On Monday, March 24, 2014 8:57:32 AM UTC+5:30, Chris Angelico wrote: > On Mon, Mar 24, 2014 at 1:35 PM, Rhodri James wrote: > >> Would you not consider this to be declarative? > >> x = [1, 2, 3] > > I'm not sure I would. I look at that line of code and think of it as > > "Create a list...", very much in an imperative manner. Then again, compared > > with C structs and typedefs and actual honest-to-God type declarations, > > there's precious little in Python I would consider truly declarative.
> I'm in the declarative group here. Yes, it has to be implemented as > creating a list and adding three elements to it, but conceptually, it > means "Bind x to a new list with these elements". And as long as > that's the end result, I don't care how it's done; the interpreter's > most welcome to have a "template list" that it copies, or maybe a > literal tuple that gets passed to the list() constructor, or > whatever's most efficient. > It gets a bit messier when there's stuff with side effects, though. > This has to be a bit more imperative: > x = [foo(y), bar(y), quux(y)] > That means "Call foo, bar, and quux, in that order, each with y as an > argument, and bind x to a new list with their return values". And if > Python had a simple notation for calling a series of functions, that > could be written something like this: > funcs = [foo, bar, quux] > x = funcs(y) > which is looking more declarative again. It's now "Take this list of > functions and call them all, and bind x to a list of their return > values". (This could be done, with a callable subclass of list. Call > it a sort of "implicit map" if you like.) Python doesn't have that > syntax, but it does have this: > x = [f(y) for f in funcs] > and I'd say that's reasonably declarative; it should be read like the > previous one: "Take this list of funcs, call each one, and bind x to a > list of their return values". And I could imagine a > parallel-processing version of a list comp that functions like > multiprocessing.Pool.map() and doesn't promise order, which would > *definitely* be declarative. You have described nicely a slippery slope! The list [(1, 1), (1, 2), (1, 3), (1, 4), (2, 1), (2, 2), (2, 3), (2, 4), (3, 1), (3, 2), (3, 3), (3, 4)] looks neater written (and *thought of* ) as [(x,y) for x in range(1,4) for y in range(1,5)] Neat! So I play around... Change it to [(x,y) for x in range(1,10000) for y in range(1,10000)] and I dont have an answer but a thrashing machine!! (*) IOW everything we write/read as programmers has a declarative and an imperative side. Which one one wants to focus on requires good sense and taste. (*) Example also shows inter alia how some things -- range -- have gone from imperative to declarative from python 2 to 3. Some people have made languages whose main focus is generalizing this idea to more dimensions and restrictions: http://www.irisa.fr/cosi/Rajopadhye/dag-talk.ps -- https://mail.python.org/mailman/listinfo/python-list
