At 09:37 AM 6/22/2006 -0700, Guido van Rossum wrote: >On 6/22/06, Phillip J. Eby <[EMAIL PROTECTED]> wrote: > > This hypothetical "const" would be a *statement*, > > executed like any other statement. It binds a name to a value -- and > > produces an error if the value changes. The compiler doesn't need to know > > what it evaluates to at runtime; that's what LOAD_NAME or LOAD_DEREF are > > for. ;) > >Please think this through more. How do you implement the "produces an >error if the value changes" part? Is the const property you're >thinking of part of the name or of the object it refers to? > >The only way I can see it work is if const-ness is a compile-time >property of names, just like global. But that requires too much >repetition when a constant is imported.
Right; MAL pointed that out in the message I was replying to, and I conceded his point. Of course, if you consider constness to be an implicit property of imported names that aren't rebound, the repetition problem goes away. And if you then require all "case" expressions to be either literals or constant names, we can also duck the "when does the expression get evaluated?" question. The obvious answer is that it's evaluated wherever you bound the name, and the compiler can either optimize the switch statement (or not), depending on where the assignment took place. A switch that's in a loop or a function call can only be optimized if all its constants are declared outside the loop or function body; otherwise it degrades to an if/elif chain. There's actually an in-between possibility, too: you could generate if's for constants declared in the loop or function body, and use a dictionary for any literals or constants declared outside the loop or function body. The only problem that raises is the possibility of an "inner constant" being equal to an "outer constant", creating an ambiguity. But we could just say that nearer constants take precedence over later ones, or force you to introduce the cases such that inner constants appear first. (This approach doesn't really need an explicit "const foo=bar" declaration, though; it just restricts cases to using names that are bound only once in the code of the scope they're obtained from.) _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
