This post originated from an RSS feed registered with Agile Buzz by James Robertson. Original Post: Continuation based smalltalk Feed Title: Michael Lucas-Smith Feed URL: http://www.michaellucassmith.com/site.atom Feed Description: Smalltalk and my misinterpretations of life Latest Agile Buzz Posts Latest Agile Buzz Posts by James Robertson Latest Posts From Michael Lucas-Smith

As a curiosity, I wondered what a simple piece of Smalltalk might look like if expanded out in to Continuation-Passing-Style. The real question of whether CPS is viable in Smalltalk is this: How many method permutations are required

What do I mean by that? Take the method isNil. In smalltalk, this method takes no arguments, down at the VM level it takes one argument - the receiver. In CPS there is no 'return', so any information you wish to use ahead of you must be passed as an argument:

a := 6.
myBool isNil ifTrue: [self doSomething].
a doSomethingElse

In this scenario, the variable a needs to be passed through all the calls that can get to a doSomethingElse. So we'd end up with a new variant of isNil that looked something like:

Object>>isNil: continuationReturnLocation theVariableA: a
	continuationReturnLocation(false,a)

Smalltalk already has a Message object which can be used to tell some code where to go and what to pass it, so instead we can do the following:

Object>>isNil: continuation
continuation value: false

And to call it:

myBool isNil: (Message receiver: self selector: #nextBit:continuation: arguments: (Array with: #returnValueGoesHere with: continuation)

As noisy as this is, it shows us that we don't need a million and one variations of every method in the system to implement CPS - we just need to make lots and lots and lots of Continuation/Message objects.

So! What would the world look like if we had a VM create lots and lots of Continuation objects instead of making lots and lots of method call variants? Well, lets take the following piece of code:

asRpcXml
	| aStream numberOfArguments |
	numberOfArguments := arguments isNil ifTrue: [1] ifFalse: [arguments size].
	aStream := WriteStream on: (String new: numberOfArguments * 100 + 100).
	self printRpcXmlOn: aStream.
	^aStream contents

Then we bloat it out using Continuation/Message objects

asRpcXml: continuation
	arguments isNilContinuation: (Message receiver: self selector: #asRpcXmlBranch:with: arguments: ? and: continuation)

asRpcXmlBranch: isNil continuation: continuation
	isNil
		ifTrue: (Message receiver: self selector: #asRpcXmlBranchTrue: argument: continuation)
		ifFalse: (Message receiver: self selector: #asRpcXmlBrancFalse: argument: continuation)
		continuation: (Message receiver: self selector: #nowhere)

asRpcXmlBranchTrue: continuation
	1 times: 100 continuation: (Message receiver: self selector: #asRpcXmlBranchTruePlus:continuation: arguments: ? and: continuation)

asRpcXmlBranchFalse: continuation
	arguments sizeContinuation: (Message receiver: self selector: #asRpcXmlNewString:with: arguments: ? and: continuation)

asRpcXmlBranchTruePlus: number continuation: continuation
	number plus: 100 continuation: (Message receiver: self selector: #asRpcXmlNewString:continuation: arguments: ? and: continuation)

asRpcXmlNewString: size continuation: continuation
	String new: size continuation: (Message receiver: self selector: #asRpcXmlWriteStreamOn:continuation: arguments: ? and: continuation)

asRpcXmlWriteStreamOn: string continuation: continuation
	WriteStream on: string continuation: (Message receiver: self selector: #asRpcXmlPrintOn:continuation: arguments: ? and: continuation)

asRpcXmlPrintOn: stream continuation: continuation
	self printRpcXmlOn: stream with: (Message receiver: self selector: #asRpcXmlReturn:continuation: arguments: stream and: continuation)

asRpcXmlReturn: stream continuation: continuation
	stream contentsContinuation: continuation

This a thought experiment. What I'd really like to do is to find some easy way of measuring how many method permutations are required to implement the VisualWorks code base using non-context object based CPS. Thoughts?