On Wed, Apr 21, 2010 at 4:34 PM, Hernán Morales Durand < hernan.morales@gmail.com> wrote:
2010/4/21 Lukas Renggli renggli@gmail.com:
thisContext is a special object for representing an activation in a stack frame in a stack-based VM.
Actually "thisContext" represents *the current* activation/stack-frame.
foo: anObject ^ thisContext at: 1
is the same as
foo: anObject ^ anObject
Of course, "this" refers to "the current" in common language usage.
There are two kinds of contexts: Method Contexts and Block Contexts.
Actually in Pharo images there are only instances of MethodContext. Though you can ask the context if it comes from a block by sending the message #isExecutingBlock.
Actually in Pharo there is a BlockContext class, which is not instantiated anymore after the introduction of the closure compiler?
Right.
In other Smalltalks aBlockContext is the resulting context of a block activation during its evaluation and is activated by sending #value,.this fills thisContext with the execution information inside the block.
and it is now with the closure compiler. Or rather a new context is created when a BlockClosure is sent the value message. The context is an instance of methodContext though.
In Squeak (or the old compiler) block contexts were created using #blockCopy:, now I see #closureNumCopied:numArgs:. Does this means BlockContext could be completely removed and replaced with MethodContext semantics? I've removed the BlockContext class and used Pharo a little bit with no problems, maybe some Decompiler issues in the Debugger...
Yes. But MethodContext behaves differently depending on whether its closureOrNil inst var is nil (a normal method) or a BlockClosure (an activation of a block). If nil, ^-returns return to the context's sender. If not nil ^-returns return from the home context, found by following the outerContext chain through the closureOrNil inst var. So we no longer need BlockContext.
This also means we only need one context class, and some time I'd like to merge the two ContextPart and MethodContext classes into a single Context class. Possibly the name should be ExecutionContext or MethodOrBlockContext or? Suggestions?
Now, test yourself before evaluating :) what should be the result of this expression?
[: arg | arg perform: #isExecutingBlock ] value: thisContext
It depends on whether the expression is executed at method level or at block level. As a test this should be:
testIsExecutingBlock self assert: ([: arg | arg perform: #isExecutingBlock ] value: thisContext) == false. [self assert: ([: arg | arg perform: #isExecutingBlock ] value: thisContext) == true] value
Can somebody justify the result?
isExecutingBlock answers if the receiver is executing a block or not. It does not answer whether the context in which isExecutingBlock is sent is executing a block or not. Remember that
[:arg| arg == thisContext] value: thisContext
is false. thisContext is rebound within every actual method or block scope.
HTH
Eliot
Context creation is optimized in the VM in most Smalltalks, so it's only really created as an object in the environment (reified) when it's specifically needed through "thisContext".
In Pharo contexts are not reified like that. Stack-frames are actual objects at all times. However, for speed reasons, their creation and garbage-collection is optimized by the VM. Stack frames get automatically recycled if nobody refers to them.
I think is what I said :)
But its not a property of Pharo. It is a property of the VM. In Cog contexts are not actual objects all the time, only when needed. And Cog runs Pharo images just as it runs Squeak images. So it would be incorrect to say "in Pharo" and better to say "in the standard Squeak VM".
There are several applications related with computational reflection (Reflective Programming, Meta-Programming, MOP, etc) which makes use of the current context.
Also: exception handling, generators, continuations, co-routines, ...
For another fun use of "thisContext" check this Stack-Overflow question:
http://stackoverflow.com/questions/2500483/is-there-a-way-in-a-message-only-...
Lukas
-- Lukas Renggli www.lukas-renggli.ch
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On 2010-04-21, at 5:04 PM, Eliot Miranda eliot.miranda@gmail.com wrote:
This also means we only need one context class, and some time I'd like to merge the two ContextPart and MethodContext classes into a single Context class. Possibly the name should be ExecutionContext or MethodOrBlockContext or? Suggestions?
I suggest ActivationContext.
Colin
Yes this is a nice name
On Apr 22, 2010, at 6:06 AM, Colin Putney wrote:
On 2010-04-21, at 5:04 PM, Eliot Miranda eliot.miranda@gmail.com wrote:
This also means we only need one context class, and some time I'd like to merge the two ContextPart and MethodContext classes into a single Context class. Possibly the name should be ExecutionContext or MethodOrBlockContext or? Suggestions?
I suggest ActivationContext.
