Re: Request for comments: use-cases for delayed evaluation
Steven D'Aprano writes: > Suppose Python had a mechanism to delay the evaluation of expressions > until needed. What would you use it for? Delayed evaluation is some form of "lazy evaluation": evaluate an expression (only) at the time, it is needed (maybe for the first time). The avocates of "lazy evaluation" bring forward that it gives you a convenient new modularization facility: you can separate the description for the construction of (potentially huge or even infinite) data structures from the algorithms operating on those structures. The "lazy evaluation" ensures that the structure is "unfolded" only so far as a concrete algorithm requires it. An example could be a game. Lazy evaluation would allow to describe once and for all how to compute the tree of game states reachable from a given game state (usually a huge, maybe even an infinite tree). Various algorithms for the selection of the next game move could all work on tree (and would be some form of tree search). With lazy evaluation, Python might become more attractive in areas where conceptually huge data structures can be involved: games, articial intelligence, proving systems, ... -- https://mail.python.org/mailman/listinfo/python-list
RE: Request for comments: use-cases for delayed evaluation
I could easily see using all of the examples; I run into this pretty regularly. What about something like the following (which, honestly is really a combination of other examples). If I have a function that has multiple parameters, each of which might be expensive, but it might break out earlier depending on how each one is evaluated... for example: (pretending that "$" is a flag that says "evaluate later", and $(arg) is how you say "evaluate now") def combine_unless_none(*$args, sep=', '): """ if any arg evaluates to None, return '', otherwise join""" for arg in args: tmp_arg = $(arg) If tmp_arg is None: return " return sep.join(args) -- https://mail.python.org/mailman/listinfo/python-list
