[stack] Concatenative versus Compositionality

On Dec 3, 2007 10:17 AM, William Tanksley, Jr <[email protected]> wrote:
>
> Christopher Diggins <[email protected]> wrote:
> > FWIW I like your definition.
>
> In your blog you mention,

For those who haven't seen it:
http://cdiggins.com/2007/12/01/compositional-programming-languages-not-concatenative/
And some reddit discussion is at:
http://programming.reddit.com/info/61win/comments/

> reasonably, that because we're uncertain
> about the precise results of the term 'concatenative', we should use
> another term; you suggest 'compositional'.

That doesn't quite characterize my reasoning. The fact that there is
uncertainty is not of concern to me. I am suggesting that the nature
of these languages is not the concatenativeness (lets define this as
meaning a language where whitespace corresponds to an operation) but
rather the compositionality. Haskell is a language where concatenation
denotes function application, and Joy is a language where
concatenation denotes function composition. What unifies the langauges
so far labelled as concatenative is that they are compositional
(whitespace denotes function composition).

> I disagree. Our uncertainty about the precise meaning of the term
> mirrors the fact that these languages are very little studied; the
> fact that we're a bit murky on the precise boundaries of our field
> therefore only makes sense.

Yes of course.

> At the same time, your suggested term, 'compositional', is good; the
> obvious problem is that it's already been used. A compositional
> computer language is a language used only to bring together code
> written in other languages;
>
> the language Piccola
> http://www.iam.unibe.ch/~scg/Research/Piccola/ is a 'pure' example,

From the Piccola paper "a composition language — a language for
building applications from
software components implemented in another, host programming language."

IMO Piccola would be much better described as a "component oriented language".
Howerver, that said: there is an overloading of the term "composition"
in Computer science to mean either object/component composition and
function composition.
We could offer the distinction of "function composition language" for
Joy et. al. whereas Piccola would be an "object composition language".

> and I suppose the various shell scripts are impure examples.  But
> that's not broad use, and I don't see a severe problem with using it
> anyhow.
>
> I also don't agree with your claim that Haskell qualifies as a
> concatenative language. Am I wrong? Do any two valid Haskell programs
> concatenated together form a valid Haskell program?

No it is not true in the general case. But does the general case hold
for Forth or Postscript? Does it also hold for all Joy programs
without exception?

I think however, the concatenation of programs is of less relevance
than what it means to concatenate terms (i.e. expressions) in the
language. Only looking at whether whole programs can be concatenated
is too restrictive. Consider if we were to add some features to Joy
(e.g. some kind of modularization constructs or scoping constructs)
which meant that arbitary programs could no longer be concatenated,
but the core semantics of the language were preserved: would that make
the language an entirely new category? This an indicator that
concatenation defined at strictly the program level is too narrow to
merit a categorization.

> My knowledge here
> is limited, but I don't _think_ currying is that powerful -- _some_
> Haskell programs are 'concatenative' in that sense, but it's not a
> general feature of the language, to the best of my knowledge.
> Furthermore, if Haskell were concatenative, would that be bad?

Nope, in fact it would be very good. Bridging the gap between
compositional concatenative languages, and applicative concatenative
languages would be an excellent thing for the community. There is a
lot for members of Haskell and ML community to bring to the table.

Nonetheless Jot (http://barker.linguistics.fas.nyu.edu/Stuff/Iota/) is
an example of the a language where concatenation denotes application.

This naturally leads to interesting ideas: what else could whitespace
denote? Pick a combinator, and you have a very interesting esoteric
language to study. (Hmmm: a language where whitespace could be changed
dynamically ...).

>  I admit
> that I did invent 'concatenative' with some vague idea of being
> 'opposite' to applicative languages,  but I don't think the category is
> useless even if it's orthogonal rather than opposite.

Yes, perhaps I overstated the case. If we broadened the term
"concatenative" to mean something like a "language where the
concatenation of terms is valid, and has semantic meaning (e.g.
function application or function composition)". It would be a useful
and interesting categorization, and one which I think is much more
intuitively correct. I could see it as being a nice way to classify
languages where whitespace has semantic meaning.

It may be late in the game, where there is already a fair amount of
momentum behind the usage of the term concatenation (and this
wonderful community), but I still think it is worthwhile to try and be
as accurate and intutiviely descriptive as possible with our
terminology.

Perhaps you are wondering why I am pushing back now. I am currently
writing and submitting numerous articles for computer science
conferences. I would like to write a paper introducing the
"concatenative" category of languages, as something worth studying,
but in fact I am only interested in compositional concatenative
languages. I fear that the referees in the academic computer science
community will bring up the same concerns that I am pointing out here.

> > - Christopher
>
> -Wm

Cheers,
Christopher


 
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