Reflection: Stack as an Object

Just to Reveal a Bit of It

To let you know that it exists

• On demand the stack can be turned into an object
• We can walk but also modify it

Coding in the Debugger

• When a method is not found, we may end up with a debugger
• In the debugger, we can ask for the creation of a method on the fly
• The system compiles on the spot a special method
• Then it reexecutes the method
• It raises a shouldBeImplemented exception
• Then you can edit the method in the debugger
• Then proceed and the program continues to run

Operation Supporting Coding in the Debugger

• On the fly method definition
• Stack navigation / manipulation

Stack Frame as an Object

• Execution stack can be turned into a live object
• By default the stack is not an object
  • Can be reified on demand
• Support exception definitions from within the language
• Basis for continuations and web serving

thisContext

• thisContext is one of the three pseudovariables with self and super
• Returns an object that represents the method activation
• Can walk and change the stack
• Put self halt in the code to see it and walk

Example: Better Deprecated Message

We declare deprecation

A >> foo
   self deprecated: 'Use bar.' on: '31/12/2015' in: 'Pharo50'.
   self bar

should print: 'Message foo is deprecated in Pharo50. Use bar'

Example: Better Deprecated Message

deprecated: anExplanationString on: date in: version
	"Warn that the sending method has been deprecated"
	
	(Deprecation
		method: thisContext sender method
		explanation: anExplanationString
		on: date
		in: version) signal

thisContext sender method returns compiled method A>>#foo

Example: Conditional Halt and Test

• How to halt a method that is heavily used?
• How to stop execution only from a given execution flow?

foo 
  ...
  Halt if: #testSetInitialized
  ...

Only halt if executed from testSetInitialized

if: Implementation

Halt class >> if: condition
  "This is the typical message to use for inserting breakpoints during debugging.
  The argument can be one of the following:
    - a block: if the Block has one arg, the calling object is bound to that.
    - an expression
    - a selector: Halt if found in the call chain"

  condition isSymbol ifTrue: [ ^ self haltIfCallChainContains: condition ].
  condition isBlock ifTrue: [ ^ self haltIfBlockWithCallingObject: condition].
  condition ifTrue: [self signal].

How if: is implemented?

Halt class >> haltIfCallChainContains: aSelector
  | cntxt |
  cntxt := thisContext.
  [ cntxt sender isNil ] whileFalse: [
    cntxt := cntxt sender. 
    (cntxt selector = aSelector) ifTrue: [ self signal ] ].

Foundation for Innovation

• Continuations
• Seaside: Powerful dynamic web framework for dynamic web applications
  • http://www.seaside.st
  • http://book.seaside.st