Re: [Chicken-users] On parameterize and fluid-let

[Kon Lovett]

On Oct 14, 2007, at 10:05 AM, John Cowan wrote:

> Tobia Conforto scripsit:
>
> > Question: does (parameterize) have any hidden or special meaning  
> > that's
> > not obvious?  Are these two forms equivalent, except maybe for the
> > introduction of a few symbols?
> >
> > (parameterize ((param1 value1) ...)
> >   body ...)
> >
> > (let ((old-value1 (param1)) ...)  ;save parameters
> >   (param1 value1) ...             ;change parameters
> >   (let ((ret (begin body ...)))   ;execute body and save return value
> >     (param1 old-value1) ...       ;restore parameters
> >     ret))                         ;return value
> >
> > Is (parameterize) just a handy macro for the latter form?
>
> If you use parameters only in the "standard" way, then it can  
> appear so.
> But parameters are first-class objects, lexical variables are not.
> In (parameterize ((n1 v1) (n2 v2) ...) ...), the n1, n2, ...  are  
> general
> Scheme expressions that evaluate to parameter objects, not names.
> In the typical use, global variables are permanently bound to  
> parameter
> objects, and then the global variable is an expression which evaluates
> to the parameter object.  But you can equally well have parameter- 
> valued
> procedures or procedures which accept parameters as arguments.

It is implemented as above, except for the 'dynamic-wind' safety.

> Although parameters are defined by SRFI-39 (see
> http://srfi.schemers.org/srfi-39/srfi-39.html ), their interaction  
> with
> threads is not.  Different Schemes may make parameters thread-global,
> thread-local with copying (like Chicken) or thread-local but  
> uninitialized
> in new threads.
>
> > (fluid-let), on the other hand, operates on non-thread-local  
> > variables,
> > so its effect is very different from these two forms.  Is this  
> > correct?
>
> Fluid-let really is what you thought parameters were: just a hack to
> shallow-bind local variables.  Its behavior in the presence of non- 
> local
> transfers of control can't be counted on.

It will bind globals also, uses '##sys#set!'. Also uses 'dynamic-wind'.

> --
> Even the best of friends cannot                 John Cowan
> attend each others' funeral.                    address@hidden
>         --Kehlog Albran, The Profit             http://www.ccil.org/ 
> ~cowan
>
>
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Best Wishes,
Kon