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00:42 <lpaste> colt44 pasted “esqueleto” at http://lpaste.net/355660
00:42 <colt44> guys, is it possible to write that query usin esqueleto's dsl?
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01:03 <Squarism> anyone used https://hackage.haskell.org/package/lrucaching-0.3.1 ? I wonder how i can obtain all keys of the cache?
01:04 <Squarism> all type classes only seem to expose values
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01:07 <geekosaur> https://hackage.haskell.org/package/lrucaching-0.3.1/docs/Data-LruCache-Internal.html you must access the internal HashPSQ and use that type's access mechanisms
01:08 <geekosaur> https://jaspervdj.be/posts/2015-02-24-lru-cache.html linked in the README is also relevant
01:08 <Squarism> ok.. so not really meant to be used that way then i guess
01:08 <geekosaur> no, I'd expect not
01:09 <geekosaur> if you want to access via key or via priority/age, you want the PSQ directly
01:09 <geekosaur> because that's what a PSQ is
01:09 <geekosaur> so skip this package and go straight to the PSQ it wraps :)
01:09 <geekosaur> (psqueues package)
01:09 <Squarism> ah ok
01:10 <Squarism> what does it mean if a field has "!" infront of type like : "lruCapacity :: !Int"
01:10 <geekosaur> it means the field is strict (will be forced to WHNF)
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01:11 <Squarism> ah ok
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01:18 <jle`> Squarism: it means that if you ever resolve the constructor, you also resolve the field (to whnf)
01:18 <jle`> @let data NoStrict = NS { ss :: !Int }
01:18 <lambdabot> Defined.
01:19 <jle`> um i definitely messed that up
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01:21 <geekosaur> that definition looks slightly oxymoronic to me >.>
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01:22 <Squarism> laziness turned off
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01:22 <geekosaur> only partially though
01:23 <geekosaur> > take 1 . ss $ NS [1,undefined]
01:23 <lambdabot> error:
01:23 <lambdabot> • Couldn't match type ‘Int’ with ‘[a]’
01:23 <lambdabot> Expected type: NoStrict -> [a]
01:23 <geekosaur> whoops
01:23 <geekosaur> well, right, I need a different type for that
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01:24 <geekosaur> anyway if you make a ![Int] field, the only thing forced is [] for an empty list or the first 'cons' for a non-empty list
01:24 <geekosaur> (1:undefined) can be a value of such a field
01:24 <geekosaur> but undefined itself cannot
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01:27 <jle`> yes sorry i did mess up the deifnition, heh
01:27 <jle`> @undefine
01:27 <lambdabot> Undefined.
01:27 <jle`> @let data YesStrict = YS { ys :: !Int }; data NoStrict = NS { ns :: Int }
01:28 <lambdabot> Defined.
01:28 <jle`> > case YS undefined of YS _ -> True
01:28 <lambdabot> *Exception: Prelude.undefined
01:28 <jle`> > case NS undefined of YS _ -> True
01:28 <lambdabot> error:
01:28 <lambdabot> • Couldn't match expected type ‘NoStrict’
01:28 <lambdabot> with actual type ‘YesStrict’
01:28 <jle`> > case NS undefined of NS _ -> True
01:28 <lambdabot> True
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01:28 <jle`> normally, if you want to resolve that something was made with a given constructor, you don't have to look at the fields
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01:29 <jle`> but with !, it's like a runtime hook that says evaluate the field when you evaluate the constructor
01:29 <jle`> > case NS (sum [1..]) of NS _ -> True
01:29 <lambdabot> True
01:29 <jle`> > case YS (sum [1..]) of YS _ -> True
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01:29 <lambdabot> mueval-core: Time limit exceeded
01:29 <lambdabot> mueval: ExitFailure 1
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01:30 <jle`> my examples were also kind of silly because NoStrict and YesStrict only had one constructor, so it might be seen as "obvious" that they were made using NS/YS
01:30 <jle`> but just assume that they had other constructors, heh
01:30 <Squarism> thanks for trying! =D
01:31 <jle`> (you can aslo make a YesStrict w/o the YS constructor by using an infinite loop or something, so in that case, resolving the constructor is impossible even though there's only one, but that's a different thing)
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01:50 <Squarism> jle`, i get it.
