Lecture. Advanced Reflection: MetaLinks
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(1) MetaLinks allow attaching metaobjects to nodes in a method's AST that are called before, after, or instead of the node executing. (2) This enables fine-grained behavioral reflection by reflecting on individual nodes in the AST. (3) MetaLinks were demonstrated to add logging, breakpoints, code coverage tracking, and a way to "accept methods for testing" without modifying the original code.
![AST Integration
• We can navigate from execution to AST
• Example:
[ 1 + 2 ] sourceNode ==
thisContext method sourceNode blockNodes first](https://image.slidesharecdn.com/metalinks-220916114838-f608bffc/85/Lecture-Advanced-Reflection-MetaLinks-16-320.jpg)
![Plugin: Example
• We get all ifTrue: sends
• replace them with true
DemoPlugin>>transform
transform
| sends |
sends := ast sendNodes.
sends := sends select: [ :each | each selector = #ifTrue: ].
sends do: [:each | each replaceWith:
(RBLiteralNode value: true)].
^ast](https://image.slidesharecdn.com/metalinks-220916114838-f608bffc/85/Lecture-Advanced-Reflection-MetaLinks-21-320.jpg)
![MetaLink: MetaObject
• MetaObject can be any object
• Even a Block: [Transcript show ‘hello’]
• Install on any Node with #link:
• de-install a link with #uninstall](https://image.slidesharecdn.com/metalinks-220916114838-f608bffc/85/Lecture-Advanced-Reflection-MetaLinks-27-320.jpg)
![MetaLink: Condition
• We can specify a condition for the MetaLink
• Link is active if the condition evaluates to true
• We can pass rei
fi
cations as arguments
link := MetaLink new
metaObject: Halt;
selector: #once;
condition: [:object | object == 5] arguments: #(object).
(Number>>#sin) ast link: link.](https://image.slidesharecdn.com/metalinks-220916114838-f608bffc/85/Lecture-Advanced-Reflection-MetaLinks-33-320.jpg)
![Example: Log
• We want to just print something to the Transcript
link := MetaLink new
metaObject: [Transcript show: 'Reached Here'].
(Number>>#sin) ast link: link](https://image.slidesharecdn.com/metalinks-220916114838-f608bffc/85/Lecture-Advanced-Reflection-MetaLinks-35-320.jpg)
![Recursion Problem
• Solution: Meta-Level
• We encode the a level in the execution of the system
• Every Link Activation increases the level
• A meta-link is just active for one level. (e.g. 0)
link := MetaLink new
metaObject: [ Object new ];
level: 0.
(Behavior>>#new) ast link: link.](https://image.slidesharecdn.com/metalinks-220916114838-f608bffc/85/Lecture-Advanced-Reflection-MetaLinks-37-320.jpg)
![Example: Log
• Better use #level: 0
• Nevertheless: be careful! If you add this to method called
often it can be very slow.
link := MetaLink new
metaObject: [Transcript show: 'Reached Here’];
level: 0.](https://image.slidesharecdn.com/metalinks-220916114838-f608bffc/85/Lecture-Advanced-Reflection-MetaLinks-38-320.jpg)
![Example: WatchPoint
• Implementation: class Watchpoint, method install
• example of a #after link with a condition
link := MetaLink new
metaObject: self;
selector: #addValue:;
arguments: #(value);
control: #after;
condition: [ recording ].](https://image.slidesharecdn.com/metalinks-220916114838-f608bffc/85/Lecture-Advanced-Reflection-MetaLinks-42-320.jpg)
![Example: Accept for Test
• Add this code to the beginning of the method:
[:aContext :args |
CurrentExecutionEnvironment value isTest ifTrue: [
aContext return: (newMethod
valueWithReceiver: aContext
receiver
arguments: args) ]]
• Let’s do that with a MetaLink!](https://image.slidesharecdn.com/metalinks-220916114838-f608bffc/85/Lecture-Advanced-Reflection-MetaLinks-49-320.jpg)
![Example: Accept for Test
execute
| newMethod metaLink |
newMethod := method methodClass compiler
source: source;
options: #(+ optionParseErrors);
compile.
"the link executes the method we just created and returns"
metaLink := MetaLink new
metaObject: [ :aContext :args |
CurrentExecutionEnvironment value isTest
ifTrue: [ aContext return: (newMethod
valueWithReceiver: aContext receiver
arguments: args) ] ];
selector: #value:value:;
arguments: #(context arguments).
