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04:25 <mac10688> https://stackoverflow.com/questions/43533947/getline-within-list-comprehension
04:25 <mac10688> if anyone knows the answer, I would appreciate it
04:28 <glguy> mac10688: [ getLine | y <- [0..4]] isn't a function, is your goal just to produce an action that reads 5 lines from that?
04:28 <glguy> There's sequence [ getLine | y <- [0..4]], and there's replicateM 5 getLine
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04:29 <glguy> but you won't be executing getLine from inside the *list* comprehension
04:30 <mac10688> i'm working on a codeingame problem. I want to build a matrix using a list comprehension. I get the values by using the getLine function
04:30 <mac10688> and using a list comprehension is the only way I know to keep up with the index so I can mutate the cells into coordinates
04:31 <mac10688> I think the sequence thing is doing the trick!
04:32 <mac10688> this is awesome, thanks!
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05:00 <Cale> mac10688: If you haven't looked at how sequence is implemented, it might be quite instructive
05:00 <Cale> mac10688: The basic idea is that we want to turn a list full of IO actions into a single IO action which will run them all, and collect up a list of the results
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05:01 <Cale> sequence [] = ... -- in the case that there are no actions, we want to produce an action which does nothing, except to result in an empty list, so that's what return will get us:
05:01 <Cale> sequence [] = return []
05:02 <Cale> sequence (x:xs) = ... -- in the case of a nonempty list which starts off with x...
05:02 <Cale> sequence (x:xs) = do v <- x; ... -- we'll first run x, getting some result v
05:02 <Cale> and now we want to run the rest of the list of actions, collecting up a list of results -- thankfully, we have a perfectly good function to do that:
05:02 <Cale> sequence (x:xs) = do v <- x; vs <- sequence xs; ... -- the function we're writing of course
05:03 <Cale> and then we just need to put the initial result and the rest of the results together:
05:03 <Cale> sequence (x:xs) = do v <- x; vs <- sequence xs; return (v:vs)
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05:20 <mac10688> Oh thanks Cale , I'm just now seeing your response
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05:21 <mac10688> I've worked through sequence in the haskell book, but I guess it's been a while and I wasn't used to seeing it in action with an IO monad. It's still not totally intuitive but let me see
05:22 <mac10688> that's a smooth implementation, totally makes sense now
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06:14 <eatman> Hi! Good morning.
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06:19 <trudyjane> Good morning, Have a question about the traversable instance for the Constant type.
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06:23 <trudyjane> In traverse _ (Constant x) = pure (Constant x), it seems it would just return a Constant. How does traverse know to embed a Constant in the 'f' passed to it?
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06:25 <jle`> trudyjane: hm, what do you mean?
06:26 <jle`> trudyjane: what 'f' do you mean?
06:26 <jle`> the Applicative, or the function?
06:26 <trudyjane> pure embeds the Constant a in a structure, but Constant's applicative instance just returns a Constant.
06:26 <jle`> we aren't using Constant's Applicative instance
06:27 <trudyjane> Oh, so the pure that is being called is based on the 'f' passed in?
06:27 <jle`> it's based on the type
06:27 <jle`> traverse :: Applicative f => (a -> f b) -> Constant c a -> f (Constant c b)
06:27 <jle`> it's the f there
06:28 <jle`> the f in the type signature
06:28 <trudyjane> I see now it makes sense, thank you. Was turned around by looking at Constant's applicative pure definition.
06:28 <jle`> no problem!
06:29 <trudyjane> Helps to look at type signatures too : )
06:29 <jle`> it's indirectly potentially related to the function passed in, because the type of the function passed in may help Haskell with type inference for what the 'f' type variable is instantiated to
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06:30 <jle`> but haskell might also be able to infer what 'f' is from the context in which the result is used, as well
06:30 <trudyjane> That's what I was wondering about.
06:30 <jle`> > traverse pure (Const 10) :: Maybe (Const 10)
06:30 <lambdabot> error:
06:30 <lambdabot> • Expecting one more argument to ‘Const 10’
06:30 <lambdabot> Expected a type, but ‘Const 10’ has kind ‘k0 -> *’
06:30 <jle`> aw
06:30 <jle`> > traverse pure (Const 10) :: Maybe (Const Int Double)
06:30 <lambdabot> Just (Const 10)
06:30 <jle`> there it knows that 'f' is Maybe because of the return type to expect
06:30 <jle`> > traverse Just (Const 10)
06:30 <lambdabot> Just (Const 10)
06:31 <jle`> there it knows that 'f' is Maybe because of the type of the function given to it
06:31 <jle`> type inference is fun :)
06:31 <trudyjane> Yes!
06:31 <jle`> probably should have used a better function
06:31 <jle`> > traverse (\_ -> Nothing) (Const 10)
06:31 <lambdabot> Just (Const 10)
06:31 <trudyjane> That's nice, I was using (\x -> Just x)
06:32 <jle`> yeah, but that example is a little misleading because it makes it look like traverse = ($), heh
06:32 <trudyjane> Ha!
