Values and objects

[Steven D'Aprano]

On Sun, 11 May 2014 11:26:41 +0300, Jussi Piitulainen wrote:

> Marko Rauhamaa writes:
>> Rustom Mody:
>> 
>> > On Saturday, May 10, 2014 2:39:31 PM UTC+5:30, Steven D'Aprano wrote:
>> >> 
>> >> Personally, I don't imagine that there ever could be a language
>> >> where variables were first class values *exactly* the same as ints,
>> >> strings, floats etc.
>> >
>> > [...]
>> >
>> > What you mean by *exactly* the same mean, I am not sure...
>> 
>> Lisp variables (symbols) are on an equal footing with other objects.
>> IOW, lisp variables are objects in the heap.
> 
> Only some, or only in quite old or special members of the family. But
> yes, I suppose when Lisp was still LISP, it was the kind of language
> that Steven fails to imagine in the quotation above. Variables really
> were symbols, which still are objects that can be passed around and
> stored in data structures. Or maybe not - wasn't the essential binding
> component (originally an "association list", later a more abstract
> "environment", called "namespace" in Python culture) separate from the
> symbol even then? Global bindings aside.

I'm not sure if you are agreeing or disagreeing that variables are values 
in Common Lisp or not. First you say they are, they you say "maybe not".

The point is, it is *logically impossible* for a language to use 
precisely the same syntax for value-assignment and variable-assignment. 
Consider the variable called "x", which is bound to the value 23. If the 
language has a single assignment operator or statement:

let y := name;

that cannot be used for *both* binding the value 23 to y and binding the 
variable "x" to y (making y an alias for x).

To use Pascal as an example, you cannot use the same declaration for pass-
by-value and pass-by-reference, one or the other must use different 
syntax.

function foo(x: integer, var y: integer): integer;

Given that declaration, and the function call foo(a, b), x is bound to 
the value of a, while y is bound to the variable b.

In the case of Common List, I count at least four different assignment 
forms: 

    set, setq, setf, psetq, let

plus two unbinding forms:

    makunbound, fmakunbound


but I don't know enough about Lisp to tell you which ones implement 
binding-to-values and which binding-to-variables, if any.



-- 
Steven D'Aprano
http://import-that.dreamwidth.org/