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2020-10-03 05:41:04 <dsal> Yeah.
2020-10-03 05:41:25 ski . o O ( (`-Wmissing-fields',`-Woverlapping-patterns',)`-Wincomplete-patterns',`-Wincomplete-uni-patterns' )
2020-10-03 05:41:54 <dsal> Same as in go and other languages. That's not the revolutionary benefit of haskell's type system. :)
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2020-10-03 05:42:24 <ski> > let foo Node{} = () in foo Node{} -- not really fond of such patterns
2020-10-03 05:42:26 <lambdabot> ()
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2020-10-03 05:45:04 <p0a> Why do I get this error? file-library-exe: ./library.database: openFile: does not exist (No such file or directory)
2020-10-03 05:45:48 <p0a> https://paste.tomsmeding.com/Um3sNGUr
2020-10-03 05:46:41 <p0a> (I know the code is long, but readDatabase and writeDatabase are relevant on Lib.hs
2020-10-03 05:47:04 <p0a> tomsmeding: your paste would be nicer if it had line numbers :) I think
2020-10-03 05:47:21 <dsal> Does the file exist?
2020-10-03 05:47:29 <p0a> No it does not
2020-10-03 05:47:33 <p0a> but I use readMaybe
2020-10-03 05:47:52 <dsal> That's not what readMaybe does. heh
2020-10-03 05:47:54 <dsal> :t readMaybe
2020-10-03 05:47:56 <lambdabot> error: Variable not in scope: readMaybe
2020-10-03 05:48:06 <p0a> oh wait
2020-10-03 05:48:18 <p0a> huh I wonder what it is I used readMaybe for then
2020-10-03 05:48:53 <dsal> You're trying to fmap readMaybe into an IO action that is failing.
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2020-10-03 05:49:43 <p0a> okay I see now
2020-10-03 05:49:54 <p0a> I remember what happened
2020-10-03 05:50:04 <p0a> There were two issues, I fixed one, and I forgot about the other
2020-10-03 05:50:04 <p0a> :)
2020-10-03 05:50:12 <p0a> Thank yuo
2020-10-03 05:50:48 <p0a> (This is my little project which I hope to improve, but right now trying to get version 0.01)
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2020-10-03 05:52:20 <p0a> I wanted to move to RIO soon
2020-10-03 05:52:36 <p0a> I guess for now I'll leave this bug unfixed and assume the database exists. I think I'd have to use exceptions otherwise
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2020-10-03 06:03:58 hackage arch-hs 0.2.0.0 - Distribute hackage packages to archlinux https://hackage.haskell.org/package/arch-hs-0.2.0.0 (berberman)
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2020-10-03 06:29:17 <p0a> if I have a Maybe (IO ()) how am I supposed to act on it?
2020-10-03 06:29:43 <p0a> What's the idiomatic way? Sorry, I meant Maybe (IO T)
2020-10-03 06:30:16 <ski> `sequence'/`sequenceA' ?
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2020-10-03 06:31:57 <p0a> so say I have a file I read. It's a Maybe (IO Database) and I'd like to print the entries of the database
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2020-10-03 06:32:34 <p0a> if I had a database I'd just do map putStrLn (contents db)
2020-10-03 06:32:58 <p0a> sorry, I meant mapM_
2020-10-03 06:33:08 <c_wraith> It's vastly unlikely something that interacts with the filesystem is going to produce Maybe (IO Database)
2020-10-03 06:33:19 <p0a> Is it a mistake c_wraith ?
2020-10-03 06:33:33 <p0a> That's how I wrote it. I have a readDatabase that returns a Maybe (IO Database)
2020-10-03 06:33:35 <c_wraith> IO (Maybe Database) would make much more sense.
2020-10-03 06:33:48 <p0a> Yes, that's what it is, sorry
2020-10-03 06:34:54 <ski> that's quite different
2020-10-03 06:35:38 <ski> what should happen, if you get `Nothing' ?
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2020-10-03 06:35:44 <p0a> Right
2020-10-03 06:36:32 <p0a> I see it's a problem with types. So is there an idiomatic way or do I need to do a conditional?
2020-10-03 06:36:33 <taurux> p0a: you can use my_io_maybe_db >>= (maybe what_to_do_when_nothing (\db -> what_to_do_when_db))
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2020-10-03 06:37:40 <p0a> What does -> do?
2020-10-03 06:37:43 <p0a> oh it's a lambda
2020-10-03 06:37:46 <taurux> it's a lambda
2020-10-03 06:38:09 <taurux> what_to_do_when_nothing :: IO something
2020-10-03 06:38:10 <c_wraith> p0a: it's not a "problem" with types. It's just types. At most there's a puzzle involved. bind the value so you can use it in an IO block, then deal with the Maybe appropriately
2020-10-03 06:38:12 <ski> do you want to catch the failure, and do something else, here ?
2020-10-03 06:38:18 <ski> or just propagate the failure ?
2020-10-03 06:38:26 <p0a> propagate the failure
2020-10-03 06:38:35 <taurux> You should use a Monad transformer
2020-10-03 06:38:36 <taurux> MaybeT
2020-10-03 06:38:37 <p0a> so that I never deal with it, I'm at top level :P
2020-10-03 06:38:38 <ski> use `MaybeT IO', possibly
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2020-10-03 06:39:09 <ski> it will short-circuit, if something fails (with `Nothing')
2020-10-03 06:39:24 <taurux> monad transformers make semantics of failure propagation much more clear. Take a look at the dedicate chapter in LYAH
2020-10-03 06:39:28 <taurux> *dedicated
2020-10-03 06:40:26 <p0a> for one thing I realized it's just Maybe Database because I do bind it in a do block
2020-10-03 06:40:32 <p0a> I'm just confused as usual
2020-10-03 06:41:47 <p0a> ah I know the issue, I actually have a Maybe [String] that I wish to print
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