I'm wrong or Will we fix the ducks limp?

BartC bc at freeuk.com
Tue Jun 7 06:49:13 EDT 2016


On 07/06/2016 08:56, Marko Rauhamaa wrote:
> Gregory Ewing <greg.ewing at canterbury.ac.nz>:
>
>> Marko Rauhamaa wrote:
>>> Seriously, though, it is notable that the high-level programming
>>> languages pretty unanimously refuse to make variables first-class
>>> objects. I wonder why.
>>
>> That's an interesting question. One reason might be
>> that in the absence of static type analysis, assigning
>> to a variable holding a reference to another variable
>> would be ambiguous. For example, suppose Python had
>> an & operator that gives you an object referring to
>> a variable somehow. Then, after
>>
>> a = 42
>> b = 17
>> c = &a
>> c = &b
>>
>> does 'c' now hold a reference to the variable 'b', or
>> does it still hold a reference to 'a' and 'a' now
>> holds a reference to 'b'?

c points to b. For the latter part of your statement to be true, the 
last line might have to be something like:

  *c = &b

>> Somehow these two operations would have to be spelled different ways,
>> which means you would need to know whether you were dealing with a
>> variable reference or not. So they wouldn't really be first-class, in
>> the sense of being treated on an equal footing with ordinary
>> variables.
>
> It's not that ambiguous.
>
>    >>> a = 3
>    >>> c = &a
>    >>> c
>    <global variable a>
>    >>> *c
>    3
>    >>> c is a
>    False
>    >>> *c is a
>    True
>    >>> c is &a
>    True
>    >>> a = 4
>    >>> *c
>    4
>    >>> *c is a
>    True
>    >>> c = &c
>    >>> c
>    <global variable c>
>    >>> *c
>    <global variable c>
>    >>> **c
>    <global variable c>


Here are some results in another, non-Python language. While the object 
reference module is similar to Python's, the language retains explicit 
pointers which can be used for variable references.

In this code, ^x means pointer to x, x^ means dereference c, while := is 
assignment and = means equality:

a := 3
c := ^a

println c               # refvar: 02176228

println c = a           # error (compare pointer to int)
println c = ^a          # True

a := 4
println c^              # 4

c^ := 12
println a               # 12

c := ^c
println c               # refvar: 02176248
println c^              # refvar: 02176248
println c^^             # refvar: 02176248

Associating a pointer value to a variable symbol table entry is a 
separate step. But a pointer needn't point at just a named variable:

a := (10,20,30,40,50)
c := ^a[3]

println c^              # 30 (was 1-based)

c^ := 77
println a               # (10,20,77,40,50)


And here is the above example:

a := 42
b := 17
c := ^a
c^ := ^b

println a               # refvar: 21A16F0
println a^              # 17

-- 
Bartc


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