| --- Log opened Sun Jun 09 00:00:39 2019 |
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| 15:45 | | * ToxicFrog screams at python |
| 15:46 | <&ToxicFrog> | a = []; for n in range(0,10): a += [lambda: n]; a[0]() |
| 15:46 | <&ToxicFrog> | What do you expect this to return? |
| 15:48 | <~Vornicus> | 10 |
| 15:48 | <&ToxicFrog> | WHY?! |
| 15:49 | <~Vornicus> | (but I'm well aware of Python's semantics here; they're quite similar to javascript's.) |
| 15:49 | <&ToxicFrog> | I mean, I know why, it's because all the lambdas close over the same "n" and retroactively get the same final value for it when called |
| 15:50 | <&ToxicFrog> | But as someone used to lisp and lua semantics this is both surprising and unpleasant, and leaves me rummaging around in functools to figure out a way to implement this basic functionality |
| 15:50 | <&ToxicFrog> | Probably using partial() |
| 15:51 | <&[R]> | Is that: a = []; for (var n = 0; n < 10; n++) { a.push(function() { return n }) }; a[0]() //? |
| 15:52 | <&[R]> | (JS equiv, I think) |
| 15:52 | <~Vornicus> | yes |
| 15:53 | <&ToxicFrog> | Yeah, and in Lua the n gets rebound on each loop iteration, so each function closes over a different n and a[0]() returns 0. |
| 15:53 | <&[R]> | You could just make a function that returns a function though to fix the issue, yes? |
| 15:54 | <&ToxicFrog> | (the actual problem I'm trying to solve is: for command in [long list of commands with trivial handler code]: locals()[handler_name(command] = lambda ...: (some stuff involving command)) |
| 15:54 | <&[R]> | function F(n) { return function() { return n } }; a = []; for (var n = 0; n < 10; n++) { a.push(F(n)) }; a[0]() |
| 15:54 | <&ToxicFrog> | [R]: yeah but at that point I've just reimplemented a hyperspecialized version of partial(), so I might as well just use partial(). |
| 15:55 | <&[R]> | Ah, I have no idea what partial() is |
| 15:57 | <&ToxicFrog> | partial(f, args...) returns a version of f with the args baked in |
| 15:58 | <&ToxicFrog> | E.g. def mul(x,y) return x*y; double = partial(mul, 2) |
| 15:58 | <&ToxicFrog> | double is now a function of one argument that's equivalent to calling mul(2, <argument to double>) |
| 15:58 | <&ToxicFrog> | (the general term for this is "partial application") |
| 16:01 | <&[R]> | Ah |
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| 19:40 | <&jeroud> | ToxicFrog: The loop variable belongs to the outer scope and is mutated for each iteration. The typical solution is to put "n = n" at the top of the loop body. |
| 19:48 | <&jeroud> | (This is also why "for n in range(10): ...; print(n)" is legal.) |
| 19:58 | <&ToxicFrog> | jeroud: Python doesn't use block scope, though, so I thought n=n was reassignment, not creating a new binding |
| 19:58 | <&ToxicFrog> | If it does create a new binding, that's convenient, but also surprisingly inconsistent with the rest of the language :/ |
| 19:59 | <&jeroud> | I think there are special scoping rules for closures. |
| 19:59 | <&ToxicFrog> | [screaming intensifies] |
| 20:00 | <&McMartin> | Note that in Lisp you get ten 10s as well, but you'd never structure the loop like that. |
| 20:00 | <&jeroud> | And now that I think about it, I suspect any variable defined in the loop body is available after the loop. |
| 20:00 | <~Vornicus> | oh, I should correct myself |
| 20:01 | <~Vornicus> | I said 10 up there |
| 20:01 | <~Vornicus> | you only get 9 |
| 20:03 | <&ToxicFrog> | McMartin: yeah, I lump lisp in with lua here because the idiomatic way of phrasing it both languages results in each closure closing over a different n. |
| 20:03 | <&ToxicFrog> | Even if the actual details of that are different |
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| --- Log closed Mon Jun 10 00:00:40 2019 |