Colin
Yes. But MethodContext behaves differently depending on whether its closureOrNil inst var is nil (a normal method) or a BlockClosure (an activation of a block). If nil, ^-returns return to the context's sender. If not nil ^-returns return from the home context, found by following the outerContext chain through the closureOrNil inst var. So we no longer need BlockContext.
Hi Elliot
I was wondering why you do not use two classes and you do the nil check?
This also means we only need one context class, and some time I'd like to merge the two ContextPart and MethodContext classes into a single Context class. Possibly the name should be ExecutionContext or MethodOrBlockContext or? Suggestions?
Now, test yourself before evaluating :) what should be the result of this expression?
[: arg | arg perform: #isExecutingBlock ] value: thisContext
It depends on whether the expression is executed at method level or at block level. As a test this should be:
testIsExecutingBlock self assert: ([: arg | arg perform: #isExecutingBlock ] value: thisContext) == false. [self assert: ([: arg | arg perform: #isExecutingBlock ] value: thisContext) == true] value
Can somebody justify the result?
isExecutingBlock answers if the receiver is executing a block or not. It does not answer whether the context in which isExecutingBlock is sent is executing a block or not. Remember that
[:arg| arg == thisContext] value: thisContextis false. thisContext is rebound within every actual method or block scope.
HTH
Eliot
Context creation is optimized in the VM in most Smalltalks, so it's only really created as an object in the environment (reified) when it's specifically needed through "thisContext".
In Pharo contexts are not reified like that. Stack-frames are actual objects at all times. However, for speed reasons, their creation and garbage-collection is optimized by the VM. Stack frames get automatically recycled if nobody refers to them.
I think is what I said :)
But its not a property of Pharo. It is a property of the VM. In Cog contexts are not actual objects all the time, only when needed. And Cog runs Pharo images just as it runs Squeak images. So it would be incorrect to say "in Pharo" and better to say "in the standard Squeak VM".
There are several applications related with computational reflection (Reflective Programming, Meta-Programming, MOP, etc) which makes use of the current context.
Also: exception handling, generators, continuations, co-routines, ...
For another fun use of "thisContext" check this Stack-Overflow question:
http://stackoverflow.com/questions/2500483/is-there-a-way-in-a-message-only-...
Lukas
-- Lukas Renggli www.lukas-renggli.ch
Pharo-project mailing list Pharo-project@lists.gforge.inria.fr http://lists.gforge.inria.fr/cgi-bin/mailman/listinfo/pharo-project
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On Thu, Apr 22, 2010 at 12:49 AM, stephane ducasse < stephane.ducasse@gmail.com> wrote:
Yes. But MethodContext behaves differently depending on whether its
closureOrNil inst var is nil (a normal method) or a BlockClosure (an activation of a block). If nil, ^-returns return to the context's sender. If not nil ^-returns return from the home context, found by following the outerContext chain through the closureOrNil inst var. So we no longer need BlockContext.
Hi Elliot
I was wondering why you do not use two classes and you do the nil check?
One reason is backward compatibility. The existing VM ensures that closureOrNil (which used to be called receiverMap) is nil. The other is the internal workings of the VM. The nil test is done by the VM on every ^-return. The VM isn't going to waste time sending a message. It is going to do a nil test.
In fact in Cog the VM doesn't even do a nil test since in machine code the JIT knows at compile time whether an ^-return is within a block or not, and in the stack interpreter whether a frame is a block activation is indicated by a status bit in the method field of a frame.
So don't think Smalltalk code, think optimized VM internals. We're not going to waste time instantiating some object just to tag a structure as not being a block activation.
HTH Eliot
This also means we only need one context class, and some time I'd like to
merge the two ContextPart and MethodContext classes into a single Context class. Possibly the name should be ExecutionContext or MethodOrBlockContext or? Suggestions?
Now, test yourself before evaluating :) what should be the result of this expression?
[: arg | arg perform: #isExecutingBlock ] value: thisContext
It depends on whether the expression is executed at method level or at
block level. As a test this should be:
testIsExecutingBlock self assert: ([: arg | arg perform: #isExecutingBlock ] value:
thisContext) == false.
[self assert: ([: arg | arg perform: #isExecutingBlock ] value:thisContext) == true] value
Can somebody justify the result?
isExecutingBlock answers if the receiver is executing a block or not. It
does not answer whether the context in which isExecutingBlock is sent is executing a block or not. Remember that
[:arg| arg == thisContext] value: thisContextis false. thisContext is rebound within every actual method or block
scope.
HTH
Eliot
Context creation is optimized in the VM in most Smalltalks, so it's only really created as an object in the environment (reified) when it's specifically needed through "thisContext".
