[ZPT] Re: TAL's lack of an else...

[Fergal Daly]

On Saturday 02 August 2003 10:51, Chris Withers wrote:
> Fergal Daly wrote:
> > Yep, but that's programming in XML for you. Anyway, the alternative if
> > even worse
>
> I don't agree ;-)

I presume you are not claiming that programming in XML is progress...

> > <tal:x tal:define="temp_cond_var user/isLoggedIn">
> >   <tal:x tal:condition="temp_cond_var">
> >     welcome
> >   </tal:x>
> >   <tal:x tal:condition="not:temp_cond_var">
> >     please login
> >   </tal:x>
> > </tal:x>
>
> This contains the same level of complexity as the tal:condition-group stuff
> you and Evan are proposing, but doesn't require knowledge of more syntax.

I am not propose anything with Evan. The first time I read the condition group 
stuff I didn't get it. Now I get it and I can see some uses for it but I 
think it's much too easy to make a mistake with it.

What I was proposing was a way to do if/elsif/else, no more no less. I am 
quite surprised to see people defending TAL's lack of else. Would you be 
happy with Python if instead of

if user.isLoggedIn:
  print "You are logged in"
elif user.hasCookie:
  print "Welcome back please log in"
else
  print "Please register"

you had to write

isLoggedIn = user.isLoggedIn
if isLoggedIn:
  print "You are logged in"
if not isLoggedIn:
  hasCookie = user.hasCookie
  if hasCookie:
    print "Welcome back please log in"
  if not hasCookie:
    print "Please register"

I doubt you would be happy with that. So why are people happy with it in TAL? 
It would be undestandable if it was an uncommon construct but it's not, it's 
very common and the whole purpose of a good language is to make the very 
common as easy as possible.

Some other comments:

> Stuff used in this context should _never_ have side effects!
> If it does, you deserve the complexity you get!

Whether due to side effects or expense, the most common practice seems to use 
a temporary variable so template authors must be worried about something.

> That said, Cache Managers are suc ha doddle to use in Zope, I think Jean's 
> suggestion is fine.

I'm not familiar with them but I would guess wrapping all of your objects in 
cache managers so that they can be used safely in your templates is not a 
good idea.

> And end up with Perl 7.XML? No thanks....

Perhaps we should deprecate use of named variables and tal:condition and start 
using registers, branch-if and gotos?

F