Home liberachat/#haskell: Logs Calendar

Logs on 2026-02-24 (liberachat/#haskell)

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03:25:06 <Leary> Upper bounds aside, `cabal check` doesn't seem to care about lower bounds. Does that mean I can give up on the archaeology necessary to find them?
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03:34:26 <monochrom> I usually do. I set my lower bounds to be what I happen to have when I start the project. Then I don't change them until someone wants it changed.
03:39:28 <Leary> Well, in that case you're setting them conservatively. I was more wondering about the opposite strategy of omitting them entirely.
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03:45:11 <monochrom> Then one day you will run into something like this recent example: https://discourse.haskell.org/t/breaking-changes-in-time-1-15-ghc-9-14/13711
03:45:53 <monochrom> (time-1.15 adds new pattern synonyms that name-clashes with someone's existing code that assumes <=1.14)
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03:47:26 <monochrom> So if you set a lower bound like <=1.14 or 1.13, the worst that can happen is cabal downloads and builds an appropriate version of time. If you don't set a lower bound, now you have to set a lower bound.
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04:08:53 <Leary> In that particular case, I think the actual issue is importing without concern for namespace pollution. Also, that's an upper bound?
04:09:15 <Leary> Reflecting things a bit, your point seems to be that an older version of a dependency (than is implicitly assumed) may be used to build the package, causing it to fail.
04:09:40 <Leary> But the alternative of refusing to build what would have succeeded doesn't really seem better; either way the bound is wrong and you're relying on intrepid explorers to eventually correct it.
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04:18:18 <monochrom> Oh oops yeah that's an upper bound.
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04:32:27 <geekosaur> ithis is a long-running argument, and there are no real solutions because you can't reliably infer upper or lower bounds; you or someone else must experiment or actively test
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04:32:49 <geekosaur> and leaving either off forces others to do it for you by their tripping over it
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07:08:15 <gentauro> adding this to a file works -> `{-# OPTIONS_GHC -Wall -Werror -fpackage-trust #-}`. However, this does not: `{-# OPTIONS_GHC -Wall -Werror -fpackage-trust -trust=base #-}`.
07:08:39 gentauro error msg: "Unknown flag in {-# OPTIONS_GHC #-} pragma: -trust=base".
07:09:08 <gentauro> anybody know how to pass trusted packages in the `OPTIONS_GHC` pragma?
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07:11:36 <gentauro> there seem to be `GHC` cli flags for that -> https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/safe_haskell.html#package-trust
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07:11:53 <gentauro> however, they don't work with `runghc` :-\
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07:20:09 <gentauro> Is there built-in logic in Haskell such as in F# `Enum.GetValues(typeof<tags>)` where you can get a sequence of the values for an (enum) datatype? For example for `data FooBarEnum = Foo | Bar deriving (Eq, Show)`.
07:20:29 gentauro which would result in [ Foo, Bar ]
07:21:44 <Leary> @let data FooBarEnum = Foo | Bar deriving (Eq, Show, Enum)
07:21:46 <lambdabot> Defined.
07:21:56 <Leary> > [minBound .. maxBound] :: [FooBarEnum]
07:21:57 <lambdabot> No instance for ‘Bounded FooBarEnum’
07:21:57 <lambdabot> arising from a use of ‘minBound’
07:21:58 <lambdabot> In the expression: minBound
07:22:24 <Leary> Ah, Forgot to derive `Bounded`. Well, you get the idea.
07:23:04 <Leary> > [minBound .. maxBound] :: [FooBarEnum]
07:23:05 <lambdabot> [Foo,Bar]
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07:27:01 <gentauro> Leary: now that was nifty <3
07:27:32 <gentauro> many thx
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09:31:27 <dminuoso> % :t minBound
09:31:27 <yahb2> minBound :: Bounded a => a
09:31:58 <dminuoso> Curious, why is Ord not a superclass of Bounded?
09:32:19 <dminuoso> What does `minimum/maximum` even mean in the absence of an ordering?
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09:33:40 <tomsmeding> perhaps it makes sense that an Enum has a first and a last value for enumeration, but while it has an ordering compatible with that Enum instance, that ordering makes no semantical sense and is hence not defined?
09:33:53 <tomsmeding> but then you could argue that you should give that Ord instance anyway
09:34:47 <humasect> it says right in the first sentence https://hackage-content.haskell.org/package/base-4.22.0.0/docs/Data-Bounded.html
09:35:05 <tomsmeding> lol, dminuoso ^
09:35:24 tomsmeding should have thought of that
09:35:32 <dminuoso> humasect: Yes, and its not much of an explanation.
09:35:43 <dminuoso> It reads nonsensical to me.
09:35:48 <humasect> perhaps look into its meaning, disassemble it to understanding
09:35:50 <tomsmeding> dminuoso: not every ordering is total
09:35:54 <dminuoso> Oh.
09:35:54 <tomsmeding> partial orders are a thing
09:35:55 <humasect> how to learn ?