self method ast link: metaLink](https://image.slidesharecdn.com/metalinks-220916114838-f608bffc/85/Lecture-Advanced-Reflection-MetaLinks-50-320.jpg)
Lecture. Advanced Reflection: MetaLinks
- 1. Advanced Re fl ection: MetaLinks Marcus Denker, Inria http://marcusdenker.de Lecture at VUB Brussels, March 31, 2022
- 2. What we know… • Smalltalk is re fl ective • Classes, Methods, Stack-Frames… are Objects • Re fl ective API on all Objects
- 3. Re fl ection in Smalltalk • Re fl ection is based on the Metaclass model, thus it is inherently structural • Behavioral Re fl ection limited to: • Method lookup on failure (#doesNotUndersand:) • Rei fi ed stack (thisContext)
- 4. Can we do better? • A more fi ne-grained re fl ective mechanism seems to be missing • Let’s look again at a Method in the Inspector
- 5. Inspector on a Method
- 6. The AST • AST = Abstract Syntax Tree • Tree Representation of the Method • Produced by the Parser (part of the Compiler) • Used by all tools (refactoring, syntax-highlighting,…) Smalltalk compiler parse: 'test ^(1+2)'
- 7. AST • RBMethodNode Root • RBVariableNode Variable (read and write) • RBAssignmentNode Assignment • RBMessageNode A Message (most of them) • RBReturnNode Return
- 8. Inspect a simple AST • A very simple Example Smalltalk compiler parse: 'test ^(1+2)'
- 9. AST: Navigation • To make it easy to fi nd and enumerate nodes, there are some helper methods • CompiledMethod has: #sendNodes, #variableNodes, #assignmentNodes • Every AST node has #nodesDo: and #allChildren
- 10. AST: Visitor • RBProgramNodeVisitor: Visitor Pattern for the AST • Make subclass, override visit… methods • Let’s see it in action: Count Message sends
- 11. Demo: Visitor
- 12. Repeat:The AST • AST = Abstract Syntax Tree • Tree Representation of the Method • Produced by the Parser (part of the Compiler) • Used by all tools (refactoring, syntax-highlighting,…) Smalltalk compiler parse: 'test ^(1+2)'
- 13. The Compiler • Smalltalk compiler -> Compiler Facade • Classes de fi ne the compiler to use • You can override method #compiler • Behind: Compiler Chain
- 14. The Compiler Source AST Annotated AST IR Annotated AST Bytecode RBParser OCSemanticAnalyzer OCASTTranslator/ IRBuilder IRBytecodeGenerator
- 15. AST Integration • Originally just internal to the compiler • Pharo: • send #ast to a method to get the AST • Cached for persistency. (Point>>#x) ast == (Point>>#x) ast —> true
- 16. AST Integration • We can navigate from execution to AST • Example: [ 1 + 2 ] sourceNode == thisContext method sourceNode blockNodes first
- 17. Compiler: Extensible • All parts can be subclassed • Compiler instance can be setup to use the subclass for any part (parser, name analysis, translator…) • enable for a class only by implementing #compiler on the class side
- 18. Compiler Plugins • The AST can be easily transformed • We added a Plugin architecture to the Compiler • enable for a class only by implementing: compiler ^super compiler addPlugin: MyPlugin
- 19. The Compiler Source AST Annotated AST IR Annotated AST Bytecode RBParser OCSemanticAnalyzer OCASTTranslator/ IRBuilder IRBytecodeGenerator
- 21. Plugin: Example • We get all ifTrue: sends • replace them with true DemoPlugin>>transform transform | sends | sends := ast sendNodes. sends := sends select: [ :each | each selector = #ifTrue: ]. sends do: [:each | each replaceWith: (RBLiteralNode value: true)]. ^ast
- 22. Back to the topic… • A more fi ne-grained re fl ective mechanism seems to be missing • Can’t we do something with the AST?
- 23. Wouldn’t it be nice.. • With the AST, wouldn’t it be nice if we could use this structure for Behavioural Re fl ection? • If we could somehow attach a “arrow to the code” that points to a meta-object test ^( 1 + 2 ) meta-object for this Send
- 24. We have all pieces… • We have the AST for each method • It is quite simple • We have a compiler in the system • So this should be possible…
- 25. The MetaLink • MetaLink points to metaObject • De fi nes a selector to call • And a control attribute: #before, #after, #instead • Installed on a AST node: link := MetaLink new metaObject: Halt; selector: #once; control: #before. (Number>>#sin) ast link: link
- 26. The MetaLink • Can be installed on any AST Node • Methods will be re-compiled on the fl y just before next execution • Link installation is very fast • Changing a method removes all links from this method • Managing link re-installation has to be done by the user
- 27. MetaLink: MetaObject • MetaObject can be any object • Even a Block: [Transcript show ‘hello’] • Install on any Node with #link: • de-install a link with #uninstall
- 28. MetaLink: Selector • MetaLink de fi nes a message send to the MetaObject • #selector de fi nes which one • Default is #value • Yes, a selector with arguments is supported • We can pass information to the meta-object
- 29. MetaLink: Argument • The arguments de fi ne which arguments to pass • We support a number of rei fi cations
- 30. Rei fi cations • Rei fi cations de fi ne data to be passed as arguments • Reify —> Make something into an object that is not one normally • Example: “All arguments of this message”
- 31. Rei fi cations: examples • All nodes: #object #context #class #node #link • Sends: #arguments #receiver #selector • Method: #arguments #selector • Variable: #value They are de fi ned as subclasses of class RFRei fi cation
- 32. Rei fi cations as MetaObject • We support some special metaObjects: • #node The AST Node we are installed on • #object self at runtime • #class The class the links is installed in
- 33. MetaLink: Condition • We can specify a condition for the MetaLink • Link is active if the condition evaluates to true • We can pass rei fi cations as arguments link := MetaLink new metaObject: Halt; selector: #once; condition: [:object | object == 5] arguments: #(object). (Number>>#sin) ast link: link.
