1. Reflective Programming in Smalltalk eXterme Metaprogramming
Brian Foote
University of Illinois at Urbana-Champaign
7 May 1998
Smalltalk Solutions ‘98
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

2. Reflective Programming in Smalltalk
Definitions
Building a Language Out of Objects
Examples
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

3. Frequently Asked Questions
Q: What is reflection?
A: It’s about building your language out of first-class, dynamic objects that you can look at and change at runtime.
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

4. Frequently Asked Questions
Q: Haven’t people really been doing this for a long time?
A: They certainly have. Smalltalk programmers have been doing this since the ‘70s.
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

5. Reflection from the Eisenhower Era
Hence, [the machine] can, in particular, change the orders (since these are in memory!)--the very orders that control its actions
--John Von Neumann 1958
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

6. Frequently Asked Questions
Q: What about efficiency? A:
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

7. Frequently Asked Questions
Q: What about efficiency?
A: There are a variety of techniques that can be used to make reflective systems faster.
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

8. Frequently Asked Questions
Q: Isn’t reflection dangerous?
A: Yes! You bet it is!
A: Yes, if you are not careful.
A: Yes, but you can make it safer.
A: Yes, but so is crossing the street.
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

9. Frequently Asked Questions
Q: Isn’t reflection just a fancy name for a few clever hacks?
A: Well, that and so much more...
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

10. Frequently Asked Questions
Q: Why is so much of the reflection literature so hard to read?
A: This is due, in part, to the area’s AI heritage...
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

11. Reflective Terminology
introspection
reflection
reification
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Reflective Programming in Smalltalk

12. Reflective Terminology
infinite regress
causal connection
reflective tower
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Reflective Programming in Smalltalk

13. Introspection
When a program can look at at the objects from which it is built
Smalltalk has a rich, comprehensive, indeed, unrivaled collection of introspective facilities
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

14. Reflection
When a program can change the objects from which it is built
Smalltalk represents as much as it can as as first-class dynamic objects, and you can change them at runtime.
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

15. Meta considered harmful “Meta” considered harmful
Consider these
Meta considered harmful
“Meta” considered harmful
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Reflective Programming in Smalltalk

16. Reflective Programming in Smalltalk
Neat Hack Hall of Fame
Parentless Objects
Does Not Understand
Metaobjects
Lightweight Classes
Method Wrappers
Byte Code Manipulation
Compiled Method Copying
Context Manipulation
Source Generation
Association Hacks
Becomes
Class Change
Instance Variable At
Methods On-Demand
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Reflective Programming in Smalltalk

17. Reflective Programming in Smalltalk
Objects themselves
Object
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

18. Reflective Programming in Smalltalk
Object Introspection
Object size == basicSize hash identityHash printOn: storeOn:
dependents ...
allOwners firstOwner nextInstance ownerAfter:
instVarAt:
isKindOf: class isMemberOf: respondsTo:
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

19. Example: AccessibleObjects
Demonstrates:
doesNotUnderstand:
instVarAt: instVarAt:put:
at: at:put:
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

20. Reflective Programming in Smalltalk
Accessible Objects
AccessibleObject class methods for: examples
example
"AccessibleObject example"
| temp |
temp := AccessibleObject new.
temp dog: 'Fido'.
temp cat: 'Tabby'.
Transcript print: temp dog; cr.
Transcript print: temp items; cr.
temp keysDo: [:key | Transcript print: key; cr].
Transcript print: (temp variableAt: #items); cr.
Transcript endEntry
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

21. Reflective Programming in Smalltalk
Accessible Objects
AccessibleObject methods for: accessing
at: key
"Return the object associated with the given key..."
^self valueAt: key
at: key put: value
"Store the indicated value at the designated place in our item
dictionary... "
^self valueAt: key put: value
size
"Let's say our size is the size of our item dictionary
plus our number of instance variables..."
^self items size + self instVarNames size
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

22. Reflective Programming in Smalltalk
Accessible Objects
AccessibleObject methods for: accessing
valueAt: key
"Return the object associated with the given key..."
^self valueAt: key ifAbsent: [self errorKeyNotFound]
valueAt: key put: value
"Store the indicated value at the designated place in our item
dictionary, unless there is an instance var by that name..."
items isNil ifTrue: [items := IdentityDictionary new: 16].
(self hasVariableNamed: key)
ifTrue: [^self variableAt: key put: value]
ifFalse: [^items at: key put: value]
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

