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

Don Groves <[email protected]> wrote:
> William Tanksley, Jr wrote:
>> Don Groves <[email protected]> wrote:
>>>>>> "William Tanksley, Jr" <wtanksleyjr@> wrote:
>>>>>>> Laziness is one of the things that can't easily be imported to a
>>>>>>> concatenative language.
>>>  It seems to me that laziness can be implemented simply by
>>> encapsulating all data in functions. If a particular function
>>> is not called, that data is not evaluated. What am I missing?

>> Data isn't supposed to be evaluated, of course; I think you meant
>> encapsulating all functions in data rather than the other way around.

> OK, guess I may need some hand-holding here:

No... It's just unclear presuppositions. Thanks for making them clear
by asking about them.

> Isn't call-by-reference essentially equivalent to lazy evaluation?

No; C++ allows call by reference semantics (explicitly specified, of
course) without being lazy at all. An early Fortran had implicit call
by reference, again not lazy (but, I'm told, very dangerous). I don't
use Perl, but I'm told it's call-by-reference.

Wikipedia just reminded me that you're thinking of "call by name" or
"call by need", but those are simply ways to achieve lazy evaluation.

> If the called function doesn't need a certain parameter during a
> particular call, that parameter will not be evaluated. So, parameters
> (data) are only evaluated when needed, i.e., they are lazy.

That IS what lazy evaluation is, yes.

> 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.

Now, if your language is lazy (or non-strict) this doesn't matter; you
can avoid executing the function by never mentioning its parameter on
an execution path. This allows languages that don't provide
higher-order functions to still be very expressive.

> don

-Wm