That code from other lisp implementations needs rewritten to be ported, is really not that much different from how many other lisp implementations were to each other.
CL and Scheme did a lot to unify things, such that any scheme should run another scheme's code. Same for any CL implementation. But, for example, running emacs lisp in any other lisp just isn't going to work.
CL-USER> (defun fact (n) (do* ((i n (- i 1)) (j i (* j i))) ((= i 1) j)))
FACT
CL-USER> (fact 15)
1307674368000
ELISP> (defun fact (n) (do* ((i n (- i 1)) (j i (* j i))) ((= i 1) j)))
fact
ELISP> (fact 15)
1307674368000 (#o23016735654000, #x13077775800)
That does not say anything about whether or not a language is an assembly. Just as this says nothing about whether a language is a lisp.
Emacs Lisp is directly in the tradition of McCarthy's Lisp. It shares names, concepts, literature, even code.
Logo, Dylan, ML, Clojure don't. They were derived or inspired by Lisp, but they are now they own language family...
The result of that design is that Clojure shares literally zero lines of code with other Lisps: Autolisp, ISLisp, Emacs Lisp, Standard Lisp, Common Lisp. Many basic concepts are absent, renamed or redesigned ('Atom', 'Linked List', ...). Clojure is fully incompatible to any other language with Lisp in its name. Programs have to be re-architectured, because the concepts are different: no TCO, but 'functional', different approach about side effects (-> avoid), lazy persistent datastructure at its core, different idea of OOP (-> avoid), Lisp infrastructure (-> avoid), ...
There is no software I know of, which is shared. The number of tools shared is 'one': GNU Emacs. It's a different community with different goals, different software, different applications, different libraries.
Just think of it: zero lines of code is shared.
My problem with that broader idea of 'Lisp': it is fully vague and it has no practical implications. Sometimes it's only a marketing slogan. It has the same vague meaning like 'object-oriented' or 'Functional'. There is no consensus what OOP in a broad sense means and how it should be defined. 'Lisp' is just as useless.
It does not mean that Clojure is useless or worse than Lisp. Totally not. Clojure has new ideas on data structures put to work and enabled some very productive people enjoying their tools. They are doing wonderful stuff.
> Programs have to be re-architectured, because the concepts are different:
"Re-architectured" can be read in a multitude of ways. Changing a few datatypes because Clojure prefers vectors instead of lists, etc., feels like it falls well below the bar for "re-architectured".
> no TCO, but 'functional'
This would hold weight if CL mandated TCO, but it doesn't.
With this in mind, I'd like to examine a few of your points from a different angle:
Racket:
- Basic concepts are the same, (atom, linked lists, '...')
- Restructuring not really needed (up for debate, depends entirely on your prefered initial design choices in either language. Racket isn't that opinionated.)
- Tail call optimization required as per Scheme standard, so is present. Note that this is not actually something Lisp mandates.
- Doesn't 'care' about side effects, community is pragmatic and will generally advise you to do whatever is practical
- Has an object oriented sub-language with message passing and so on
There are a lot of points here that, according to you, makes Racket essentially a Lisp, even your non-point about TCO. I'm curious to know what you feel about people saying Scheme is a Lisp, considering the above.
The whole debate of "Is X (a) Lisp?" reminds me of nationality debates that essentially boil down to some people saying blood is more important than culture. You seem to be taking both sides, however; arguing culture and blood (source code). You cherry pick the cultural differences like most people standing on one side of the nationality debate would and argue that just those specific differences are the most important.
That's the thing, though; Swedish people could argue however much they want that they're very different from Norwegian people. To the rest of the world, though, they're essentially the same. Especially when you start comparing them to people from Peru, Venezuela, South Africa, and so on. When you're in the bubble the very small differences are much bigger to you, but if you zoom out to get some perspective these differences are much smaller than the commonalities.
> Racket is Racket. It's its own language.
You've also been nothing but vague in terms of what makes Racket not a Lisp, which is ironic considering your previous argument that we should have more clearly defined requirements for what makes a Lisp, but I'm beginning to think that this was mostly hot air.