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06:34 <eatman> Hi! I'm trying to use Data.Graph but I'm havin issues when it comes to use a graph as a Functor instance.
06:34 <eatman> My Graph represent a Game Of Life board.
06:35 <eatman> (Every cell connected to its neighbors.)
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06:35 <eatman> Is there any *easy* way to map over the cells without having to make many functions call to retrieve neigbhors?
06:36 <eatman> I thought the cells (state, key, neighbors) would have benn accessible.
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06:36 <jle`> eatman: what package are you using?
06:36 <eatman> where neighbors == [key].
06:36 <eatman> Data.Graph.
06:36 <jle`> fgl?
06:37 <jle`> that's a module
06:37 <jle`> but what package provides it?
06:37 <eatman> Well, I've no idea.
06:37 <jle`> what library?
06:37 <jle`> because it's not a part of the base library
06:37 <eatman> I added http://ix.io/rkl
06:38 <eatman> And then *import Data.Graph*.
06:38 <jle`> ah it's probably from 'containers' then
06:38 <jle`> since your library depends on 'containers'
06:38 <jle`> and 'containers' does export a module called Data.Graph
06:38 <eatman> Yep, you're right.
06:38 <jle`> what is your graph type?
06:38 <eatman> Have I done it the wrong way?
06:38 <jle`> is it 'Graph' ?
06:39 <eatman> http://ix.io/rkn/hs
06:39 <eatman> Yep.
06:39 <jle`> i've never used container's graphs myself; the typical graph library people use is the graphs from fgl, but i'm sure it's fine for simple use cases
06:40 <eatman> I didn't know there would be more than one Data.Graph.
06:40 <eatman> (Noob)
06:40 <jle`> well, there's only one Data.Graph that i know of
06:40 <eatman> Anyway, I'm using the graphFromEdges :: Ord key => [(node, key, [key])] -> (Graph, Vertex -> (node, key, [key]), key -> Maybe Vertex)
06:41 <eatman> Source
06:41 <jle`> but there are multiple packages that implement graphs
06:41 <jle`> but, graphs in haskell really aren't all that common either
06:41 <eatman> Well, I couldn't think of something better to represent a rectangular board game.
06:42 <jle`> are you talking about conway's game of life
06:42 <jle`> or like, the board game
06:42 <eatman> Yep.
06:42 <jle`> oh
06:42 <jle`> it's probably better to just use a 2d array
06:42 <jle`> or even a vector of vectors
06:42 <eatman> Well... sounds easer but not FP-friendly.
06:42 <jle`> how do figure?
06:43 <eatman> ?
06:43 <jle`> why do you think so?
06:43 <jle`> sorry, i meant to say how do you figure, heh
06:44 <eatman> Because I come from C++ and 2D Arrays of vectors of vectors sounds pretty much like nested loops to me.
06:44 <jle`> hm
06:44 <jle`> nested loops is control flow
06:44 <jle`> vectors and arrays are just data
06:44 <jle`> it's very haskell :)
06:44 <jle`> pure vectors and pure arrays
06:44 <jle`> a nested vector type might be Vector (Vector Double)
06:45 <jle`> er sorry, Vector (Vector Bool)
06:45 <jle`> and you can fmap over it
06:45 <jle`> (fmap . fmap) :: (a -> b) -> Vector (Vector a) -> Vector (Vector b)
06:45 <jle`> etc.
06:45 <jle`> you can even generate vectors purely using the generate function
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06:45 <jle`> generate :: Int -> (Int -> a) -> Vector a
06:46 <jle`> give a size, and a function from index to value, and you get a vector! :)
06:46 <jle`> generate 10 (\x -> x**2) gives you a vector of squares from 0 to 81
06:46 <jle`> super functional :)
06:46 <jle`> no nested loops anywhere
06:46 <eatman> But, I need to access each neighbors.
06:47 <jle`> yes, which is why vectors support indexing
06:47 <eatman> Yeah, this is the ugly part to me.
06:47 <jle`> indexing?
06:47 <jle`> indexing is what vectors are best at :)
06:47 <eatman> Given an index, generate the neighbors indexes.
06:47 <jle`> sounds like a pure function to me
06:48 <eatman> You're probably right :-)
06:48 <jle`> in fact this is probably much uglier in an imperative language
06:48 <jle`> which might be why you are averse to it
06:48 <eatman> Have to go for a coffe with coworkers, will be back in 15.
06:48 <eatman> Thanks a lt for the advices
06:48 <jle`> have a nice coffee! and no problem
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07:01 <eatman> Back.
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07:02 <eatman> So, in order to avoid the same "error" as with Graphs.
07:02 <eatman> Wich Vector am I suppoed to use?
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07:08 <jle`> you can either use nested vectors or 2d arrays
07:08 <jle`> or you can use a flat array and do some math to manage the indexing
07:09 <jle`> one simple way would be to make your update function a Vector (Vector Bool) -> Vector (Vector Bool)
07:10 <jle`> updateGame v = generate 10 (\x -> generate 10 (\y -> ...use logic to generate the tile at (x,y).....))
