On 30.09.2016, at 02:02, Frank Shearar <frank.shearar at gmail.com> wrote:

> On 29 September 2016 at 12:19, Tobias Pape <Das.Linux at gmx.de> wrote:
> 
> On 29.09.2016, at 20:57, Frank Shearar <frank.shearar at gmail.com> wrote:
> >
> >>
> >>      Environment current
> >>
> >> is parallel to
> >>
> >>      Project current
> >>
> >> I think this is OK.
> >>
> >> @Frank, could you elaborate please what you mean with '_delimited
> >> dynamic variable_' ?
> >
> > Well, I like to be precise with my terminology. The tl;dr is that a delimited dynamic variable is just like what other people call a dynamic variable, except it plays nicely with stack slicing. The throw-a-Notification-get-an-answer idiom is exactly equivalent to delimited dynamic variables
> >
> > http://lists.squeakfoundation.org/pipermail/squeak-dev/2014-February/176777.html has a bit of a conversation around the topic, and has pointers to in-depth articles on delimited dynamic binding, and I've done my small part in trying to dig into the topic here: http://www.lshift.net/blog/2012/06/27/resumable-exceptions-can-macro-express-delimited-dynamic-variables/.
> >
> > Anyway, the fundamental point I was trying to make is that using a Notification (CurrentEnvironment) is exactly avoiding global state, but it seems I missed the point that you and Nicolas were making, around various things having a #environment property.
> 
> A note: The first time I had seen a dynamic variable was in seaside where
> it is/was used for accessing the 'request context' (Being a web framework, this thingy described the HTTP request received by the framework).
> 
> To access that, you call
> 
>         self requestContext zork
> 
> in you seaside class, which boils down to
> 
>         requestContext
> 
>                 ^ CurrentRequestContext value
> 
> And CurrentRequestContext being a Dynamic Variable implemented as Notification.
> 
> However, since then, we have merged in the DynamicVariable as ProcessSpecificVariable contributed by Martin von Löwis.
> This provides a more efficient way to 'resolve' the DV to a value, because the Notification-based DV needs to peel the stack and re-built it (particularly hurting Cog) whereas the ProcessSpecificVariable-based DV 'just' needs some Dict-lookups in the current Process.
> 
> This is why I like being specific in terminology. Yes, the thing called DynamicVariable in Squeak trunk is a dynamic variable, and not a delimited dynamic variable, which is exactly why you cannot combine using DynamicVariable and stack manipulations. Debugging happens to do said stack manipulations.

Ok, I think I understand, but doesn't the [ ] ensure: [] in DynamicVariable class>>value:during: take care of the delimition?

> 
> Yes, the Notification idiom is much less efficient (in its current implementation (*)) than DynamicVariable, but DynamicVariable fundamentally cannot work correctly with stack manipulations. Oleg Kiselyov covers the issue in depth in the various articles linked to from http://okmij.org/ftp/Computation/dynamic-binding.html#DDBinding.
> 
> (*) I would imagine that VM support would allow for efficient delimited dynamic variables, because Henry Baker wrote a whole pile of papers back in the late 70s on the subject (see http://www.pipeline.com/~hbaker1/home.html, for instance "Shallow Binding Makes Functional Arrays Fast".
> 
> frank
>  
> 
> ========
> 
> I quite like accessing the current Environment via a dynamic variable, as it exactly provides the control-flow scope that is necessary for this kind of thing.
> 
> We should however adopt a proper DV here, increasing efficiency and readability.
> No more 'CurrentEnvironment signal ifNil: […]'
> 
> but rather
> 
>         Object>>environment
> 
>                 ^ CurrentEnvironment value
> 
> and
> 
>         DynamicVariable subclass: #CurrentEnvironment
>         instanceVariableNames: ''
>         classVariableNames: ''
>         poolDictionaries: ''
>         category: '...'
> and
>         CurrentEnvironment class>>default
> 
>                 ^ Smalltalk globals
> 
> and then, for example
> 
>         EnvironmentLoader>>evaluate: chunk
> 
>                 ^ CurrentEnvironment use: environment during:
>                         [Compiler evaluate: chunk environment: environment]
> 
> 
> (not tested, tho)
> 
> Best regards
>         -Tobias
> 
> PS: I sometimes call DVs semi-globals. not really global, not really non-global…
> 
> 
> 
>