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09:36:22 <dminuoso> tomsmeding: Is there a notion of `minimum` in a partially ordered set, then?
09:36:30 <dminuoso> Im not even sure what that would mean.
09:36:32 <__monty__> tomsmeding: But Bounded doesn't deal with just any partial order.
09:36:43 <tomsmeding> dminuoso: yes, a lower bound :) https://en.wikipedia.org/wiki/Lattice_(order)
09:36:44 <__monty__> It's restricted to partial orders that have a single lower and upper bound.
09:37:15 <tomsmeding> sure, but even those need not have a total ordering
09:37:27 <tomsmeding> perhaps "lower bound" is not the technically correct term here
09:37:38 <tomsmeding> (but then, it's probably "minimum", which would not have helped dminuoso)
09:38:17 <tomsmeding> dminuoso: all subset of a given set S are partially ordered by inclusion, and they have a unique minimum ({}) and maximum (S), but they are not totally ordered
09:38:47 <dminuoso> tomsmeding: Okay so what you're saying is that Bounded is just synonmous for Lattice?
09:38:55 <tomsmeding> perhaps?
09:39:16 <tomsmeding> well, a bounded lattice
09:39:46 <dminuoso> tomsmeding: Aren't all lattices bounded by their suprema?
09:40:04 <dminuoso> Ohh wait.
09:40:06 <dminuoso> I see.
09:40:07 <tomsmeding> according to the wikipedia definition that I'm currently reading, a plain "lattice" requires joins and meets of _two_ elements
09:40:10 <tomsmeding> and thus finite sets
09:40:27 <tomsmeding> a _complete lattice_ additionally requires suprema and infima of larger sets
09:40:38 <tomsmeding> thus all complete lattices are bounded, but not all bounded lattices are complete? Perhaps?
09:40:55 dminuoso wonders why lattices are called lattices
09:41:19 <tomsmeding> because of this picture I always assumed https://commons.wikimedia.org/wiki/File:Pow3nonlattice.svg
09:41:39 <jreicher> dminuoso: hasse diagrams
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09:42:06 <jreicher> https://en.wikipedia.org/wiki/Hasse_diagram
09:42:21 <tomsmeding> which that picture is an example of
09:42:40 <Leary> It's a bounded partial order with minimal and maximal elements---that doesn't imply unique suprema or infima.
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09:43:09 <tomsmeding> fair! Even pairs need not have unique joins or meets
09:43:48 <dminuoso> Leary: Are you saying that Bounded does not have uniqueness requirements?
09:44:16 <tomsmeding> dminuoso: even if there is a maximal element of the entire set, that does not necessarily mean there is a _unique_ smallest upper bound of any subset of the set
09:44:31 <__monty__> dminuoso: I think they're talking about lattices.
09:44:33 <tomsmeding> "lattice" implies that such upper bounds (and lower bounds) are unique
09:44:57 <tomsmeding> so Bounded is not a "bounded lattice", it's a "partial order with minimal and maximal elements"
09:45:07 <tomsmeding> which... we knew 10 minutes ago already
09:45:08 <dminuoso> tomsmeding: Trying to understand what "maximal element of the entire set" would mean, if it was not a unique smallest supremum of any subset of the set.
09:45:20 <tomsmeding> welcome to math where there are unintuitive counterexamples
09:45:27 <dminuoso> Oh hold on.
09:45:35 <dminuoso> The favourite person in a group could work I suppose.
09:46:10 <jreicher> tomsmeding: clopen is my favourite
09:46:44 <dminuoso> Hold on what I just said makes no sense.
09:46:52 <jreicher> dminuoso: are you wondering what the difference between lattice and having max and min is?
09:47:06 <dminuoso> The uniqueness is not of any subset, but a *pair* of elements.
09:47:11 <jreicher> Yes
09:47:18 <jreicher> (But I'm still not sure what the initial question was)
09:47:24 <tomsmeding> (which are, in particular, subsets)
09:47:29 <tomsmeding> jreicher: I think yes
09:47:46 <tomsmeding> do you have an example of a bounded partial order that is not a lattice?
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09:47:53 <tomsmeding> (I don't)
09:48:02 <Leary> dminuoso: Maximal means nothing else is bigger than it, though perhaps the /greatest/ element is a better notion for a bound, in which case its bigger than everything.
09:48:22 <int-e> I'm confused; Bounded just defined two points called minBound and maxBound? So it would work for partial orders too (and thus, lattices)
09:48:44 <tomsmeding> int-e: the original original question was why Bounded does not have Ord as a superclass, which had answer "partial orders are a thing"
09:48:45 <Leary> tomsmeding: You can just contrive a small finite partial order on paper.
09:48:58 <Leary> It's not hard to make a counterexample.
09:49:03 <int-e> tomsmeding: Ah. I see, lack of context strikes again.
09:49:08 <tomsmeding> int-e: then we thought "but is Bounded then a bounded lattice?", to which the answer is no, as no unique joins/meets are required
09:49:16 <tomsmeding> now we want an example of that :p
09:49:54 <jreicher> tomsmeding: by "bounded" do we just mean the entire set (rather than any pair) as an infimum and supremum?
