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2020-10-03 19:12:53 → knupfer joins (~Thunderbi@i59F7FF93.versanet.de)
2020-10-03 19:12:53 × machinedgod quits (~machinedg@45.78.189.122) (Read error: Connection reset by peer)
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2020-10-03 19:14:29 × fluturel quits (~fluturel@79.119.85.203) (Read error: Connection reset by peer)
2020-10-03 19:15:49 <ixlun> Hi all, I'm trying to do a fold with a cutoff.. i.e. at some point during the computation of the fold, an intermediate value should be retunrned, even though the entire input list may not have been traversed.
2020-10-03 19:16:09 → heatsink joins (~heatsink@107-136-5-69.lightspeed.sntcca.sbcglobal.net)
2020-10-03 19:16:15 <ixlun> I'm currently trying: `head . takeWhile cutoff . scanr'
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2020-10-03 19:16:35 × elliott__ quits (~elliott@141.156.194.190) (Ping timeout: 240 seconds)
2020-10-03 19:16:53 <dsal> :t scanr
2020-10-03 19:16:55 <lambdabot> (a -> b -> b) -> b -> [a] -> [b]
2020-10-03 19:17:17 <ixlun> I'm not sure if that's doing as I expect... if I traceShowId on the value returned by cutoff, it appears to only be evaluated once
2020-10-03 19:17:58 <dsal> I'm a bit confused as to what you're doing. Can you just use foldr?
2020-10-03 19:18:23 <ixlun> I can kind of see why it's doing that since I'm calliing `head', but that's only to get the value where the the cutoff predicate became true.
2020-10-03 19:18:48 → machinedgod joins (~machinedg@45.78.189.122)
2020-10-03 19:18:57 <geekosaur> it's lazy so the scan would end when you stop consuming values
2020-10-03 19:18:59 <dsal> If you move that predicate into the argument to foldr, I would expect it to do the right thing.
2020-10-03 19:21:05 <dsal> Does that function differently from `f i . head` ?
2020-10-03 19:21:10 dsal needs to have lunch
2020-10-03 19:21:14 <ixlun> Say I have the list: [a,b,c,d] and I also have a predicate function `cutoff :: x -> Bool'. Part way through the list that preciate could become true, so I want to stop the fold at that point and return the value
2020-10-03 19:21:51 <dsal> Is the predicate on the input or the output? If it's the output, then it doesn't seem to do anything at all.
2020-10-03 19:22:13 <ixlun> The predicate is on the output
2020-10-03 19:22:53 <dsal> But if you're asking for head, so it's a partial function that either returns the first output, or there's no output.
2020-10-03 19:23:48 → macrover joins (~macrover@ip70-189-231-35.lv.lv.cox.net)
2020-10-03 19:23:59 <ixlun> Yeah, that's not what I want I guess
2020-10-03 19:25:37 <ixlun> As an example, suppose I predicate: `x > 10' and then I have: `foldl (+) 0 [2,3,4,5]' I'd want the return value to be: 9
2020-10-03 19:26:00 <ixlun> is there such a thing?
2020-10-03 19:27:06 → thir joins (~thir@p200300f27f0fc60004d129737887aa72.dip0.t-ipconnect.de)
2020-10-03 19:28:12 <ahf> jag
2020-10-03 19:29:51 <ixlun> I suppose what I'm after is a terminating fold
2020-10-03 19:30:04 <dsal> Well, foldr terminates.
2020-10-03 19:30:16 <dsal> But it's a bit hard in this exact situation.
2020-10-03 19:31:09 → Sgeo joins (~Sgeo@ool-18b982ad.dyn.optonline.net)
2020-10-03 19:31:48 <dsal> foldr will stop if you don't look at the output, but your condition depends on that here.
2020-10-03 19:32:08 → kernelmethod joins (~kernelmet@c-73-181-114-129.hsd1.co.comcast.net)
2020-10-03 19:32:25 × thir quits (~thir@p200300f27f0fc60004d129737887aa72.dip0.t-ipconnect.de) (Ping timeout: 240 seconds)
2020-10-03 19:34:02 <dsal> > last . takeWhile (<10) . scanl (+) 0 $ [2..] -- this is pretty close to what you described.