01:50 <Squarism> thanks
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01:55 <Squarism> someone told me haskell evaluates "as far as it needs". Ie pattern matching a constructor doesnt evaluate fields. I see this as a way to go around that. =D
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01:56 <jle`> mhm, i think of it as a hook you can hack into the runtime system, sometimes
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04:02 <n_blownapart> hi is using atom haskell ide plugins now working well with stack? a few months ago it was buggy
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06:24 <thang1> That intermediate haskell book looks interesting
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06:34 <glguy> why's that?
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06:52 <thang1> Well it's always nice to have more books and stuff to look at for Haskell
06:53 <thang1> the more high quality learning materials, the better. Plus, Haskell seems to have a ton of beginner materials relative to the intermediate level
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06:57 <thang1> On the other hand, I don't know who Vlad and Artyom are and how good they are at writing books, so I'm cautiously optimsitic but not much more than that. Further, I don't know if they're just writing to make a profit or writing because they want to put better information out. If it's the latter, I'm more inclined to make the book open source so that the book's content is dynamically generated from
06:57 <thang1> a continuously evolving crowd-sourced material. Otherwise a book emphasizing standard practices for working with certain problems and "standard" libraries is going to be hopelessly out of date real damn quick.
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07:18 <LiaoTao> Is there any way to make a function a -> b call f.ex. read :: b, depending on the resolved type b?
07:18 <LiaoTao> Template Haskell territory?
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07:27 <LiaoTao> The function wouldn't be (a -> b), but rather (a -> MyType b), where I'd like to use read :: b
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07:31 <mjora7> Hi, why does stack take so much longer to install packages than something like npm? I don't think I've ever waited longer than 60 seconds for a node application to fully install all of its dependencies. It seems like waiting for stack to install all the dependencies of something (like Scotty or Yesod) can take many minutes (like it just took 5 minutes to install the latter).
07:31 <mjora7> Is there just something wrong with my setup? Or is this expected.
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07:34 <jle`> LiaoTao: not sure what you mean exactly
07:34 <jle`> 'read' already can be implemented for different types
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07:37 <LiaoTao> jle`: I'm doing parsing with readMaybe and would like to match against a series of Symbol type constructors
07:37 <LiaoTao> It's probably not a very good way of going about this
07:37 <jle`> still not super sure what you want, sorry :|
07:37 <jle`> can you give an example
07:38 <LiaoTao> Sure, hold on
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07:41 <lpaste> LiaoTao pasted “Parsing example” at http://lpaste.net/355661
07:41 <LiaoTao> jle`: ^
07:41 <LiaoTao> Ignore the -3>, don't know where it came from
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07:43 <jle`> hm, what are some example inputs and outputs from parse?
07:43 <LiaoTao> jle`: It's supposed to be parseLiteral
07:43 <jle`> btw, Maybe ('MkConstant a) is a kind error
07:43 <LiaoTao> Did some refactoring and forgot to rename it
07:43 <jle`> yeah, what are some example inputs and ouputs?
07:43 <jle`> Maybe :: * -> *
07:44 <jle`> but MkConstant :: (Show a, Num a, Ord a) => a -> Symbol
07:44 <jle`> so the kind of `MkConstant a` is Symbol
07:44 <LiaoTao> Like "45678" would turn into the corresponding Integer
07:44 <LiaoTao> Right
07:44 <LiaoTao> I'm on very shaky ground here, I realize
07:45 <jle`> ok so if i gave `parseLiteral "45678"`, what would the result be?
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07:46 <LiaoTao> jle`: MkConstant 45678
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07:47 <LiaoTao> I'll try to rewrite it for a moment - please hold the help for now!
07:47 <LiaoTao> Thanks!
07:47 <jle`> np :)
07:48 <LiaoTao> Also, it probably should be 'Just MkConstant 45678'
07:48 <LiaoTao> Forgot the Maybe
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07:50 <jle`> so :: Maybe Symbol ?