09:49:59 <tomsmeding> yes
09:50:02 <tomsmeding> or at least, I do
09:50:11 <__monty__> dminuoso: If you're looking for any example of it whatsoever I think the one tomsmeding mentioned is the easiest. Take any set with more than one element and all subsets of that set, the partial order relation is subset-of, the minBound would be the empty set, the maxBound the set you started with, some pairs of subsets won't satisfy the subset-of relation in either order. So this can implement Bounded
09:50:17 <__monty__> but not Ord.
09:50:30 <jreicher> Then just "duplicate" a meet or join. It will no longer be a lattice.
09:50:48 <tomsmeding> __monty__: yes, we figured that out, but we're already at the next question: that thing is clearly a lattice, but we want a not-lattice that is nevertheless a bounded partial order
09:51:04 <int-e> Is it okay if it's? 0 < 1,2 < 3,4 < 5 has minimal and maximal elements but 3,4 have no infimum and 1,2 have no supremum
09:51:29 <tomsmeding> but there's no unique maximum
09:51:30 <__monty__> tomsmeding: The conversation has moved on but I'm not sure it's not leaving dminuoso behind ; )
09:51:42 <tomsmeding> ah :p
09:51:43 <int-e> tomsmeding: 5 is the unique maximal element here
09:51:47 <Leary> tomsmeding: https://gist.github.com/LSLeary/c52be6739f5815d13a6774d46dcbde75
09:51:53 <tomsmeding> int-e: no, because 2 !< 5
09:52:00 <int-e> tomsmeding: it's transitive
09:52:07 <tomsmeding> oh notation
09:52:14 <tomsmeding> yes
09:52:27 <tomsmeding> yes perfect
09:52:36 <jreicher> I think int-e and I are thinking the same. 0 is one end and 5 is the other. The join of 1 and 2 is not unique
09:52:37 tomsmeding closes the image editor
09:52:41 <int-e> tomsmeding: sorry, I'm trying to cast a Hesse diagram in ASCII here
09:52:51 <int-e> the translation is a bit lossy ;)
09:52:55 <jreicher> int-e: have I understood you correctly?
09:53:18 <tomsmeding> Leary: that notation in your gist is a bit ambiguous, what do the /\ on line 4 mean :p
09:53:19 <int-e> Hasse diagram, meh.
09:54:13 <dminuoso> Leary: Did you mean it like https://paste.tomsmeding.com/ayvscggm ?
09:54:34 <__monty__> I'm also interested in kind of the reverse question, are there structures that can implement Ord but not Bounded? Something like a cyclical ordering? I suppose only if we put the "laws" of the class aside.
09:54:43 <dminuoso> Or well, maybe without that x even
09:54:51 <merijn> __monty__: Clearly we need a PartialOrd superclass of Ord that implement "a -> a -> Maybe Ordering"
09:54:53 <dminuoso> Not sure if it makes it less or more ambiguous
09:54:55 <tomsmeding> dminuoso: the x must be a crossing of lines without a node, yes
09:55:11 <tomsmeding> __monty__: Integer?
09:56:14 <jreicher> __monty__: if the order is transitive and antisymmetric it can't be cyclical
09:56:24 <Leary> tomsmeding: Cross connections. Bit hard in slapdash asci. I revised it with an X if that's clearer.
09:56:52 <tomsmeding> yes, which is also what dminuoso made of it
09:57:21 <__monty__> tomsmeding: Oh, obviously, darn, less interesting than I thought.
09:59:28 <int-e> tomsmeding: here is what it looks like properly: https://int-e.eu/~bf3/tmp/hapo.png
10:01:03 <__monty__> TIL, min and max can return a value that is distinct from their arguments, as long as it is == to one of their arguments.
10:01:25 <__monty__> I assume that's for some sort of canonicalization?
10:01:58 <tomsmeding> int-e: yes
10:02:13 <Leary> __monty__: `==` values aren't supposed to be distinguishable in the first place.
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10:08:16 <__monty__> Leary: But things like Arg exist and they exist because they are useful, I assume.
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10:11:25 <Leary> It's occasionally pragmatic to break the law, yes. `Arg` actually wouldn't be useful if people always remembered to write `fooBy` instead of just `foo :: Ord a => ...`.
10:11:52 <Leary> At least, not useful enough to justify its existence.
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10:17:10 <int-e> `fooBy` is awful when you have data structures with invariants (consider an implementation of containers that implements Data.Map using Data.Set with `Arg k v` elements)
10:18:20 <int-e> (So it's not just programmer laziness.)
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10:20:56 int-e ponders extending this argument to saying that type classes are useles; people could just pass around the right operations explicitly. :-P
10:21:57 <Leary> Even in such a case, `Arg` should be handrolled in the guts of containers, not exposed from base.
10:23:43 <Leary> Re the extended argument, it's really about typeclasses with laws being used to implement operations that /don't require/ those laws and are useful in their absence; the typeclass method merely being a default choice.
