Hi Tobias,

...

Hi, I can't figure this one out - is the value of /complete / in #ifCurtailed method used somewhere?

Yes.

That’s a great help, thanks! So I understand “complete” marks contexts that have already been processed (unwound) during an abnormal return (in #resume:through:) or, in case of a normal return it just marks normal completion of #ensure/#ifCurtailed.

Just to be sure: there’s a close interplay between e.g. #ensure and #resume:through: but NO interplay between #ensure and #ifCurtailed, right?

And finally, is there a simple example where “complete” plays a role? It seems to me that if #resume:through: unwinds contexts it never gets to unwind the same context twice and if the #ensure protected block returns normally no unwinding even happens.

I tried to remove “complete” from the #ensure/#ifCurtailed code and all my naive examples work as before :)

But first thing to notice is that #ensure: uses the same primitive and the same names and oder of local variables.

In both methods, the comment states: " The VM uses prim 198 in a context's method as the mark for an ensure:/ifCurtailed: activation"

But more so:

Look at Context>>unwindTo:

unwindTo: aContext

​ | ctx unwindBlock | ​ ctx := self. ​ [(ctx := ctx findNextUnwindContextUpTo: aContext) isNil] whileFalse: [ ​ (ctx tempAt: 2) ifNil:[ ​ ctx tempAt: 2 put: true. ​ unwindBlock := ctx tempAt: 1. ​ unwindBlock value] ​ ].

In a Context, that is the "running" form of a Block, tempAt: 1 is the argument of #ifCurtailed:/#ensure:. and tempAt: 2 is "completed". So: the context will be marked as being completed either via #ifCurtailed: directly or via #unwindTo:. However, in #ensure:

ensure: aBlock | complete returnValue | <primitive: 198> returnValue := self valueNoContextSwitch. complete ifNil:[ complete := true. aBlock value. ]. ^ returnValue

The aBlock will only be run if the action was not completed yet.

Now this confused me: if the protected block returns normally, that means no unwinding has happened and thus “complete” must be nil and aBlock will always be evaluated.

Conversely, if the protected block returned abnormally I thought it meant the process would never reach the condition and the aBlock would never be run here (but on the unwind path instead).

Where am I wrong?

(I used this to explore: [x := thisContext sender explore. ^1] ensure: [x explore] )

Thanks again! Jaromir

"complete" can be set from the outside as can be seen in #unwindTo:. It is also visible, that #unwindTo: and #ensure: have a similar structure, but #unwindTo: operates on the data structure of the "running" code.

See also in Context: #restart #resume: #resume:through: or Process>>#terminate.

I hope this helps :)

Best regards -Tobias

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