On Thu, Dec 2, 2021 at 6:27 PM Brendan Barnwell <brenb...@brenbarn.net> wrote:
>
> On 2021-12-01 18:35, Chris Angelico wrote:
> > In my reference implementation, there is no object that stores it;
> > it's simply part of the function. A good parallel is the if/else
> > expression:
> >
> > x = float("inf") if z == 0 else y/z
> >
> > Is there an object that represents the if/else expression (or the
> > deferred "y/z" part)? No, although it could be implemented that way if
> > you chose:
>
> This is not a good parallel. There is nothing deferred there. The
> entire line is evaluated when it is encountered and you get a result and
> no part of the if/else expression can ever impact anything else again
> unless, via some external control flow, execution returns and
> re-executes the entire line. That is not comparable to a function
> default, which is STORED and evaluated later independently of the
> context in which it was originally written (i.e., the function default
> is re-executed but the function definition itself is not re-executed).
>
> The ternary expression vanishes without a trace by the next line,
> leaving only its evaluated result. There would be no use in being able
> to access some part of it, since the whole (i.e., the ternary
> expression) is completely finished by the time you would be able to
> access it. This is not the case with a function definition. The
> function definition leaves behind a function object, and that function
> object needs to "know" about the late-bound default as an independent
> entity so that it can be evaluated later. It is bad for the function to
> store that late-bound default only in some private format for its
> exclusive future use without providing any means for other code to
> access it as a first-class value.
>
That's exactly why it's such a close parallel. The late-evaluated
default is just code, nothing else. It's not "stored" in any way - it
is evaluated as part of the function beginning execution.
There is no "first class object" for a late-evaluated default any more
than there is one for the "y/z" part of the ternary.
ChrisA
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