macports_version

[Clemens Lang]

Hi,

----- On 22 May, 2018, at 09:04, Ryan Schmidt ryandesign at macports.org wrote:

>> The cleanest way of defining a global const variable that I’ve come across is
>> with trace, tying the variable to a write command with an explicit error
>> message.
>> 
>> But the Tcl wiki page that I linked to previously notes that "command names are
>> the natural choice for constants:
>> 	• they live in a separate namespace to variables;
>> 	• they are rarely redefined, and few commands do so;
>> 	• global commands are available everywhere without importing;
>> 	• they offer the possibility of byte-code optimization, i.e. inlining (no idea
>> 	if this is or could be done).”
> 
> Yes, I know that variables can be made read-only through a trace. MacPorts base
> has a feature built around traces called option_proc. So my question is why it
> wasn't done that way.
> 
> Clemens, do you remember?

Simple, declaring a variable gives you the option of changing it. Making it a proc
doesn't. It's about runtime vs. compile time failure if you're trying to do something
unsupported.

IMHO, using a proc for read-only variables is much cleaner than using a write trace,
which feels like a hack to me.

Why would you need it to be a variable? Could you not just call the proc instead?

-- 
Clemens Lang