[stack] Re: Barebone implementation of concatenative language in c or c++

--- In [email protected], Don Groves <dgroves@...> wrote:
>
> On 20 Jul 2010, at 21:18, William Tanksley, Jr wrote:
> 
> > Don Groves <dgroves@...> wrote:
.
.
.
> >> In a call-by-value environment, It seems to me that encapsulating
> >> all data in functions yields the same effect as call-by-reference. If
> >> a parameter is not needed, its evaluating function will not be  
> >> called.
> >
> > This gets confusing to me, so hold on tight... :-)
> >
> > In order to pass a function to a procedure, the function has to be
> > available as data. If it's not, you can't pass it.
> 
> I don't understand this -- I do it all the time in my C code by using  
> the
> address operator, &, and the same is easily done in Forth, et al.

That's pretty much the same indirection approach I use in Furphy (see http://users.beagle.com.au/peterl/furphy.html), to achieve the same effect with a workaround even when what actually happens is done eagerly. I even describe it there with an analogy to what you are thinking of in C:-

Conditional code is mostly achieved by IF and a complementary pair of things, freezing and thawing. This is a bit like using indirection in C to pass parameters by name instead of the default by value. The analogy here is that IF does eager evaluation, and freezing and thawing change the behaviour to lazy evaluation. IF has this behaviour, using Forth documenting standards: (  true/false_flag  true_option  false_option  ---  result  ).

FREEZE and THAW behave like an anonymous label (if that isn't a contradiction in terms - it really returns early and pushes a re-entry address to the stack) and a dynamic subroutine call, different from the Forth keyword EXECUTE that it resembles. Typically, you would put FREEZE at the beginning of each keyword that you might want to select with IF and then you would use THAW on whichever one got chosen, like this:-

FREEZE TRUE_STUFF TRUE_OPTION

FREEZE FALSE_STUFF FALSE_OPTION

GET_FLAG TRUE_OPTION FALSE_OPTION IF THAW DEMO

Later on I describe some syntactic sugar to make coding and using the indirection easier, too. P.M.Lawrence.