Re: [TCLCORE] Empty list is not a list -- using twoPtrValue forNULL type

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Neil Madden wrote:
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 On 17 Jan 2008, at 23:53, Alexandre Ferrieux wrote: 
&gt; 
&gt; OK, sorry for the delay :-) Here it goes. 
&gt; Sorry in advance for the obvious introduction, but let's start from 
&gt; the basics to be sure. 
&gt; 
&gt; Mutability - principle 
&gt; ----------------------------- 
&gt; 
&gt; Values in Tcl, be they "scalars" like integers or strings, or 
&gt; "containers" like lists or dicts, are so far strictly independent
from 
&gt; one another.&nbsp; Hence, in 
&gt; 
&gt;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; # assuming no trick like upvar #0 x y 
&gt;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; set x [function call returning some value] 
&gt;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; set y $x 
&gt;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; do anything to y 
&gt; 
&gt; the $x value will *never* be touched by whatever is done to the 
&gt; variable y, including "in-place" operations like [lappend], [lset], 
&gt; etc. In other words, as seen from the script level, values can
just be 
&gt; copied, never passed "by reference". 
 
Nit picking, but values, strictly speaking, are always immutable.&nbsp; 
It's variables that are passed by reference in most languages, or&nbsp; 
some kind of first-class reference cell or "object". This is an&nbsp; 
important point, because presumably it doesn't make sense to make the&nbsp; 
Tcl_Obj representation of "2" mutable. I mean, if "2" gets changed to&nbsp; 
"3", what has happened? What *is* the explanation of: 
 
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One of my first really nasty programming experiences was when I passed
the 
value "1" into a FORTRAN call that treated it as a variable. When the
variable 
got set to "0" all the "if" statements started behaving very strangely.
FORTRAN 
(at least) allows values to be mutable. 
 
Tom K. 
 
... snip ... 

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Re: [TCLCORE] Empty list is not a list -- using twoPtrValue forNULL type

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Re: [TCLCORE] Empty list is not a list -- using twoPtrValue forNULL type