[Python-ideas] And now for something completely different
cliff at develix.com
Thu Sep 18 02:27:38 CEST 2008
On Wed, 2008-09-17 at 16:18 -0700, Bruce Leban wrote:
> On Wed, Sep 17, 2008 at 3:41 PM, Cliff Wells <cliff at develix.com>
> I'd prefer to write
> def foo ( lambda: arg ):
> return arg # evaluation happens here
> foo ( x )
> ("lambda" not necessarily being the actual name of the token
> in the
> second case).
> I'd rather use the time machine. You can already do this quite
> def foo (arg):
> return arg() # evaluation happens here
> foo(lambda: x)
Except I'd prefer not having lambda sprinkled throughout the calling
code (or worse, forgotten).
> The idea of something automagically changing from an unevaluated to an
> evaluated value strikes me as a very bad one so requiring explicit
> evaluation here is a *good* thing. Aside from that, why should arg be
> evaluated on return as opposed to being passed along unevaluated until
> its value is really needed?
Didn't suggest that. That was merely an example. What I'm suggesting
is that the *first* time it's used, evaluate then (my example used
return for simplicity).
> How would I check to see if an arg was provided? I certainly can't
> use a default value of None and then check for None since the check
> would cause it to be evaluated.
I don't consider that a problem. You either care about the value of an
argument or you don't. If you care then you will eventually evaluate it
anyway. If you don't, then don't evaluate it, ever.
> Maybe we have a special operator that allows us to look at the
> unevaluated value. What's the context of the evaluation? Can it access
> variables in the place where it's evaluated? Maybe there's a special
> way to do that too. Let's not go down that path.
Agreed we shouldn't go down that path, but don't agree that it's needed.
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