07:50 <LiaoTao> jle`: Already changed it
07:50 <LiaoTao> I figure I need to match it while readMaybe returns Nothing, so a do-expression would be a good idea
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07:51 <jle`> do, or maybe just a pattern match
07:51 <jle`> w/ a case statement
07:52 <LiaoTao> I first tried the case statement, but wouldn't that make an ugly indentation mess?
07:52 <jle`> hm yeah, depends on how many nests you have
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07:55 <merijn> If you find yourself nesting a lot of cases you can usually invent a helper for that
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07:57 <LiaoTao> Hmm, I think the issue arises when trying to differentiate between different types of numbers
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07:58 <LiaoTao> I.e. I'm trying to implement parseLiteral that works for both Integers and Floats
07:59 <LiaoTao> parseLiteral "123" would return Just MkConstant 123 and parseLiteral "1.23" would return Just MkConstant 1.23
07:59 <LiaoTao> parseLiteral "abc" would return Nothing
08:00 <merijn> LiaoTao: that's not something you can easily do
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08:00 <merijn> This requires the type returned by parseLiteral to depend on it's input, which isn't possible in Haskell
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08:05 <LiaoTao> jle`,merijn a nested case statement worked
08:05 <LiaoTao> I just hope I don't need any more types!
08:06 <lpaste> LiaoTao pasted “Ugh” at http://lpaste.net/355664
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08:07 <LiaoTao> merijn: That's a good answer to my original question, thanks!
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08:07 <merijn> What's the type of MkConstant? Seems weird that this typechecks, tbh :p
08:08 <LiaoTao> merijn: Symbol :D
08:08 <LiaoTao> GADTs
08:08 <LiaoTao> merijn: http://lpaste.net/355661
08:08 <LiaoTao> That's the original paste
08:09 <merijn> ah, right :)
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08:10 <LiaoTao> I daren't imagine how much twiddling you'd have to do in C to get this
08:10 <LiaoTao> Haskell less than three
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10:36 <LiaoTao> So I've got a function that returns (Maybe a) and want to use it to create a StateT [b] Maybe a value
10:36 <LiaoTao> return makes it a StateT [b] Maybe (Maybe a) value, though
10:36 <LiaoTao> Can you join these in some way?
10:36 <merijn> :t StateT
10:36 <lambdabot> (s -> m (a, s)) -> StateT s m a
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10:37 <merijn> LiaoTao: You have to wrap it via StateT explicitly
10:37 <merijn> Or maybe lift can work?
10:37 <merijn> :t lift
10:37 <lambdabot> (Monad m, MonadTrans t) => m a -> t m a
10:37 <merijn> right
10:37 <merijn> :t lift `asAppliedTo` (Just True)
10:37 <lambdabot> MonadTrans t => Maybe Bool -> t Maybe Bool
10:40 <LiaoTao> It works, but I'm not sure how it works
10:40 <LiaoTao> Is it like pure and return, except for wrapping monads?
10:40 <merijn> It's for lifting existing monadic values into a monad transformer
10:41 <LiaoTao> So exactly what I wanted? :D
10:41 <merijn> LiaoTao: "StateT s" is a transformer (and instance of MonadTrans)
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10:41 <merijn> So you get "lift :: Maybe a => (StateT s) Maybe a" i.e. 'StateT s' corresponding to 't' in the original type
10:42 <LiaoTao> Yeah
10:42 <LiaoTao> Thanks!
10:43 <LiaoTao> runStateT popOperator "+Hello+"
10:43 <LiaoTao> Just ('+',"Hello+")
10:43 <LiaoTao> \o/
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10:58 <Phillemann> I have [String] and I'd like a [Int] from that. When there's an error, I'd like to have the String that failed. Is it best to write a function "String -> Either String Int" and then fold over the list?
10:58 <Phillemann> Not sure if that's the right approach.
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10:59 <Iceland_jack> you could map that function over the list, yes
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11:00 <merijn> Phillemann: Sounds reasonable
11:00 <Iceland_jack> Do you want spoilers for String -> Either String Int?
11:00 <Phillemann> Map, okay, and then look for a Left and otherwise collect the Rights...
11:00 <Iceland_jack> or to implement it yourself
11:00 <merijn> Phillemann: Oh, you wanna fail if any of them fail?