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11:54:53 <gentauro> tomsmeding: have you road the functional pearl from Alejandro Russo on how to use `lattices` to secure apps?
11:54:58 <gentauro> it's a pretty good one !!!
11:55:50 gentauro I actually live coded an implementation for a Spanish audience (HackMadrid / HaskellMAD). Nobody understood anything xD
11:58:10 gentauro for Spanish speaking peeps -> https://www.youtube.com/watch?v=iU5dfVRREUo&t=7806s (02:10:06 - 03:11:59)
12:02:13 gentauro merijn and tomsmeding perhaps that talk will provide "some" endorsment for Hackage? xD
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14:04:58 <tomsmeding> gentauro: I'm pretty sure I have not read that pearl
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15:17:08 <gentauro> tomsmeding: let me see if I can find it (Chalmers moved their stuff when they made the new website)
15:18:18 <gentauro> tomsmeding: there you go -> https://www.cse.chalmers.se/~russo/publications_files/pearl-russo.pdf
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15:20:38 <tomsmeding> thanks, will have a look :)
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15:22:05 <gentauro> tomsmeding: there is also the ICFP 2015 Vancouver: https://www.youtube.com/watch?v=K6Uf61U0fvo (but it's recorded with a toaster …) :-\
15:22:42 gentauro less than 19 min, if you ears can handle the toaster A/V
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17:04:12 <Milan_Vanca> Hello guyz.. Stupid quesiton incoming! We can nicely nest "case of" fn a b c = case a of.. case b of... Lovely but sadly I can't pattern match and need guards. Which I don't know how to nest. I don't want to duplicate e1 && e2 && e3 when only e4 changes.
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17:09:39 <ncf> guards don't do anything that let and case can't
17:10:10 <ncf> if you really want a RHS equivalent of guards there is MultiWayIf https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/multiway_if.html
17:18:56 <Milan_Vanca> So is there other way how to rewrite this more nicely? https://paste.tomsmeding.com/NokCDmVI
17:20:48 <Milan_Vanca> Imagine I want to have another otherwise with under counter path.
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17:21:07 <Milan_Vanca> I probably need whole new function for it
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17:32:27 <Milan_Vanca> I think I can do this https://paste.tomsmeding.com/FfemQcpX but this case of is ugly?
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17:36:14 <ncf> i don't see a reason not to use an if statement there
17:36:23 <ncf> well, an if expression
17:37:29 <Milan_Vanca> What about now? https://paste.tomsmeding.com/ptnApMeq
17:38:28 <Milan_Vanca> Like I can do it with if but it does not look like nicer solution.
17:39:13 <Milan_Vanca> If I had only 2 then I would use an if expression which is perfect solution for that case
17:40:27 <ncf> i mean that's fine, if you have a lot of those you might want to define a list or a map like [(methodGet, counterGet), (methodPut, counterPut), ...] and then do a lookup into that
17:42:16 <Milan_Vanca> Hmm map with lookup sounds interesting too.
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18:08:45 <haskellbridge> <loonycyborg> Milan_Vanca: not sure if it applies to your situation but you can do deep nesting in the pattern too, so if you just want to look deeper in a nested structure you can fold nested "case" into a single case.
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18:15:23 <Milan_Vanca> loonycyborg: Do you mean something like (v1, (Just v2), _) ?
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18:15:45 <haskellbridge> <loonycyborg> yep
18:18:04 <Milan_Vanca> Yeah :) This is nice.. and If I wanted to modify v2 I could use lenses for that. Is that correct?
18:18:41 <EvanR> that is one way
18:20:46 <EvanR> you can also write an update function f g (v1, Just x, v3) = (v1, Just (g x), v3)
18:21:39 <Milan_Vanca> Little more verbose but one does not need another lib.
18:21:40 <EvanR> for a one off case, this takes a lot less support
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21:04:18 <lantti> is there a faster way to print a list of Int, separated by spaces, than "putStrLn (unwords $ map show ints)"?
21:05:14 <Rembane> lantti: Faster as in time from first character to last character?
21:05:27 <Rembane> ...minimizing that time that is
21:05:31 <lantti> yes
21:06:06 <Rembane> Converting it to a chunked bytestring could be faster, but I haven't compared.
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21:06:39 <lantti> I'm doing an algorithms course where I have to choice to do it in C, Rust or Haskell and I chose Haskell, but in my current task just printing the result takes me to the limit on acceptable time use
21:06:49 <newmind> depending on the length of ints, maybe with a builder, but i doubt for any reasonable length outputs there is that much difference to be had here
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21:07:38 <dolio> You could try `mapM_` with something. But I'd be surprised if it were a lot faster.
21:07:44 <newmind> so, since haskell is a lazy language... how are you calculating ints?
21:07:45 <lantti> although on a positive note, I did pass already. I'm just trying to improve my result from barely passing to something better
21:07:54 <Rembane> lantti: How many ints do you print?
