03.07.2015, 15:45, "Martin Simmons" martin@lispworks.com:
On Fri, 03 Jul 2015 12:15:32 +0300, Anton Vodonosov said:
Envelope-From: avodonosov@yandex.by
I want to correct myself. Unlike numbers or any other objects, symbols _are_ about names, so we can say that the name CL-USER::FOO represents the "nature" of symbol.
I think Common Lisp wants to save memory and speedup comparison, so when we use the same name we get the same object, as implemented by INTERN (this trick even has name - the Flyweight pattern).
So, this is just an optimization trick, and UNITERN is a maintenance, system tool, not designed to express programs. We are encouraged to operate as if the symbol name means the same object.
I disagree about it being to save memory -- a CL symbol is an object with mutable attributes, so identity is important.
This is part of the optimization. Functions like SYMBOL-VALUE, GET could be, for example, backed by hash maps from symbol name, thus returning the same value for equally named symbols.
I mean on the level of abstraction mathematicians use when they say "let X = 10" it means that the textual name X is bound to 10. In Common Lisp it means that the symbol object with name X is bound to 10.
So, in general, abstract sense, symbols need not to be EQ. But Common Lisp distinguishes symbols up to their object instance identity. I still suppose this choice is an optimization.
Also, the identity of uninterned symbols is just as important (e.g. for macros) as interned ones
I think if symbols were compared by their names instead of EQ, the were ways to satisfy needs of macros. But that would be another language, not CL.
Best regards, - Anton