In Pharo contexts are not reified like that. Stack-frames are actual objects at all times. However, for speed reasons, their creation and garbage-collection is optimized by the VM. Stack frames get automatically recycled if nobody refers to them.
I think is what I said :)
But its not a property of Pharo. It is a property of the VM. In Cog
contexts are not actual objects all the time, only when needed. And Cog runs Pharo images just as it runs Squeak images. So it would be incorrect to say "in Pharo" and better to say "in the standard Squeak VM".
There are several applications related with computational reflection (Reflective Programming, Meta-Programming, MOP, etc) which makes use of the current context.
Also: exception handling, generators, continuations, co-routines, ...
For another fun use of "thisContext" check this Stack-Overflow
question:
http://stackoverflow.com/questions/2500483/is-there-a-way-in-a-message-only-...
Lukas
-- Lukas Renggli www.lukas-renggli.ch
Pharo-project mailing list Pharo-project@lists.gforge.inria.fr http://lists.gforge.inria.fr/cgi-bin/mailman/listinfo/pharo-project
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thanks for the explanation I see the intention. I was curious.
Stef
On Apr 22, 2010, at 6:55 PM, Eliot Miranda wrote:
On Thu, Apr 22, 2010 at 12:49 AM, stephane ducasse stephane.ducasse@gmail.com wrote:
Yes. But MethodContext behaves differently depending on whether its closureOrNil inst var is nil (a normal method) or a BlockClosure (an activation of a block). If nil, ^-returns return to the context's sender. If not nil ^-returns return from the home context, found by following the outerContext chain through the closureOrNil inst var. So we no longer need BlockContext.
Hi Elliot
I was wondering why you do not use two classes and you do the nil check?
One reason is backward compatibility. The existing VM ensures that closureOrNil (which used to be called receiverMap) is nil. The other is the internal workings of the VM. The nil test is done by the VM on every ^-return. The VM isn't going to waste time sending a message. It is going to do a nil test.
In fact in Cog the VM doesn't even do a nil test since in machine code the JIT knows at compile time whether an ^-return is within a block or not, and in the stack interpreter whether a frame is a block activation is indicated by a status bit in the method field of a frame.
So don't think Smalltalk code, think optimized VM internals. We're not going to waste time instantiating some object just to tag a structure as not being a block activation.
HTH Eliot
This also means we only need one context class, and some time I'd like to merge the two ContextPart and MethodContext classes into a single Context class. Possibly the name should be ExecutionContext or MethodOrBlockContext or? Suggestions?
Now, test yourself before evaluating :) what should be the result of this expression?
[: arg | arg perform: #isExecutingBlock ] value: thisContext
It depends on whether the expression is executed at method level or at block level. As a test this should be:
testIsExecutingBlock self assert: ([: arg | arg perform: #isExecutingBlock ] value: thisContext) == false. [self assert: ([: arg | arg perform: #isExecutingBlock ] value: thisContext) == true] value
Can somebody justify the result?
isExecutingBlock answers if the receiver is executing a block or not. It does not answer whether the context in which isExecutingBlock is sent is executing a block or not. Remember that
[:arg| arg == thisContext] value: thisContextis false. thisContext is rebound within every actual method or block scope.
HTH
Eliot
Context creation is optimized in the VM in most Smalltalks, so it's only really created as an object in the environment (reified) when it's specifically needed through "thisContext".
In Pharo contexts are not reified like that. Stack-frames are actual objects at all times. However, for speed reasons, their creation and garbage-collection is optimized by the VM. Stack frames get automatically recycled if nobody refers to them.
I think is what I said :)
But its not a property of Pharo. It is a property of the VM. In Cog contexts are not actual objects all the time, only when needed. And Cog runs Pharo images just as it runs Squeak images. So it would be incorrect to say "in Pharo" and better to say "in the standard Squeak VM".
There are several applications related with computational reflection (Reflective Programming, Meta-Programming, MOP, etc) which makes use of the current context.
Also: exception handling, generators, continuations, co-routines, ...
For another fun use of "thisContext" check this Stack-Overflow question:
http://stackoverflow.com/questions/2500483/is-there-a-way-in-a-message-only-...
Lukas
-- Lukas Renggli www.lukas-renggli.ch
Pharo-project mailing list Pharo-project@lists.gforge.inria.fr http://lists.gforge.inria.fr/cgi-bin/mailman/listinfo/pharo-project
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vm-dev@lists.squeakfoundation.org
-
Colin Putney -
Eliot Miranda -
stephane ducasse