21:08:08 <lantti> 10^6
21:08:20 <mauke> :t shows
21:08:22 <lambdabot> Show a => a -> ShowS
21:08:52 <EvanR> :t mapM_ print
21:08:53 <lambdabot> (Foldable t, Show a) => t a -> IO ()
21:08:59 <mauke> that's not space separated
21:09:18 <dolio> Yeah, you need the space analogue of print.
21:09:28 <tomsmeding> lantti: are you allowed to use dependencies, like 'text' or 'bytestring'?
21:09:59 <dolio> Is there a show-to-text/bytestring that doesn't run through show?
21:10:17 <dolio> Other than the one I stopped maintaining like 10 years ago?
21:10:48 <tomsmeding> well, for a start I'd be curious how `T.putStrLn (T.intercalate " " (map T.show ints))` performs, despite still using Prelude.show
21:11:00 <mauke> :t let f [] = "\n"; f (x : xs) = shows x (' ' : f xs) in f
21:11:03 <lambdabot> Show a => [a] -> [Char]
21:11:13 <tomsmeding> dolio: https://hackage-content.haskell.org/package/text-2.1.4/docs/Data-Text-Lazy-Builder-Int.html ?
21:11:34 <dolio> Oh, nice.
21:11:48 <mauke> > let f [] = "\n"; f (x : xs) = shows x (' ' : f xs) in f [1,2,3]
21:11:49 <lambdabot> "1 2 3 \n"
21:11:51 <dolio> Also, is printing actually the bottleneck?
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21:12:24 <tomsmeding> right, possibly laziness makes it look like it is, even if it actually isn't
21:12:36 <EvanR> isn't builder in appropriate here since we already have a lazy list of the things to print, in order
21:12:45 <EvanR> just print them in order
21:12:51 <tomsmeding> well, putStrLn takes an MVar on stdout
21:13:04 <tomsmeding> taking an MVar for outputting like 5 bytes may not be optimal
21:13:16 <EvanR> facepalm
21:13:29 <tomsmeding> writing everything in a big buffer and then sending that buffer into stdout may be quite a bit faster
21:13:41 <EvanR> this sounds so backwards
21:13:58 <EvanR> kind of like buffering your own buffer before it gets to the OS buffers, which are then buffered
21:14:09 <lantti> I'm currently trying to figure out how the profiling works, so I can get a better idea of the bottlenecks
21:14:20 <tomsmeding> EvanR: output is already buffered, the actual write(2) call in the kernel is only called when stdout is flushed, which, if you don't write newlines, happens only every 4096 bytes or so
21:14:47 <dolio> It's already effectively building a 'buffer' as a lazy list of characters. Builder is just generally a better one.
21:15:08 <tomsmeding> EvanR: also, a Text Builder produces a _lazy_ text, so that doesn't actually create a whole separate buffering step
21:15:33 <dolio> I think you can directly put a builder, too.
21:15:35 <EvanR> a lazy text itself is already a lazy list
21:15:35 <tomsmeding> or, well, it does, but then it (kind of) synchronises with the one in the RTS so they kinda merge
21:15:46 <EvanR> then a builder to make a straight lazy list is not helping
21:16:31 <tomsmeding> dolio: doesn't seem like you can, but toLazyText should be fine
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21:16:49 <EvanR> builder is often recommended in case people are trying to form a big sequence from pieces other than just tacking onto the end
21:16:54 <dolio> Oh. I guess it's just bytestring builders.
21:17:10 <tomsmeding> EvanR: which i why I recommended `T.intercalate` first
21:17:20 <tomsmeding> but that doesn't have the nice Int serialiser
21:17:41 <newmind> lantti: what's the actual time you're getting? just to see how reasonable it is
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21:19:21 <tomsmeding> further tip: benchmark your code but setting `ints = replicate 100000 123456789` or something, just to double-check it's actually the printing that's slow, not the computation preceding it
21:20:01 <newmind> that was my second suggestion, replacing ints with [1..100000] or something
21:20:32 <EvanR> .oO( replace the printing with just evaluating the ints in sequence )
21:20:42 <tomsmeding> that's the dual version :p
21:23:22 <mauke> :t mapM_ evaluate
21:23:24 <lambdabot> Foldable t => t b -> IO ()
21:24:47 <EvanR> hopefully evaluate doesn't first take an MVar before evaluating xD
21:25:04 <EvanR> requiring us to do artifically additional buffering wtf
21:25:35 <lantti> I can't figure out the profiling under stack right now, but unix time command for the 10000000 Ints case gives 0.8s
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21:25:54 <EvanR> 10^7 ints
21:26:07 <lantti> oops, 1000000
21:26:32 <EvanR> are they particularly uniform ints or like up to 3 digits or
21:27:17 <lantti> the whole task is that one: https://cses.fi/alon/task/1092/
21:27:55 <lantti> so basically two runs of consecutive ints with one exception
21:28:09 <lantti> in each run
21:29:33 <EvanR> ok it doesn't seem to be trying to print out literally 10^6 ints but the program is more complex than that
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21:30:24 <haskellbridge> <ijouw> Is there a good way to define a more general Ord class that avoids troubles with functions using Ordered argumens like nubOn?
