Re: [TCLCORE] Expr evolution

[Trevor D. (Twylite) <tw...@cr...>]

Hi,

On 2012/12/13 03:16 AM, Colin McCormack wrote:
> I've not been following this too closely, but I get the impression that
> proponents of this evolution are primarily interested in making exprs
> look better.
>
> You could always alias = to expr, and then write:
>
> [= {expression text}]
>
> Is that not pretty enough?
No ;)

The motivation (for me at least) is a combination of terseness and 
reducing the likelihood of incorrect application.

We (should) all know that "expr 1 + $a" is unsafe if $a can originate 
from outside the source code, yet unbraced use of [expr] is still 
common.  Using "alias {} = {} expr" doesn't fix this, nor even "proc = 
{exp} { expr $expr }" (you could still use [= 1+$a] which is unsafe).

To avoid double-substitution we have to introduce new syntax.  The big 
question is whether the syntax should be allowed to break existing 
code.  {=}{...} doesn't break existing code, but it's also not 
intuitive.  (...) is intuitive (it's already used to group 
sub-expressions in an expression) but will break some existing code.

Terseness and prettiness are closely related, and can be illustrated 
using inline computation of indices or coordinates:

   string range $buffer [expr {$pos-2}] [expr {$pos+10}]
   string range $buffer ($pos-2) ($pos+10)
   string range $buffer $($pos-2) $($pos+10)
   string range $buffer {=}{$pos-2} {=}{$pos+10}


On 2012/12/13 03:04 PM, Andreas Leitgeb wrote:
> Making exprs look better requires one step that's more controversial
> than aliasing "expr": namely getting rid of one "enclosement"-level:
> a typical expression use involves both, braces and brackets around the
> actual expression.  That's probably, why even {=}{...} is perceived as
> a win by some: while it's just 4 chars saved, two of them are the brackets.
I think the prettiness argument (saving brackets) is less relevant than 
the correctness argument.  {=}{...} doesn't permit accidental or 
subversive evaluation of a variable.

Regards,
Twylite