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Logs: liberachat/#haskell

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2021-07-27 16:45:10 × deejaytee quits (~deejaytee@193.46-255-62.static.virginmediabusiness.co.uk) (Killed (zirconium.libera.chat (Nickname regained by services)))
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2021-07-27 17:28:09 <lechner> Hi, is there an elegant way to deal with several Maybe values in a row without increasing the indentation in the source code but while reporting errors properly (i.e no fromJust)? Thanks!
2021-07-27 17:28:47 <geekosaur> use the Monad instance?
2021-07-27 17:28:55 <monochrom> >>=
2021-07-27 17:29:21 <lechner> i am so new. is there an example on the web?
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2021-07-27 17:39:20 <sm> lechner: write a do block, where each line extracts the next Just value with <-. If any of those fail, the whole block evaluates to Nothing
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2021-07-27 17:39:44 × LukeHoersten quits (~LukeHoers@user/lukehoersten) (Quit: My MacBook has gone to sleep. ZZZzzz…)
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2021-07-27 17:40:17 <lechner> That's longhand for this, right? "By definition, when the first argument to >>= is Nothing, it just returns Nothing while ignoring whatever function it is given." https://en.m.wikibooks.org/wiki/Haskell/Understanding_monads/Maybe
2021-07-27 17:42:59 <Cale> do-notation is a shorthand syntax for a chain of uses of (>>=)
2021-07-27 17:43:21 <Cale> @undo do { u <- x; v <- y; return (f u v) }
2021-07-27 17:43:22 <lambdabot> x >>= \ u -> y >>= \ v -> return (f u v)
2021-07-27 17:43:29 × chris_ quits (~chris@81.96.113.213) (Remote host closed the connection)
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2021-07-27 17:43:56 <lechner> thanks! i am making baby steps
2021-07-27 17:44:02 <Cale> and so in the case of Maybe, you're allowed to have x and y be Maybe values there
2021-07-27 17:44:16 <Cale> and then u and v will be the contents of the Just constructor in the case that they're both Just
2021-07-27 17:44:51 <Cale> So it's a good way to unpack a bunch of Maybes and produce Nothing if any of them happens to be Nothing
2021-07-27 17:45:46 <Cale> (the do-block as a whole will be a Maybe value in the case that x and y are)
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2021-07-27 17:47:14 <Cale> There are also a bunch of functions which work with any instance of Monad, that specialise to possibly-useful functions on Maybe
2021-07-27 17:47:17 <Cale> :t sequence
2021-07-27 17:47:18 <lambdabot> (Traversable t, Monad m) => t (m a) -> m (t a)
2021-07-27 17:47:36 <Cale> (That one is even more general now...)
2021-07-27 17:47:51 <Cale> Well, if you just think of it as (Monad m) => [m a] -> m [a]
2021-07-27 17:47:55 <Cale> for Maybe, that becomes
2021-07-27 17:48:01 <Cale> [Maybe a] -> Maybe [a]
2021-07-27 17:48:20 <lechner> thanks! my code is slowly gaining style. haskell is amazing. i already found several potential bugs in my Perl code, which i am translating
2021-07-27 17:48:40 × peterhil quits (~peterhil@mobile-access-b04815-181.dhcp.inet.fi) (Ping timeout: 256 seconds)
2021-07-27 17:48:46 <Cale> i.e. take a list of Maybe things, and see if they're all Just, if so, then you get Just of a list of the contents, if any was Nothing, you get Nothing
2021-07-27 17:49:05 <DigitalKiwi> monochrom: https://twitter.com/krisajenkins/status/1420022268177027090?s=20
2021-07-27 17:50:02 <lechner> Cale: thanks!
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2021-07-27 17:52:45 <Cale> DigitalKiwi: I don't know how deterministic twitter's "More Tweets" section is, so https://twitter.com/d6/status/1419885183793344512 was pretty funny
2021-07-27 17:52:53 DigitalKiwi takes them to clout court
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2021-07-27 17:55:42 <DigitalKiwi> Cale: i follow d6 and this is the first time i've seen it lol
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2021-07-27 17:57:21 <lechner> Hi, how can a function that returns a "Maybe (IO ()) perform an IO task, please?
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2021-07-27 17:57:54 <Cale> So, it might give you an action you could run to perform some IO, and it might not
2021-07-27 17:58:30 × drd quits (~drd@93-39-151-19.ip76.fastwebnet.it) (Ping timeout: 276 seconds)
2021-07-27 17:58:32 <Cale> The most straightforward thing you should understand first is just to pattern match on it with case
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