21:31:02 <EvanR> :t nubOn
21:31:20 <EvanR> @botsnack
21:31:31 <lambdabot> :)
21:31:32 <lambdabot> error: [GHC-88464] Variable not in scope: nubOn
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21:32:26 <haskellbridge> <ijouw> i meant this: nubOrdOn :: Ord b => (a -> b) -> [a] -> [a]
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21:32:40 <EvanR> what is the trouble?
21:32:41 <haskellbridge> <ijouw> Filters duplicates by sorting
21:32:50 <lantti> It does need to print out 10^6 ints in the case n = 10^6. But thanks for the tips, i'll try some out and see if that makes a difference.
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21:33:46 <EvanR> yeah but it has to print a lot more than that in that case, so someone trying to replicate your running time is going to undershoot by a lot
21:34:07 <tomsmeding> EvanR: yes, "YES" and 2 more ints
21:34:07 <haskellbridge> <ijouw> It obviously does not work on MyOrd a => [a], and specifying MyOrd a, Ord a => [a] makes MyOrd seem pointless
21:34:21 <EvanR> oh it's only 2 lines
21:34:25 <EvanR> not N
21:34:57 <EvanR> MyOrd ?
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21:35:11 <EvanR> you want it to work on a variable class?
21:35:41 <EvanR> if you're trying to define an alternate Ord instance, that's what newtypes are for, then you use Ord
21:36:26 <haskellbridge> <ijouw> I want to use type families to allow the return type to be wrapped for DSL stuff
21:36:58 <tomsmeding> it kind of sounds like you'll need to be a bit more specific here
21:37:42 <EvanR> maybe you need ways to lift library functions like nubOrdOn into your DSL
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21:45:47 <haskellbridge> <ijouw> I want this https://hackage-content.haskell.org/package/Boolean-0.2.4/docs/Data-Boolean.html#t:OrdB and then define a function nubOrd' :: (OrdB a, BooleanOf a ~ Bool) => [a] -> [a] ; (maybe i should just not reimplement everything...)
21:46:06 <monochrom> I am unconvinced that (putStrLn . unwords . map show) is too slow. It's only 0.05 seconds on my laptop if you redirect to a file (which is the right thing to do if you're simulating an online autotester; if you let it go to a terminal, then the bottleneck is the terminal)
21:47:07 <newmind> .o( but it does say a lot about haskell as a language/ecosystem that we're discussing it )
21:47:33 <EvanR> it?
21:47:58 <newmind> why something as simple as "print out this list of numbers" might be slow
21:48:16 <monochrom> No, it says a lot about people having double standards.
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21:48:20 <EvanR> people mistake what's slow all the time, that's a haskell thing
21:49:28 <EvanR> this problem that separates a million integers into two sets that adds up to the same thing
21:49:28 <newmind> ywah, but with lazy eval, monads that need to be resolved... it's less straightforward than it would be in C.. which is perfectly fine, just... weird
21:49:38 <EvanR> printing it out doesn't sound like the crunchy part
21:50:10 <mauke> printing is slow in C, too
21:50:15 <monochrom> After compiling with -O, the IO monad doesn't exist in the machine code.
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21:50:28 <EvanR> i was going to say C is the one where you are sometimes trying to deduce how slow printing is
21:50:29 <tomsmeding> (that's a claim)
21:50:36 <tomsmeding> maybe in this tight loop that's true
21:50:44 <mauke> back when I did SPOJ, I got significant time improvements by bypassing stdio
21:50:48 <lantti> I suspected the printing because I realized there is an analytic solution to the actual problem and generating the lists is just some math operations and [1..xyz]
21:51:08 <lantti> I'm in the process of re-evaluating that assumption :)
21:51:20 <Guest81> I am reading "learn you a Haskell ..." and I am using interactive on the haskell.org homepage. But when I input a simple equality such as "a = [1,2,3]" is get a parse error. What am I doing wrong?
21:51:40 <Rembane> Guest81: Try a let a = [1,2,3]
21:51:55 <tomsmeding> that "Try it!" prompt on haskell.org is not a proper haskell interpreter
21:52:02 <EvanR> a = [1,2,3] ought to work in ghci and at top level?
21:52:08 <tomsmeding> it doesn't there
21:52:20 <Guest81> no go: let a = [1,2,3]
21:52:21 <Guest81> <no location info>: not an expression: ‘let a = [1,2,3]’
21:52:32 <EvanR> "interesting"
21:52:47 <monochrom> "a = [1,2,3]" works for me.
21:53:06 <tomsmeding> there's https://play.haskell.org if you want to upgrade to writing a full program (which doesn't have to be complicated!); otherwise, either install ghci, or there may be some other online repl
21:53:12 <monochrom> "let a = [1,2,3]" also works for me.
