Pharo3, but hidden ! • In Pharo4: un-hide and introduce some Special Slots (e.g. Boolean, Property) • e.g. for Morphic (user interface objects)
- 22. Help Wanted!!!
- 23. So this worked well…
- 24. Lets do it again!
- 26. :-)
- 27. Methods
- 28. Lets have a look
- 29. • Method are Objects, but… ! • No high-level model for sub-method elements • Message sends • Assignments • Variable access ! • Structural reflection stops at the granularity of methods!
- 30. Can we do better?
- 33. • Lets have a look at an example (Object>>#halt) ast
- 34. • Encodes the method as a tree of node-objects • Visitor Pattern • Transformations • Refactoring tool uses this! RBProgramNode RBDoItNode RBMethodNode RBReturnNode RBSequenceNode RBValueNode RBArrayNode RBAssignmentNode RBBlockNode RBCascadeNode RBLiteralNode RBMessageNode RBVariableNode
- 35. In Pharo3 • AST based Navigation in the Editor • “Suggestions” • Debugger uses AST for pc->code mapping • AST Interpreter for experiments
- 36. Future • AST everywhere! • Do we need to store strings? • Can we have an AST based editor? • Sub-Method Reflection: The MetaLink
- 37. Can we modify the behaviour of code? > Annotate the AST with meta-links Method Meta Link
- 38. Why? • Change behaviour for selected AST Nodes • “All assignments” • “this message send” But without changing the program code!
- 39. Breakpoints
- 40. DEMO: Atoms
- 43. Will be in Pharo4
- 44. Will be in Pharo4 !!help wanted!!
- 45. What did we see? • Slots! • • • Instance variables are just described with strings We can do better! Layout, Slots ASTs Everywhere! • Methods are objects, but internal structure not modelled • We can do better! AST, Sub-Method Reflection, MetaLinks
- 46. Questions???