11:00 <Phillemann> Yes.
11:00 <merijn> :t mapM -- :)
11:00 <lambdabot> (Monad m, Traversable t) => (a -> m b) -> t a -> m (t b)
11:01 <Phillemann> Sorry, I might not made it precise enough :)
11:01 <merijn> :t mapM `asAppliedTo` (undefind :: String -> Either String Int)
11:01 <lambdabot> error:
11:01 <lambdabot> • Variable not in scope: undefind :: String -> Either String Int
11:01 <lambdabot> • Perhaps you meant ‘undefined’ (imported from Prelude)
11:01 <merijn> :t mapM `asAppliedTo` (undefined :: String -> Either String Int)
11:01 <lambdabot> Traversable t => (String -> Either String Int) -> t String -> Either String (t Int)
11:01 <Iceland_jack> :t traverse Text.Read.readEither
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11:01 <lambdabot> (Read b, Traversable t) => t String -> Either String (t b)
11:01 <merijn> That only reports the first failure, though
11:01 <Phillemann> That'd be all right, though.
11:01 <merijn> rather than all
11:01 <merijn> Phillemann: If you don't care about which failure, it's even easier
11:02 <merijn> :t mapM Text.Read.readMaybe
11:02 <lambdabot> (Read b, Traversable t) => t String -> Maybe (t b)
11:02 <merijn> Phillemann: That returns Nothing if any of the reads fail with Nothing :)
11:03 <Iceland_jack> >>> readEither @Int `traverse` ["123", "43"]
11:03 <Iceland_jack> Right [123,43]
11:03 <Phillemann> I don't care about which failure, but I'd like to output the failed string.
11:03 <Phillemann> As in: String "foo" is not a number, please check
11:03 <merijn> > mapM Text.Read.readMaybe ["1","23","47"] :: Maybe [Int]
11:03 <lambdabot> error:
11:03 <lambdabot> Not in scope: ‘Text.Read.readMaybe’
11:03 <lambdabot> No module named ‘Text.Read’ is imported.
11:04 <merijn> > mapM readMaybe ["1","23","47"] :: Maybe [Int]
11:04 <lambdabot> error:
11:04 <lambdabot> Variable not in scope: readMaybe :: [Char] -> Maybe Int
11:04 <Iceland_jack> Phillemann: readEither doesn't report the String, but that's an easy exercise to write
11:04 <merijn> @let import Text.Read (readMaybe)
11:04 <lambdabot> Defined.
11:04 <merijn> > mapM readMaybe ["1","23","47"] :: Maybe [Int]
11:04 <lambdabot> Just [1,23,47]
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11:04 <merijn> > mapM readMaybe ["1","23","foo"] :: Maybe [Int]
11:04 <lambdabot> Nothing
11:04 <Phillemann> Iceland_jack: Sure, tupling it up is easy.
11:04 <merijn> Phillemann: The either will only show the first failure, though :)
11:05 <merijn> Although you could use map combined with
11:05 <merijn> :t lefts
11:05 <lambdabot> [Either a b] -> [a]
11:05 <Iceland_jack> or Lift.Const
11:05 <Iceland_jack> 'Validation' Either
11:06 <Iceland_jack> If you want multiple errors, use this to convert from Either to "Errors" https://hackage.haskell.org/package/transformers-0.5.4.0/docs/Control-Applicative-Lift.html#v:eitherToErrors
11:07 <Phillemann> Oh, that looks very nice, didn't know about that type
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11:08 <merijn> There's also Validation in the either package :)
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11:13 <colt44> I have a list of triples [(t1, t2, t3)] and I need to transform it into [(t1, [(t2, [t3])]. Is there such a function?
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11:25 <merijn> colt44: Do you care about the order of the resulting list?
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11:28 <colt44> merijn: only for the first element realy. but I can sort the list afterwards, if necessary.
