Logs: liberachat/#haskell
| 2025-09-10 11:17:08 | <kqr> | Backing up further, I want to compare the performance of using the RandT transformer layered on top of IO to drawing numbers in IO directly. |
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| 2025-09-10 11:19:48 | <kqr> | Thus I'd like to create a `shuffle :: MonadRandom m => [a] -> m [a]` to be used both from within a RandT stack and directly from IO. However, with the given tools I can only create `shuffle :: RandomGen g => [a] -> Rand g [a]` which can only be used in IO by running it with evalRandIO which goes through StateT in the end. |
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| 2025-09-10 11:23:16 | <dminuoso> | kqr: Like I said, `MonadRandom IO` exists |
| 2025-09-10 11:23:47 | <dminuoso> | So you can use `shuffle` in IO just the same as you would with RandT. |
| 2025-09-10 11:25:45 | <kqr> | Well, I certainly am missing something dumb here. The function I import is `List.Shuffle.shuffle :: RandomGen g => [a] -> g -> ([a], g)`. How would I turn that into `shuffle' :: MonadRandom m => [a] -> m [a]`? |
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| 2025-09-10 11:27:24 | <dminuoso> | kqr: These are different interfaces. |
| 2025-09-10 11:28:41 | <kqr> | Yes, and I wished to convert one into the other, but I'm starting to sense that it might not be possible, and the solution would have be `shuffle' :: (RandomGen g, MonadSplit g m) => [a] -> m [a]` |
| 2025-09-10 11:28:59 | <dminuoso> | Well Im not entirely sure what you mean by converting RandomGen into MonadRandom. |
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| 2025-09-10 11:29:28 | <dminuoso> | RandomGen is for sequential pseudorandom number generators. |
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| 2025-09-10 11:32:23 | <kqr> | I feel like anything that is an instance of MonadRandom ought to be able to take a function of the shape `RandomGen g => g -> (a, g)` and produce an a, i.e. there ought to be a function in `MonadRandom m` that is `liftRandom :: RandomGen g => (g -> (a, g)) -> m a`. |
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| 2025-09-10 11:39:20 | <kqr> | But I think my misunderstanding is in believing instances of MonadRandom have to be able to expose their generator. Turns out they do not. And there's no typeclass for those instances that do, beyond MonadSplit which is even more restricted. |
| 2025-09-10 11:43:29 | <dminuoso> | kqr: They are kind of orthotonal interfaces. |
| 2025-09-10 11:46:46 | <dminuoso> | kqr: And more to the point, MonadRandom IO is not even implemented using RandomGen. |
| 2025-09-10 11:47:02 | <dminuoso> | It uses `randomIO` from the `random` package (I dont even know why) |
| 2025-09-10 11:47:25 | <dminuoso> | Which ultimately uses `https://hackage-content.haskell.org/package/random-1.3.1/docs/src/System.Random.Internal.html#theStdGen` |
| 2025-09-10 11:48:34 | <dminuoso> | And since theStdGen is exported in that module, you can - if you want - access that for purpose of testing. |
| 2025-09-10 11:49:25 | <dminuoso> | Oh wait, I missed a key detail. That is a plain RandomGEn |
| 2025-09-10 11:50:02 | <dminuoso> | kqr: So you could think of `MonadRandom` as a kind of `State StdGen` maybe. |
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| 2025-09-10 12:03:54 | <kqr> | That's my intuition too, but since MonadRandom does not expose any way to get the StdGen, a function like shuffle cannot be lifted into a MonadRandom operation. |
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| 2025-09-10 13:14:57 | <kqr> | Unrelated to the above: I have a list of things where I want to occasionally perform the query listWithout :: Eq a => [a] -> a -> Maybe [a] which is Nothing if the element does not exist in the list, or Just xs-wihtout-element if the element did exist. This is obviously a Set, except the list can and is allowed to contain duplicates (and in that case only one of the element should be removed). |
| 2025-09-10 13:15:00 | <kqr> | What would be the most efficient way to store this list? Asking because profiling indicates that "list-except-element" query is where my program spends nearly 20 % of its time. I have already optimised it to be tail-recursive and perform a single iteration through the list, but it's still slow. |
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| 2025-09-10 13:16:23 | <kqr> | I imagine a Map element Int would do it, where the Int records the number of instances of the element, but I'm also worried that might be a lot of overhead compared to a list. (In this case, the list is usually fairly short (< 10 items) and probably rarely contains more than three duplicates.) |
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| 2025-09-10 13:26:25 | <haskellbridge> | <sm> kqr I thought of a Map also. I think it's worth a try, often Map has been used in optimisations |
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| 2025-09-10 13:27:00 | <haskellbridge> | <sm> some kind of map |
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| 2025-09-10 13:31:50 | <kqr> | I'll give it a shot. Going to take some refactoring but I'd probably need to do that at some point anyway. |
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| 2025-09-10 14:42:55 | <merijn> | kqr: How static are these lists? |
| 2025-09-10 14:43:49 | <EvanR> | how large are these lists |
| 2025-09-10 14:44:12 | <EvanR> | 10 items at most, yeah Map representing the histogram will have overhead |
| 2025-09-10 14:44:26 | <merijn> | I was more thinking: If you |
| 2025-09-10 14:44:42 | <merijn> | If you're spending so much time on them, do you not wanna switch to Vector or something |
| 2025-09-10 14:44:58 | <merijn> | You're gonna waste a lot of time pointer changing with a list |
| 2025-09-10 14:45:04 | <merijn> | *chasing |
| 2025-09-10 14:45:37 | <EvanR> | what's the distribution of importance placed on all the tasks the data structure does, assuming it does something else but track the population |
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