23. Reflective Programming in Smalltalk
Accessible Objects
AccessibleObject methods for: instance variable access
allInstVarNames
"Define a shorthand for this class method..."
^self class allInstVarNames
hasVariableNamed: name
"Say whether we have a variable by the given name..."
^(self variableIndex: name) ~= 0
instVarNames
^self class instVarNames
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

24. Reflective Programming in Smalltalk
Accessible Objects
AccessibleObject methods for: instance variable access
variableAt: name
"Return the named value..."
| index |
index := self variableIndex: name.
index = 0 ifTrue: [self error: 'Bad instance variable name...'].
^self instVarAt: index
variableAt: name put: value
"Set the named instance variable to the indicated value..."
^self instVarAt: index put: value
variableIndex: name
"Return the instance variable index for this name, or zero..."
^self class allInstVarNames indexOf: name asString.
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

25. Reflective Programming in Smalltalk
Accessible Objects
AccessibleObject methods for: error interception
doesNotUnderstand: aMessage
"Refer messages we don't understand to our item dictionary..."
| selector name args |
selector := aMessage selector.
name := (selector copyWithout: $:) asSymbol.
args := aMessage arguments.
(self hasVariableNamed: name)
ifTrue: [args size = 0
ifTrue: [^self variableAt: name]
ifFalse: [^self variableAt: name put: (args at: 1)]].
(items respondsTo: selector)
ifTrue: [^items perform: selector withArguments: args].
args size = 1
ifTrue: [^self valueAt: name put: (args at: 1)]
ifFalse: [^self valueAt: name ifAbsent:
[^super doesNotUnderstand: aMessage]]
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

26. Behavior ClassDescription Class Metaclass
Classes and Behavior
Behavior
ClassDescription
Class
Metaclass
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Reflective Programming in Smalltalk

27. ClassBuilder SystemOrganizer ClassOrganizer ClassCategoryReader
Organizations
ClassBuilder
SystemOrganizer
ClassOrganizer
ClassCategoryReader
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Reflective Programming in Smalltalk

28. MethodDictionary CompiledMethod ByteArray BlockClosure
Code Representation
MethodDictionary
CompiledMethod
ByteArray
BlockClosure
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Reflective Programming in Smalltalk

29. Context MethodContext/BlockContext Message MessageSend
Runtime Enviroment
Context
MethodContext/BlockContext
Message
MessageSend
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

30. Event Exception Signal SignalHandler SignalCollection HandlerList
Exceptions and Events
Event
Exception
Signal
SignalHandler SignalCollection HandlerList
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

31. Process ProcessScheduler Semaphore SharedQueue
Process Scheduling
Process
ProcessScheduler
Semaphore
SharedQueue
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

32. Debugger Decompiler Inspector ChangeList <Browsers>
Viewing the Program
Debugger
Decompiler
Inspector
ChangeList
<Browsers>
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Reflective Programming in Smalltalk

33. Reflective Programming in Smalltalk
Making a Promise
Future methodsFor: ‘demonstration’
demo
| f |
f := Future promising: [2+2].
f printString '4.0'
Future class methodsFor: 'instance creation'
promising: aBlock
| aFuture |
aFuture := self new.
^aFuture promising: aBlock
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

34. Reflective Programming in Smalltalk
Creating an Orphan
nil subclass: #Future
instanceVariableNames: ‘semaphore '
classVariableNames: ‘ '
poolDictionaries: ' '
category: ‘Reflection-Examples’
In VisualWorks, ClassBuilder does the rest. Default implementations of doesNotUnderstand: and class are provided.
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

35. Reflective Programming in Smalltalk
We’ll do it eventually...
Future methodsFor: 'initialization/dispatching'
promising: aBlock
"Create a semaphore, and fork a block that
will signal it. The result of this block is stored
in result..."
semaphore := Semaphore new.
[result := aBlock value.
semaphore signal] fork.
^self
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