21:53:23 <tomsmeding> monochrom: we're talking about the "Try it!" thing on haskell.org, which is stupid and broken
21:53:35 <monochrom> Oh that. Sorry!
21:53:51 <monochrom> "let a = [1,2,3] in length a"
21:54:07 <tomsmeding> ah yes, that does work Guest81 ^
21:54:18 <monochrom> But play.haskell.org is better.
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21:54:20 <tomsmeding> apparently it accepts only full expressions
21:54:31 <tomsmeding> play.haskell.org is not a repl, though :)
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21:55:00 <monochrom> Who writes 1000 lines of code on a REPL anyway? Be realistic.
21:55:02 <geekosaur> probably just hint
21:55:10 <Guest81> OK so basically "Try it!" is not a true interpreter.
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21:55:35 <tomsmeding> it is an interpreter, but not ghci
21:55:46 <mauke> a = [1,2,3] is not an expression
21:55:48 <tomsmeding> ghci does quite a bit more than just an interpreter for basic expressions, and "learn you a haskell" assumes ghci
21:56:55 <Guest81> So i need ghci. Thanks all.
21:57:07 <tomsmeding> that's probably easiest, yes (use ghcup)
21:57:44 <EvanR> repl is overrated, you want to load a file anyway. Then reload
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22:03:59 <haskellbridge> <ijouw> Isn't the two sets algorithm discussed above easy to compute? The question is whether the sum [1..n] is even (solvable in constant time) (or am I missing something?). Printing could indeed be slowest part.
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22:04:25 <tomsmeding> ijouw: do you have a proof that if sum [1..n] is even, there is such a splitting?
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22:04:44 <tomsmeding> (there is, but figuring out what the splitting is, requires a little trick)
22:04:47 <haskellbridge> <ijouw> Working on it
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22:12:45 <mauke> ah, I see it
22:13:01 <mauke> 1 + 2 = 3
22:15:12 <mauke> nvm, I'm blind
22:17:05 <mauke> hah, no. I was wrong about being wrong. I just did it backwards :-)
22:17:36 <newmind> mauke: did you get nerdsniped there?
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22:21:16 <tomsmeding> lantti: I'm not sure what you did, but it can probably be done more efficiently; this is haskell with `putStrLn . unwords . map show` https://tomsmeding.com/ss/get/tomsmeding/RnFf4z
22:23:02 <tomsmeding> (I figured out a way to compute what numbers to print in constant time; then I just have to print all those numbers)
22:23:28 <tomsmeding> it's a fun problem :)
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22:24:58 <tomsmeding> and I'm not sure where even that 0.30s comes from; it takes less than 1ms on that input on my machine
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22:25:48 <geekosaur> `sum [1..n]` isn't there a number theory approach for that?
22:26:22 <haskellbridge> <ijouw> yes, it is (n*(n+1))/2
22:26:41 <tomsmeding> it seems printing is actually rather slow on that judge server! On roughly 260000 I take 0.08s, on roughly 653000 I take 0.21s, and on 1000000 I take 0.30s
22:27:25 <tomsmeding> and I can assure you that I am actually doing work that should take <10us plus `putStrLn . unwords . map show` on a lazy list of Int
22:28:21 <tomsmeding> (I'm not sure if it's responsible to post my solution publically for people to scrutinise)
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22:29:56 <lantti> tomsmeding: I already passed so you won't boost my grade with it
22:30:21 <lantti> of course I don't know if any other students are reading this
22:31:07 <tomsmeding> lantti: do you determine what numbers to print in constant time?
22:31:19 <tomsmeding> or is there a little loop or something involved
22:31:32 <lantti> yes, but I still generate a list of them. eliminating that list now...
22:31:33 <tomsmeding> (and how do your judge times compare to mine?)
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22:31:48 <lantti> yes, to the constant time
22:31:55 <tomsmeding> interesting
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22:32:50 <lantti> I also use a delete on one of the lists to remove that one int that needs to move to the other set
22:33:12 <lantti> all that can still go away if i make a loop instead of a list
22:33:30 <lantti> but that might work against me as well, lets see
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22:36:59 <tomsmeding> turns out my lists weren't actually lazy when printed -- I was materialising them with 'length'. If I don't do that, my worst-case time on the judge server goes from 0.30s to 0.21s
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22:38:37 <tomsmeding> (that server is really slow...)
22:39:15 <mauke> you guys are using lists?
22:39:30 <tomsmeding> I am using lists only to pass to `putStrLn . unwords . map show`
22:40:31 <newmind> tomsmeding: without printing and just evaluating its fast on the server too?