- 34. MetaLink: control • We can specify when to call the meta-object • We support #before, #after and #instead • The instead is very simple: last one wins
- 35. Example: Log • We want to just print something to the Transcript link := MetaLink new metaObject: [Transcript show: 'Reached Here']. (Number>>#sin) ast link: link
- 36. Recursion Problem • Before we see more examples: There is a problem • Imagine we put a MetaLink on some method deep in the System (e.g new, +, do: ). • Our Meta-Object might use exactly that method, too Endless Loop!!
- 37. Recursion Problem • Solution: Meta-Level • We encode the a level in the execution of the system • Every Link Activation increases the level • A meta-link is just active for one level. (e.g. 0) link := MetaLink new metaObject: [ Object new ]; level: 0. (Behavior>>#new) ast link: link.
- 38. Example: Log • Better use #level: 0 • Nevertheless: be careful! If you add this to method called often it can be very slow. link := MetaLink new metaObject: [Transcript show: 'Reached Here’]; level: 0.
- 39. Example: Counter • In the Browser you can add a “counter” to the AST • See class ExecutionCounter install link := MetaLink new metaObject: self; selector: #increase. node link: link.
- 40. Example: Breakpoint • “Add Breakpoint” in AST (Suggestions) Menu • See class Breakpoint • Break Once • Conditional Break breakLink ^ MetaLink new metaObject: Break; selector: #break; options: options
- 41. Example: WatchPoint • Watchpoint: Record Value at a point in the AST • Example: Watch event in WorldMorph>>#mouseDown: Click on background -> value recorded
- 42. Example: WatchPoint • Implementation: class Watchpoint, method install • example of a #after link with a condition link := MetaLink new metaObject: self; selector: #addValue:; arguments: #(value); control: #after; condition: [ recording ].
- 43. Example: Code Coverage • Small Demo. • Start with CoverageDemo new open
- 44. Example: Code Coverage • Example of a MetaLink with a #node MetaObject • Meta-Object is the node that the link is installed on link := MetaLink new metaObject: #node; selector: #tagExecuted.
- 45. Interesting Properties • Cross Cutting • One Link can be installed multiple times • Over multiple methods and even Classes • And across operations (e.g., Send and Assignment) as long as all rei fi cations requested are compatible • Fully Dynamic: Links can be added and removed at runtime • Even by the meta-object of another meta-link!
- 46. Example: Accept for Test • Imagine we want to edit a method that is called often by the System. • How do we test it? • It would be nice if we could “Accept for Test”
- 47. Example: Accept for Test • Menu in the browser: AST menu shows for all nodes. SycSourceCodeCommand subclass: #SycAcceptForTest instanceVariableNames: 'source' classVariableNames: '' package: 'SystemCommands-SourceCodeCommands' defaultMenuItemName ^'Accept for Test' readParametersFromContext: aSourceCodeContext super readParametersFromContext: aSourceCodeContext. source := aSourceCodeContext tool pendingText • We implement our code in the #execute method
- 48. Example: Accept for Test • How we know that we are in a test? CurrentExecutionEnvironment value isTest • We can compile the current text bu ff er newMethod := method methodClass compiler source: source; options: #(+ optionParseErrors); compile.
- 49. Example: Accept for Test • Add this code to the beginning of the method: [:aContext :args | CurrentExecutionEnvironment value isTest ifTrue: [ aContext return: (newMethod valueWithReceiver: aContext receiver arguments: args) ]] • Let’s do that with a MetaLink!
- 50. Example: Accept for Test execute | newMethod metaLink | newMethod := method methodClass compiler source: source; options: #(+ optionParseErrors); compile. "the link executes the method we just created and returns" metaLink := MetaLink new metaObject: [ :aContext :args | CurrentExecutionEnvironment value isTest ifTrue: [ aContext return: (newMethod valueWithReceiver: aContext receiver arguments: args) ] ]; selector: #value:value:; arguments: #(context arguments). self method ast link: metaLink
- 51. What did we see? • ASTs and AST Visitors • Compiler and Compiler Plugins • MetaLinks • Recursion Problem • Examples: Counter, Breakpoint, Coverage • Accept for Test
- 52. Limitations • #instead needs more work (e.g to support conditions) • Keep in mind: next metaLink taken into account for next method activation • Take care with long running loops!
- 53. Help Wanted • We are always interested in improvements! • Pharo 10 will be released soon! • Pull Requests Welcome!
- 54. Questions?