Il giorno giovedì 30 gennaio 2014 23:39:26 UTC+11, skaller ha scritto:
>
>
> On 30/01/2014, at 7:49 PM, srean wrote:
>
> > > As ol...@pobox.com <javascript:> said to me,
> > > lazy evaluation IS control flow. And procedural languages are all
> about control flow.
> > >
> > > You like saying that, I think that as one of your cases of twisting
> terrmiology. I think everybody gets it that control flow is lazy
> evaluation. Thats considered default, no body, is bothered by it, or
> consider that to be insightful (water is wet, sky is blue etc), because
> when they see a control flow they expect laziness. Thats a non issue.
> >
> > Really? I spent 3 DECADES puzzling over this.
> > Trying to understand "control inversion".
> >
> > If you think it is trivial you're obviously a genius.
> >
> > That I obviously am....not :)
> >
> > Wasn't talking about control inversion but lisp's cond. I think everyone
> gets it why cond has to be a special form and cant be eager.
>
> Let try this. With eager evaluation you have a function and some
> arguments.
> So you first evaluate the arguments, then the function body.
> So the arguments consist of .. function evaluations. So you evaluate their
> arguments and then the functions. After your done, you evaluate the
> original
> function body which also consists of function evaluations..
>
> So the picture is: you evaluate the "tree" BOTTOM UP. Draw diagram.
> Arrows point from leaves to root (for some reason Computer Scientists
> always live in the Land Down Under and here, roots are at the top
> and leaves at the bottom .. :)
>
> NOTE: when you get to evaluate a parameter, you might just as well
> plug in the evaluation of the argument into tree. The TIME of evaluation
> is not relevant. The arguments are not evaluated "first". Its the
> direction
> of control flow (master/slave relation) that matters.
>
> Now consider lazy evaluation. You immediately enter the function.
> You see function applications, you immediately evaluate them.
> When you get to a parameter you evaluate the actual arguments.
> They're function applications so you evaluate their functions, then
> the parameters. The leaves in both eager and lazy are constants
> and variables. So with lazy evaluation you evaluate TOP DOWN.
>
> Same tree, arrows reversed. Lazy and eager are DUAL.
>
> So now I ask: what is control inversion??
>
> Answer: the application of the duality transform.
> Swap lazy for eager and vice-versa.
>
> So if you consider a master calling a slave (routine calling
> subroutine) the control inverse reverses the arrow.
>
> However we need Einstein and the space-time continuum now.
> We have to convert flow into memory (i.e. decisions into data
> and data into flow).
>
> How does Felix do this? Simple. instead of the master calling
> the subroutine (control flow into the subroutine), Felix
> RETURNS control to the driver. But not just that.
> It returns something: a continuation. That's the "rest of
> the program, encapsulated as data". Of course that's
> a closure. It is the closure of the subroutine.
>
> but its not just the closure of the subroutine .. its the closure
> of the subroutine LINKED to the caller so when the subroutine
> is finished it RETURNS control to the driver, passing the
> driver a continuation -- namely the continuation of the master
> which resumes it where it left off before.
>
> So the DOWN arrow of procedure calling control flow is converted
> to an UP arrow of returning control, and the CODE of the procedure
> is converted to the DATA of the continuation.
>
> That's control inversion, Felix style.
>
>
And I'm doing it manually right now.
Living the dream.
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