2020-10-03 19:34:05 <lambdabot> 9
2020-10-03 19:35:27 <Zetagon> wait I'm new here, but does lambdabot evaluate all haskell code? or just pure?
2020-10-03 19:35:52 <Rembane> > putStrLn "Maybe?"
2020-10-03 19:35:53 <dsal> It's sandboxed. yahb is less restricted.
2020-10-03 19:35:55 <lambdabot> <IO ()>
2020-10-03 19:36:02 <dsal> % putStrLn "hi"
2020-10-03 19:36:02 <yahb> dsal: hi
2020-10-03 19:36:05 → renzhi joins (~renzhi@modemcable070.17-177-173.mc.videotron.ca)
2020-10-03 19:36:29 <ixlun> dsal: Ahhh I get it, last and scanl will cause the evaluation to happen
2020-10-03 19:36:51 <dsal> That's kind of a weird way to think about it.
2020-10-03 19:37:18 <dsal> You're not worried about "causing evaluation" because there's no difference between something that didn't happen and something that wasn't observed.
2020-10-03 19:37:45 <dsal> last is just the thing you were asking for -- it's the boundary value in the produced list, as opposed to head, which is always the same.
2020-10-03 19:38:16 → ciupakabra5 joins (836f05b6@global-5-182.nat-2.net.cam.ac.uk)
2020-10-03 19:38:23 <ciupakabra5> mm
2020-10-03 19:38:58 <ciupakabra5> darn this is weird, so I've connected to Freenode through their website, and it lets me write in #haskell but not in #xmonad
2020-10-03 19:39:10 <Xnuk> % readFile "."
2020-10-03 19:39:11 <yahb> Xnuk: *** Exception: .: openFile: inappropriate type (is a directory)
2020-10-03 19:40:16 → vicfred joins (~vicfred@unaffiliated/vicfred)
2020-10-03 19:40:20 <Xnuk> % readFile "/etc/hostname"
2020-10-03 19:40:20 <yahb> Xnuk: *** Exception: /etc/hostname: openFile: does not exist (No such file or directory)
2020-10-03 19:40:49 × knupfer quits (~Thunderbi@i59F7FF93.versanet.de) (Quit: knupfer)
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2020-10-03 19:45:41 × kernelmethod quits (~kernelmet@c-73-181-114-129.hsd1.co.comcast.net) (Ping timeout: 265 seconds)
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2020-10-03 19:49:57 × ciupakabra5 quits (836f05b6@global-5-182.nat-2.net.cam.ac.uk) (Remote host closed the connection)
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2020-10-03 20:10:56 <koz_> MaybeT IO Database
2020-10-03 20:11:27 × GyroW_ quits (~GyroW@unaffiliated/gyrow) (Quit: Someone ate my pie)
2020-10-03 20:11:37 → GyroW joins (~GyroW@d54C03E98.access.telenet.be)
2020-10-03 20:11:37 × GyroW quits (~GyroW@d54C03E98.access.telenet.be) (Changing host)
2020-10-03 20:11:37 → GyroW joins (~GyroW@unaffiliated/gyrow)
2020-10-03 20:12:57 <MarcelineVQ> chad EitherT () IO Database vs simp MaybeT IO Database
2020-10-03 20:13:26 <koz_> MarcelineVQ: ReaderT DatabaseStatus IO Database, for the FP Completionists.
2020-10-03 20:13:36 × renzhi quits (~renzhi@modemcable070.17-177-173.mc.videotron.ca) (Ping timeout: 272 seconds)
2020-10-03 20:13:38 <MarcelineVQ> fp completionists use RIO
2020-10-03 20:13:48 <koz_> MarcelineVQ: My bad, lol.
2020-10-03 20:14:01 <koz_> I clearly don't know enough about that world.
2020-10-03 20:15:23 × danvet_ quits (~Daniel@2a02:168:57f4:0:efd0:b9e5:5ae6:c2fa) (Ping timeout: 272 seconds)
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2020-10-03 20:16:54 Citrouille is now known as ping
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