[ANSI-Smalltalk] Protocol extensions - Object

[Richard A. O'Keefe]

On 8 Sep 2008, at 6:04 pm, Paolo Bonzini wrote:
> Squeak overloads #perform: for a Message argument, see
>
> http://lists.squeakfoundation.org/pipermail/squeak-dev/2007-September/120697.html
>
> for an example.  What do you think about it?

I think I can't find it in Squeak 3.10.
Here's the source code for Object>>perform:

perform: aSymbol
	"Send the unary selector, aSymbol, to the receiver.
	Fail if the number of arguments expected by the selector is not zero.
	Primitive. Optional. See Object documentation whatIsAPrimitive."

	<primitive: 83>
	^ self perform: aSymbol withArguments: (Array new: 0)

I don't know where to find the implementation of a primitive,
so a test was the way forward.  If #perform: worked with Messages,
then
	1 perform: (Message selector: #+ argument: 2)

should be the same as 1 + 2, no?  In fact it produces a
MessageNotUnderstood exception.

What Squeak _does_ have is Message sentTo: anObject,
so
	(Message selector: #+ argument: 2) sentTo: 1

works.  That's not in VW, at least, not in VWNC 5, but

	Message>>
	sentTo: anObject
		^anObject perform: selector withArguments: args

does the trick, and more efficient implementations are of course
possible.

Overloading #perform: raises the question
"what should #perform:withArguments: do when the 'selector'
is a Message with arguments of its own?"

Adding Message>>sentTo: does not raise that question.
It does seem odd to have Messages with no direct way to send
them, so adding #sentTo: would seem to make sense.