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.

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…
>
>
>
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