I think you've mostly proven that the 'vague' definition serves a much more practical purpose than your seemingly arbitrary distinction between these languages, and it will continue to do so, as it implies far more than you've displayed in this thread.
True, and I have to apologize for that. But I don't have the time, nor the priority to go into full detail. The main points are: different community, different goals, almost zero code sharing, different technical solutions, different literature, ... The differences are technical and social. Each of the points would need more explanation, for which I don't have the time.
For me 'Lisp' is something practical. I have a bunch of non-trivial code -> can I compile/load it? Can I port it easily? Are there people who would be interested to share? What are they using? Can I work with them?
Example:
Macsyma is an old Lisp program. New Lisp dialects appeared. Macsyma was ported to them: Maclisp, Franz Lisp, NIL (New Implementation of Lisp), Lisp Machine Lisp, VaxLisp, Common Lisp...
This software can't be ported to Clojure or Racket, without fully re-architecting the software, and I'm only thinking about the basic Macsyma, without GUI or other system dependent parts.
> that the 'vague' definition serves a much more practical purpose
which one? to confuse people? To raise expectations of collaboration in a community, which are then not fulfilled?
Example:
Lisp-2:
(defun new-map (f l)
(dolist (e l)
(funcall f e)))
(defun new-map (f l)
(dolist (e l)
(f e)))
Lisp-2
(defun foo (l)
(flet ((new-map (f l)
(dolist (e l)
(f e))))
(new-map (function sin) l)))
(defun foo (l)
(let ((new-map (lambda (f l)
(dolist (e l)
(f e)))))
(new-map sin l)))
Neither does Emacs Lisp nor ABCL. Yet they are Lisps by your definition.
> different approach about side effects (-> avoid)
Though not precluded. You can write Clojure with side-effects, but to do so you need to be explicit about it.
> different idea of OOP (-> avoid)
Which says nothing about the language or its Lisp-nature. Clojure has generic dispatch with multimethods, supporting runtime polymorphism. As you say later, "OOP" is meaningless since it's definition is vague, so using this as a reason for Clojure to not be a Lisp is odd.
> Just think of it: zero lines of code is shared.
Are saying that a lisp is only a 'Lisp' if you can freely share code between them? Without modification? If the names used for functions aren't the same, then that disqualifies it from being a Lisp?
Your position seems to be that unless the lisp is a direct descendant of Lisp 1.5 then it cannot be called a Lisp. In addition to Clojure, this disqualifies Scheme (and its dialects.)
Different Lisps (by your definition) take different approaches to things like namespace separation (Lisp-1 vs. Lisp-2) and scope (dynamic vs. lexical). These differences can be subtle and lead to hard to find bugs when sharing code.
> My problem with that broader idea of 'Lisp': it is fully vague and it has no practical implications.
Yet you have put a stake in the ground and defined the broad idea of 'Lisp' as an entity that shares its roots with the ideas in MacLisp.
But there are a lot of languages in the Lisp family that don't share the syntax (except from the basics, maybe), nor the standard libraries. See e.g. http://blog.fogus.me/2011/05/03/the-german-school-of-lisp-2/
In any discussion involving Common Lisp, there will be the opinion that "Clojure is not a Lisp" without (in my view) providing sufficient context for this clearly confusing statement.
(FWIW, I call Clojure a Lisp. It is clearly in the Lisp tradition, not to mention more technical things like: code-is-data, serious REPL, accurate numbers like 10.00000000000000001M. I don't think "Lisp" should be reserved for mainly incremental improvements on an old movement.)
Like Prolog or Julia.
In any case, there is no definition of even "chair". (Which partitions the world into chair and not-chair. Simply imagine a continuum between a particular chair morphing into a not-chair; where exactly does it stop being a chair?) So I'm unconcerned about edge-cases, where a language is actually in a fuzzy part of that continuum. Not to mention those amusing blogposts claiming "Javascript is a Lisp!" which maybe aren't objectively true but inspiring.