Re: [stack] Re: the concatenative wikipedia article

On Dec 31, 2008, at 3:25 PM, John Nowak wrote:

>
> On Dec 31, 2008, at 6:01 PM, Adam wrote:
>
>> Slava recently created a nice description of Concatenative languages
>> in the following four articles
>
> Thanks, I had hadn't seen that before.
>
> Slava's description seems decent enough. It is, however, stack-
> focused. He says that concatenative languages and stack-based
> languages are similar, but then never clarifies the differences and
> goes on to conflate the two. I also have some minor niggles (in no
> particular order):

I haven't worked on this stuff for almost a year now, but when last
involved, I was fiddling with the terminology to eliminate the word
"stack," which is ingrained in the concatenative culture, but at times
not as descriptive as I would like. As these languages mature, their
operators become less and less stack-like.

I was leaning toward defining a concatenative (if that's the right
word) program as "a list of operators applied to a (possibly initially
empty) list of operands."

This definition seems to me to have two advantages: (1) it allows
for non-FIFO shuffle operations like swap, rot, etc..., which are fine
for a list but not so for a classically-defined stack; and (2) the order
of application is not specified.

Not being up on this topic at the moment, the order of application
may need to be specified -- [operand] [operators] -- I'm not sure
of this.

I'd appreciate any comments, as they may save me wasted effort
if/when I do return to this eternally intriguing subject (which is why
I've returned to it so often already ;-)
--
don


> 1. There's no reason a concatenative language has to support
> recursion. My initial version of 5th did not.
> 2. Not all concatenative languages have multiple programmer-accessible
> stacks. Forth and Factor do, Joy and Cat do not.
> 3. It isn't mentioned that the reduction of a function the
> transformation of one function to another. This is the critical thing
> I find lacking in FP and FL.
> 4. It says composition must be denoted by concatenation. I'd prefer to
> say that some sort of concatenation operator is the primary means of
> forming programs, be that operator whitespace, a dot, etc.
> 5. Quotation works quite differently in Factor and Joy compared to
> Cat. In Cat, quotation is an abstraction mechanism (as Chris has
> pointed out to me). In Factor and Joy, it is not. It is therefore not
> accurate to say a quotation is a sequence of objects.
> 6. There's no reason that only one stack be live at any given point.
> Joy's persistent stack semantics allow for parallel evaluation.
> Ignoring IO, you can reduce programs using rewrite rules applied in
> any order and/or simultaneously.
> 7. It it said that "literals are pushed onto the stack when
> encountered by the evaluator". This implies that '5' is an object, not
> a function, which I think is a less useful (and likely inaccurate)
> characterization.
>
> - John
>
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