Returning none

[Greg Ewing]

Bernhard Herzog wrote:
> 
> clgonsal at keeshah.penguinpowered.com (C.Laurence Gonsalves) writes:
> 
> > def new_apply( f, a ):
> >     try:
> >         result = old_apply( f, a )
> >     except NoResultError:
> >         return
> >     else:
> >         return result

It seems that solving this sort of problem properly would require
a special syntactic construct. Maybe an expression

  ?func_call

which would make the call and evaluate to whatever it returned,
including Nothing. This would allow you to make an either-procedure-
or-function call when you needed to, and test the result. E.g.

  def map(f, items):
    result_list = None
    for item in items:
      result = ?f(item)
      if result is not Nothing:
        if result_list is None:
          result_list = []
        result_list.append(result)
    if result_list is not None:
      return result_list
    else:
      return

Greg




> 
> There's one problem with this approach. If the NoResultError is raised
> whithin f, new_apply should reraise the exception and not just silently
> return nothing.
> 
> You could, of course, examine the traceback object to find out where the
> exception was raised, but that's very ugly and probably implementation
> dependent.
> 
> --
> Bernhard Herzog   | Sketch, a drawing program for Unix
> herzog at online.de  | http://www.online.de/home/sketch/