11:29 <colt44> the thing is that I'm join three tables with esqueleto, and I need to build a three level, nested, json out of it
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11:31 <merijn> colt44: I'd hack something together using Map.unionWith
11:31 <merijn> Turn [(t1, t2, t3)] into [(t1, (t2, [t3]))] (just singleton lists of t3
11:31 <merijn> And then collapse everything using unionWIth
11:32 <merijn> eh, maybe like
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11:32 <merijn> :t map (\(t1, t2, t3) -> (t1, M.singleton t2 [t3]))
11:32 <lambdabot> [(t1, k, t)] -> [(t1, M.Map k [t])]
11:33 <merijn> :t map (\(t1, t2, t3) -> M.singleton t1 (M.singleton t2 [t3]))
11:33 <lambdabot> [(k1, k, t)] -> [M.Map k1 (M.Map k [t])]
11:33 <merijn> :t M.unionWith (M.unionWith mappend)
11:33 <lambdabot> (Monoid a, Ord k, Ord k1) => M.Map k1 (M.Map k a) -> M.Map k1 (M.Map k a) -> M.Map k1 (M.Map k a)
11:33 <merijn> ANd then you can unwrap via M.toList
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21:03 <abhiroop> I am trying to run a single Haskell file
21:04 <abhiroop> which involves some multi threading
21:04 <abhiroop> I tried `ghc -threaded -rtsopts -with-rtsopts=-4 Abc.hs`
21:04 <abhiroop> it creates Abc.hi and Abc.o files
21:05 <abhiroop> But it doesn't link them
21:05 <glguy> abhiroop: Make sure the name of the main module is Main
21:05 <geekosaur> if there is no Main module containing a 'main' CAF
21:05 <abhiroop> Its just 1 file
21:05 <geekosaur> (or you can use -main-is to change the module and CAF name)
21:05 <abhiroop> Should I call it Main?
21:05 <geekosaur> doesn't matter
21:06 <geekosaur> yes
21:06 <geekosaur> haskell is not a shell, haskell is not a simple scripting language that runs commands sequentially
21:06 <geekosaur> it's more like java where you need 'public static void main' or C where you need 'int main(int argc, char **argv)'
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22:02 <saussure> Going through a Parser tutorial with the following : newtype Parser a = Parser { parse :: String -> [(a,String)] }
22:02 <saussure> This would be considered "record" syntax, right?
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22:07 <thaunatos> Hello! I'm having trouble using the interact function. Typing then pressing enter doesn't print anything - but when I ctrl-c out the string is printed.
22:07 <thaunatos> I suspected this has to do with something called "laziness" but I'm not sure how to fix it.
22:08 <jle`> thaunatos: can you paste the code you're using?
22:08 <jle`> saussure: yes
22:08 <thaunatos> Here you go: http://lpaste.net/355690
22:09 <jle`> thaunatos: ah yeah it actually is from your terminal's buffering settings
22:09 <thaunatos> I'm also using Windows, in case that's relevant.
22:09 <jle`> normally, terminals buffer outputs
22:09 <jle`> they "buffer" all of the things they are supposed to print, and then print it all out at once when it receives a newline
22:10 <thaunatos> Ah. So I could just add a \n to print?
22:10 <jle`> when you do `printf "%0.2f" (typingDistance s)`, it sends whatever the distancec is to the terminal
22:10 <jle`> but the terminal won't print anything until it receives a newline
22:10 <jle`> yeah, printf "%0.2f\n" (typingDistance s) should force the buffer to flush
22:10 <jle`> assuming normal buffering settings
22:11 <jle`> this issue is unrelated to programming language; it has to do with the terminal you're using, or what you're running your program with. but admittedly it's not something that's taught very well when someone is learning how to program
22:13 <saussure> jle: in the above, you could pass a lambda to the "parse" to construct a value?
22:13 <saussure> a lambda that takes a string and returns a list of tuples that is.
22:15 <thaunatos> Hmm. Simply adding \n to the end of the format string didn't change the behavior.
22:16 <jle`> saussure: yes, you can pass in anything to Parser that has the right type
22:16 <jle`> using a lambda is one way to make a function of the proper type
22:16 <thaunatos> If I press enter twice after I enter the string, *sometimes* the answer is printed twice when I ctrl + c.
