[Python-Dev] lazy evaluation redux (was dict "setdefault". Feaure request or bugfix?)

[Roman Suzi]

On Tue, 11 Feb 2003, Alex Martelli wrote:

> E.g., a strawman syntax might be...:
> 
> def setdefault(adict, akey, ?avalue):
>     if akey not in adict:
>         adict[akey] = evaluate_now(avalue)
>     return adict[akey]
> 
> to be called as, e.g.
> 
> setdefault(mydict, 'goo', ?makeavalue(x))
> 
> this would use ? for both formal and actual arguments to
> mean lazy evaluation, and a new builtin to force the time
> of evaluation.  Other choices are, of course, possible.

This could be good for the properties as well.
However I think omitted lambda is better and nicer:

def setdefault(adict, akey, avalue):
     if akey not in adict:
         adict[akey] = avalue(args...)
     return adict[akey]
>
> to be called as, e.g.
>

setdefault(mydict, 'goo', :makeavalue(x))

equal to 

setdefault(mydict, 'goo', lambda:makeavalue(x))


Alex, are we up to make a PEP for this?
(it's pretty simple change to grammar: no need to change anything else,
probably we can convince Guido to accept it due to simplicity.)

Horizonts for 0-arity lambdas are numerous. 

The idea is that "lambda" keyword is optional in some circumstances, like:

- as an argument in function call
- right after "=" in assignment
- in default value specification (in def statement)
(some other cases)


inline if then will be something like:

def iif(cond, true, false):
  if cond: return true()
  else: return false()

iif(cond, :1, :2)

This will clearly indicate to programmer that values are "quoted" (like 
in LISP) - not evaluated right now.

Sincerely yours, Roman A.Suzi
-- 
 - Petrozavodsk - Karelia - Russia - mailto:rnd@onego.ru -