36. Reflective Programming in Smalltalk
Keeping a Promise
doesNotUndertand: aMessage
"If this is our init message, let it by..."
aMessage selector == #promising:
ifTrue: [^super perform: aMessage selector
withArguments: aMessage arguments].
"Wait until our result is available..."
semaphore wait.
"If our result is a SmallInteger, it has no oop.."
(result isKindOf: SmallInteger)
ifTrue: [result := result asFloat].
"Become the result and do the
deferred message..."
result become: self.
^self perform: aMessage selector
withArguments: aMessage arguments
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

37. Metaobjects and Lightweight Classes
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

38. Compiler Decompiler CodeRegenerator Scanner SmalltalkCompiler
Compiler Classes
Compiler
Decompiler
CodeRegenerator
Scanner
SmalltalkCompiler
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Reflective Programming in Smalltalk

39. Compiler Support CodeStream DefineOpcodePool MethodNodeHolder
ProgramNodeEnumerator
ScannerTable
<more>
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

40. Reflective Programming in Smalltalk
Variables and Scopes
ArgumentVariable InstanceVariable LocalScope LocalVariable NameScope NullScope PseudoVariable ReceiverVariable RemoteVariable StaticScope StaticVariable TemporaryVariable UndeclaredVariable VariableDefinition
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Reflective Programming in Smalltalk

41. Reflective Programming in Smalltalk
Parse Tree Nodes
ProgramNode
MethodNode ParameterNode StatementNode ReturnNode ValueNode ArithmeticLoopNode AssignmentNode CascadeNode ConditionalNode LeafNode BlockNode LiteralNode VariableNode LoopNode SequenceNode SimpleMessageNode MessageNode
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Reflective Programming in Smalltalk

42. Smalltalk SystemDictionary Association/VariableBinding
<class and pool variables>
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

43. ObjectMemory MemoryPolicy WeakArray WeakDictionary
Storage and Garbage
ObjectMemory
MemoryPolicy
WeakArray
WeakDictionary
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

44. Levels of Representation
Source Compiler
Parse Node ProgramNode ProgramNode
Byte Code VM Decompiler
Native Code
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

45. Reflective Programming in Smalltalk
Ways to Wrap
Source Code Modifications
Byte Code Modifications
New Selectors
Dispatching Wrappers
Class Wrappers
Instance Wrappers
Method Wrappers
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

46. Reflective Programming in Smalltalk
Compiled Methods
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

47. Reflective Programming in Smalltalk
Method Wrappers
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

48. Reflective Programming in Smalltalk
Method Wrappers
valueWithReceiver: anObject arguments: args
self beforeMethod.
^[clientMethod
valueWithReceiver: anObject
arguments: args]
valueNowOrOnUnwindDo:
[self afterMethod]
originalMethodName: argument
^#() receiver: self value: argument
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

49. Reflective Programming in Smalltalk
Multimethods
OptimizingVisitor>>visitWithNode: aNode <ParseNode>
^self value optimized
OptimizingVisitor>>
visitWithNode: aNode <VariableNode>
^aNode lookupIn: self symbolTable
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

50. Reflective Programming in Smalltalk
Parse Tree
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

51. Power Safety Efficiency Comprehensibility
Issues
Power
Safety
Efficiency
Comprehensibility
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

52. Reflection in Smalltalk
Astonishing Introspective
Highly Reflective
Intelligently Factored
Uniquely Malleable
Unappoligetically Dynamic
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

53. Frequently Asked Questions
Q: What can’t I get my hands on?
A: In ‘89 we said: sending a message, receiving a message, returns, and instance variable access. Oh, and the ObjectMemory...
Progress has been made.
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

54. Metalevel Architecture
Building languages out of objects allows one to achieve the goals of reflection
Loops, Lazy Reification, and Induction
Where does the language end?
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

55. Reflective Programming in Smalltalk
Go Ye Forth and Hack
The best way to learn how do reflective programming in Smalltalk is to read code and experiment.
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk

56. Contact Information
Brian Foote
Dept. of Computer Science
3253 DCL
1304 W. Springfield
Urbana, IL 61801
(217)
11/18/2018 (C) Brian Foote 1998
Reflective Programming in Smalltalk