almost every language which has Lisp in its name is using some form of symbol tables for interning, from McCarthy's Lisp 1 implementation onwards. That's one of the defining features of the Lisp s-expression reader.
If the 'reader' reads an s-expression like (EMACS LISP IS A LISP DIALECT) then both occurrences of LISP are the same identical Lisp object, both are the same symbol.
If your language is doing something different, then it's not using symbols like Lisp-like languages usually do since the dawn of time.
I think the characterisation of Clojure is not that unfair here. It's Clojure keywords have the role that Lisp's symbols have (and I think they have better ergonomics), and symbols are mostly only used for source code representation.
In other Lisps the detailed semantics of symbols are more important including the identity/interning thing.
Rich Hickey was a Common Lisp user before making Clojure so there's a fair chance he knew how symbols worked there, so the cargo culting characterisation should be applied only light heartedly :)
That Clojure is keyword heavy is true, but it's so important to note that objects are essentially never checked for reference equality in Clojure, even when e.g. looking up keys in a hash map (see demo).
With this is mind, you could stop interning keywords and damn near every Clojure program would continue to work just fine - but with a noticeable slowdown.
Or, more sensibly and to bring it back to the theme of the thread, for adding a second non-interning Keyword type which can safely be generated while deserializing user input in a long running process, that you can use interchangeably with standard keywords, but will be garbage collected away with the reset of the deserialized data when you're done.
You do pay a hefty penalty here because you're hiding everything behind interfaces and abstractions. It's totally fine to not like the system, or believe it's not worth the performance hit.
But it does mean that a potentially equal but not identical symbol isn't some off brand low quality replacement as GP suggests, it's just... a symbol.
I wouldn't write a config file parsing library for C programs without interning, so == between two pointers could be used to test for keyword equality.
Interning is used outside of LIsp. See the XInternAtom function in the X Window system:
Atom XInternAtom(
Display *display,
char *atom_name,
Bool only_if_exists
);
or RegisterClass in Win32:
ATOM RegisterClassA(
const WNDCLASSA *lpWndClass
);
Though a typical use is in macros, where macros introduce new symbols and these should never clash with any existing symbol and to which there should be no access via the name.
Example: A macro which writes the form, the value and which returns the value. GENSYM generates a named/counted uninterned symbol.
> (defmacro debugit (form &aux (value-symbol (gensym "value")))
`(let ((,value-symbol ,form))
(format t "~%The value of ~a is ~a~%" ',form ,value-symbol)
,value-symbol))
DEBUGIT
If we look at the expanded code of an example, we can see uninterned symbols:
> (pprint (macroexpand-1 '(debugit (sin pi))))
(LET ((#:|value1093| (SIN PI)))
(FORMAT T "~%The value of ~a is ~a~%" '(SIN PI) #:|value1093|)
#:|value1093|)
We can also let the printer show us the identities of these symbols, labelling objects which are used multiple times in an s-expression:
> (setf *print-circle* t)
T
> (pprint (macroexpand-1 '(debugit (sin pi))))
(LET ((#2=#:|value1095| #1=(SIN PI)))
(FORMAT T "~%The value of ~a is ~a~%" '#1# #2#)
#2#)
Thus we can see above that it's just one uninterned symbol used in three places.
Example run:
> (debugit (sin pi))
The value of (SIN PI) is 1.2246063538223773D-16
1.2246063538223773D-16
Keep in mind that this is a guide from a Lisp using company (bought by Google) who wrote specifically two large applications partly, but significantly, in Lisp: a search engine for flight travel and an airline reservation system. Other application teams may have different rules&requirements, given that they may use Lisp in very different ways.
I'm sorry, my question was based on a four day old memory from the main thread that doesn't reflect what the guide says. It just says don't intern symbols at runtime. Presumably you can pass some kind of a flag to the reader to read lisp data structures without interning the symbols that it reads?
I know the story of ITA well, it and PG's writings are what got me interested in lisp in the first place. Which makes me feel old. But not comp.lang.lisp old, it's all relative!
It's interesting that despite this keywords are serialized all the time in Clojure land (eg in the transit format that is commonly used for frontend/backend communication).
I thought I was one of these gatekeepers; and that was before I found out that Clojure doesn't actually have symbols, but just a string type with a quote-free read syntax.
Even AutoCAD's AutoLisp (the old one from the 1980's) has interned symbols.
How symbols work goes back all the way to the original MacCarthy work, and all of its actual (not cargo-culted) descendants.
For what it's worth, I agree with you. Clojure only weakly holds on to its lisp heritage. I'd liken it to the similarity between C# and C++ (without implying any superiority to either side). Superficially quite similar, some parts near identical, others a complete detour.
With that in mind, however, basing your critique of Clojure on the extent to which it carries the lisp tradition is bizarre. Your criticisms are born of an ignorance of the value proposition Clojure provides, which would not be terribly different even if it had eschewed lisp syntax in favor of something else.
If you actually learned Clojure, there is zero chance you'd be complaining about symbol interning. It's just so ridiculous. You'd probably still think the whole thing is a waste of time, and I'm sure you'd have a big long list of actual, meaningful complaints.
I've seen people criticize TXR for its ugly syntax once or twice here on HN (I pay close attention to lisp posts here), and I thought that was dumb at the time. I'm not interested in learning it but I'm glad you're trying something new. It's a shame to see you stoop to the same level of drive by dismissal.
But whatever. Let's flip each other's bozo bits and move on.
Speaking of TXR, and of "holding on weakly", as a result of this discussion, I made a little change.
A remark was made somewhere that interned symbols are held with a non-weak reference. But it occurred to me that this isn't something engraved in stone. A package should be able to hold on to its interned symbols via weak references. This means that if the only reference to a symbol is from within a weak package, that symbol can be removed from the package and relclaimed by the garbage collector.
Since a package uses hash tables, and hash tables support weak keys, it's trivial to put the two together. I added an argument to make-package to specify a weak package.
In the following test, the symbols interned into package foo get reclaimed because it is weak. Those interned into bar don't get reclaimed:
(defun weak-package-test (name weak)
(let ((wp (make-package name weak)))
(let ((*package* wp))
(let ((obj (read "(a b c d e f)")))
(mapcar (op finalize @1 prinl) obj)))))
(weak-package-test "foo" t)
(sys:gc t)
(weak-package-test "bar" nil)
(sys:gc t)
$ ./txr weak-package.tl
foo:a
foo:b
foo:c
foo:d
foo:e
foo:f
You call Clojure a cargo cult descendant, which implies it's failing to achieve what its ancestors did, by copying features haphazardly without understanding the deeper motivations behind them.
Clojure is a shitty substitute for CL, but that's just not what it is trying to be. It is an interesting and worthy system in its own right.