22:42:19 <tomsmeding> then the server would mark my answer wrong and (presumably? didn't try) not proceed to the larger test cases
22:42:41 <mauke> it should run all of them
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22:45:57 <tomsmeding> ok if I replace the `putStrLn . unwords . map show` with `mapM_ evaluate` then maximum runtime goes from 0.21s to 0.02s
22:46:14 <lantti> ah, ok, I got it to 0.21s too by eliminating two calls to length and one to delete
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22:46:49 <lantti> ah, so there it is :)
22:47:01 <tomsmeding> and if I do `evaluate . length . unwords . map show` instead, I get 0.16s
22:47:21 <tomsmeding> so that's 0.02s for the computation, 0.14s for the serialisation, and 0.05s for the printing
22:47:36 <tomsmeding> this is all <0.001s on my machine lol
22:48:52 <haskellbridge> <ijouw> I now have a formal proof by induction with step 4
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22:49:17 <tomsmeding> that sounds like mauke's solution (exchanged ideas in private chat)
22:49:28 <tomsmeding> mine is overengineered, I used a sqrt
22:50:41 <mauke> my solution doesn't use lists :-)
22:50:42 <haskellbridge> <ijouw> I am debating whether i want to put it in an automatic solver
22:51:52 <tomsmeding> mauke: I tool the liberty to submit your solution to the judge; it takes 0.59s
22:52:11 <tomsmeding> stdout handling is slow, told you
22:52:26 <tomsmeding> *took
22:53:22 <mauke> spoilers: https://cses.fi/paste/7167882c695ce46ff9ce6b/
22:54:57 <tomsmeding> overengineered spoilers: https://cses.fi/paste/b23bf94c9440505af9ce60/ (if we're sharing anyway...)
22:56:05 <haskellbridge> <ijouw> There is div from Integral, no need for double cast for /2
22:56:18 <tomsmeding> yes, because there's also sqrt
22:57:40 <tomsmeding> *yes I do because
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23:07:49 <lantti> mine was https://privatebin.net/?97447d6805a0ea49#CfnZntNYLfYtSvpCod6DSKRYS8xDhzqycvhjmKNhppjv
23:07:59 <haskellbridge> <ijouw> My solution fails locally at readLn? I am confused.
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23:10:36 <haskellbridge> <ijouw> I was using a library which wraps reads with Maybe and forgot to adjust the type...
23:10:50 <haskellbridge> <ijouw> When removing it so i can submit with fewer deps
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23:19:15 <mauke> lantti: I suggest changing "otherwise" to "print" for even more confusion
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23:21:44 <lantti> mauke: is it not meant to be used like that? is it only for gurads then?
23:22:21 <lantti> guards, even
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23:25:00 <lantti> but looking at my code now I do admit that there are some decisions that only make sense if you consider that they were the smallest change to the previous version, like the decision to include the lengths of the sets in the same list as the sets themselves
23:25:38 <lantti> (the previous version used length on the generated lists)
23:26:53 <lantti> (and the term "step squad" comes from the BBC childrens series called Numberblocks)
23:29:25 <lantti> (which I can recommend to anyone with children in the 1-3 primary school or earlier)
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23:32:16 <lantti> should I use _ -> as the catch all case then? I'm not working with haskell all that much as one may notice
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23:36:05 <int-e> mauke: Yeah that looks like the best solution. Kind of sad; the simple greedy algorithm for the knapsack problem would work here.
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23:37:02 <int-e> (but then you'd have to accumulate a full result before printing anything)
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23:44:58 <Comstar> I can't think of a way to implement breadth-first searches (for the sake of discussion assume for a binary tree) that doesn't use a `Control.Monad.State` implementation
23:45:29 <Comstar> do y'all know of a more pure approach? I guess I could do like, some folding magic
23:45:49 <Comstar> I feel like I could probably find some neat code examples searching around github &al
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23:47:13 <int-e> Comstar: If you have a state S and function branch :: S -> [S] then the list monad's bind (aka concatMap) can produce the next level of a tree from the previous one. (This gets a bit awkward if you also want to record history, but you can make that part of S)
23:47:36 <Comstar> yeah that makes sense
23:47:54 <int-e> "history" being the path through the underlying tree
23:48:15 <Leary> Comstar: There was a haskell challenge for this a while back. This was my solution: https://gist.github.com/LSLeary/5083d1dfe403d1562e7778713d97b22a
23:49:31 <int-e> Leary: that task is related but abuses Foldable on an infinite tree that you already have as a value.
23:50:18 <int-e> (and it's an abuse because that Semigroup instance isn't lawful)
23:50:40 <Comstar> Leary, oh wow line 25 in `Search.hs` is really interesting, I didn't know you could do `\case` like that, does that trick have a name?
23:50:51 <Comstar> oh it's prob the LambdaCase language extension
23:50:56 <int-e> it is
23:50:56 <Leary> int-e: Do we not have a (foldable) binary tree to search? And yes, I agree, but specialising the code to a binary tree means we can kill the evil.
23:50:57 <Comstar> that'll be fun to explore
23:51:32 <int-e> Leary: I don't know whether we have one. My main point was that it's abuse.
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23:53:22 <lantti> mauke: oh, now i see. so the "otherwise" only worked because any identifier would have worked there
23:53:41 <int-e> I've implemented quite a few breadth first searches in Haskell and I've never materialized the tree; only levels (usually as lists because it's convenient, or as sets because I expected a significant number of transpositions.)

All times are in UTC on 2026-02-24.