22:16 <jle`> but there are many others
22:21 <jle`> thaunatos: huh oh
22:21 <jle`> thaunatos: it looks like 'interact' does something different than what i expected
22:21 <jle`> it doesn't run the String -> String line-by-line
22:21 <jle`> it runs it on the entire input of stdin
22:21 <jle`> does not sound very useful
22:21 <jle`> you can use Ctrl+D to end your input
22:22 <jle`> you might have to write your own interact if you want line-by-line behavior
22:22 <thaunatos> Maybe. I tried forcing line buffering (using https://mail.haskell.org/pipermail/haskell/2003-September/012779.html) but that didn't seem to change anything.
22:22 <jle`> interact f = do l <- getLine; print (f l); interact f
22:22 <jle`> yeah it's not a line buffering issue, it's the fact that 'interact' is only run on the entire contents of stdin
22:22 <jle`> sorry about my previous guess earlier
22:22 <jle`> so even if you press enter, it takes the string you press as "blahblah\nblah blah"
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22:26 <thaunatos> That makes sense. I think that interact might have the behavior I expect when I run main with ghci.
22:27 <jle`> interact uses getContents, which has different behavior in different settings
22:27 <jle`> rather unfortunate
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22:28 <thaunatos> I'm going to run hlint on my program - I would be very grateful if you'd do a code review after I've made the suggested changes.
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22:33 <jle`> just paste it here :) if i'm not here, someone else probably will be
22:33 <thaunatos> Can do! Thank you so much for your help :-)
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22:40 <saussure> data Parser a = Parser { runParser :: String -> Maybe (a, String) }
22:40 <saussure> oneChar :: Parser Char
22:40 <saussure> oneChar = Parser $ \s -> case s of
22:40 <saussure> [] -> Nothing
22:40 <saussure> (c:xs) -> Just (c, xs)
22:41 <thaunatos> Hello! I've been working on daily programmer challenge #259 (link: https://www.reddit.com/r/dailyprogrammer/comments/4bc3el/)
22:41 <thaunatos> My solution is here: http://lpaste.net/355693
22:41 <thaunatos> I'd love to have a code review.
22:43 <saussure> I'm having difficulty following why runParser oneChar "Hello World" produces Just ("H", "ello world")
22:43 <saussure> I guess the sequence of calling runParser, the oneChar, then the String.
22:44 <jle`> saussure: what part do you not understnad?
22:44 <jle`> thaunatos: looks good :)
22:45 <jle`> i would have just used `printf " -> %.2fcm\n" (typingDistance s)` but i guess that's a style thing
22:45 <jle`> instead of 'tail', use 'drop 1'
22:45 <thaunatos> Why?
22:45 <jle`> tail is partial
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22:46 <saussure> can you explain how "runParser" works? It almost seems like Parser is getting passed a Parser
22:46 <jle`> saussure: do you know how record syntax works?
22:46 <saussure> That might be my confusion
22:46 <saussure> I just can't follow this in my head...
22:46 <jle`> something like data Person = P { name :: String, age :: Int }
22:46 <jle`> do you know what that means?
22:47 <saussure> right: let x = P "Saussure" 31
22:47 <saussure> It builds a type of Person
22:47 <jle`> yeah
22:47 <jle`> what about name, age?
22:47 <jle`> what does name do?
22:48 <saussure> hmm. doesn't it just provide a function to retrieve the name value?
22:48 <jle`> mhm
22:48 <saussure> x name
22:48 <jle`> not quite
22:48 <jle`> 'name x'
22:48 <jle`> name :: Person -> String
22:48 <saussure> ah.
22:48 <jle`> so runParser is a function to retrieve the (String -> Maybe (a, String)) from the Parser a
22:49 <saussure> Yeah, I'm laughing to myself, because it's just not computing in my head.
22:50 <saussure> oneChar is of type Parser Char
22:50 <saussure> oneChar :: Parser Char
22:52 <thaunatos> Ooh zipWith exists
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22:57 <saussure> oh ok: so, runParser :: Parser a -> String -> Maybe (a, String)
22:59 <saussure> This could be read as: runParser is a function which takes a Parser a, and a String, and returns a Maybe (a, String) ??
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23:23 <glguy> saussure: basically. More specifically it's a function that takes a (Parser a) and returns a function that takes a String and returns a (Maybe (a, String))
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