07:11 <jle`> for example, if you wanted to just have the new board be identical to the previous board
07:11 <jle`> updateGame v = generate 100 $ \x ->
07:11 <jle`> generate 100 $ \y ->
07:12 <jle`> (v ! x) ! y)
07:12 <jle`> er the last parentheses should be there, but yeah
07:12 <jle`> assuming your boards are 100 x 100
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07:12 <jle`> that's just one simple way to do it :)
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10:23 <Akii> IO is scary af
10:23 <Akii> really having problems sticking my app together now
10:23 <Akii> because eveything can go to hell everywhere
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11:39 <Akii> how does one manage dependencies on a larger scale?
11:39 <merijn> Akii: What do you mean?
11:40 <Akii> well I've a somewhat bigger application now with little data types flying around everywhere that express the dependency of a sub component
11:40 <Akii> it's all a mess, really
11:41 <merijn> Akii: What kinda dependencies are you expressing?
11:41 <merijn> (brb, meeting)
11:42 <Akii> anyone got an example of a larger Haskell application?
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11:51 <yushyin> ghc
11:51 <tuturto> akii: https://github.com/LambdaHack/LambdaHack
12:05 <merijn> Akii: GHC, Pandoc, Xmonad? :)
12:06 <Akii> ye that's not what I need right now
12:06 <Akii> nvm
12:06 <Akii> looking at the recently released wire server was benefitial
12:06 <Akii> but that's just small stuff
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13:05 <evilmaid> i think rumuki was written in haskell aswel, hold on
13:06 <evilmaid> https://github.com/rumuki/rumuki-server not sure if it qualifies as large though
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13:18 <Akii> evilmaid: thanks, will check it out
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13:19 <Akii> random question: anyone in here working for codecentric?
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13:29 <Akii> evilmaid: bit small indeed
13:30 <Akii> I think the issue I'm having is purely with composition
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13:30 <Akii> and that's one of the things where coming up with right questions is so hard
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13:49 <ThomasLocke> Anybody got some good links on acid-state and some good patterns for usage? I'm dabbling and having some difficulties figuring out how to design my types.
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13:54 <merijn> ThomasLocke: Well, what are you trying to store in it?
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13:56 <ThomasLocke> merijn, Lists of things really. Imagine People, Deps, Orgs and Docs.
13:57 <merijn> What's the difficulty?
13:57 <ThomasLocke> Many many many Docs, fewer People, even fewer Deps and perhaps a few hundred Orgs.
13:57 <ThomasLocke> Should I just dump all of it in the same acid-state db or one for each type?
13:58 <merijn> I generally recommend people to not consider acid-state a database at all
13:58 <ThomasLocke> I understand. I'm just trying to learn and experiment.
13:58 <ThomasLocke> I have done it with SQLite, now I'm looking to learn something different. :)
13:59 <* ThomasLocke> is very bad at Haskell
13:59 <merijn> acid-state isn't very well suited to the relational model of "storing bunches of lists"
14:01 <ThomasLocke> merijn, What is it good for then? I'd like to learn, so what kind of types should I mock to learn to use it?
14:01 <merijn> ThomasLocke: It's good for situations where you have some Haskell datatype for which you want ACID guarantees, like the state of a daemon or something
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14:03 <ThomasLocke> So if a have a map type, and I'd like to have ACID and disk-backing, then it could also fit?
14:04 <ThomasLocke> Basically a simple key-value store with a few extra bells and whistles?
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14:06 <Akii> ThomasLocke: was it you who was working on this kind of queue thing?
14:06 <ThomasLocke> Akii, Ah yes, my queue experiement!
14:06 <ThomasLocke> Akii, That was fun, albeit completely useless. :D
14:07 <Akii> haha
14:07 <Akii> :D
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14:07 <ThomasLocke> But it worked, and that was a proud moment for me: I'd made something in Haskell, and it did what I wanted. WOoooooo
14:07 <eatman> jle`: Thanks for the 2D Array advice, it works like a charm.
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14:09 <ThomasLocke> Akii, You can see the entire thing in all its "glory" here: http://hub.darcs.net/ThomasLocke/haskell-playground/browse/jobQueue
14:09 <eatman> Hey, I now have a **splendid** working state monad playing the Conway's game of life.
14:09 <ThomasLocke> eatman, Congrats!
14:10 <eatman> How can I make it run "forever" and pick the results one after the other?
14:10 <eatman> ThomasLocke: Thanks.
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14:11 <eatman> ThomasLocke: pretty much like you said : "I've a useless working piece of code that make me feel **Wooooooo**"
14:11 <ThomasLocke> eatman, It's a damn good feeling!
14:11 <ThomasLocke> It might be the most sucky Haskell ever written, but it is mine dammit! :D
14:16 <Akii> ThomasLocke: nice :D
14:17 <ThomasLocke> I'm fairly happy with it. Not too much dark magic going on.
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14:30 <eatman> Am I wrong when thinking about the State monad when I want to run my "game of life" indefinitely and get an infinite list of the states as a result?
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14:33 <ski> @type iterate
14:33 <lambdabot> (a -> a) -> a -> [a]
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14:36 <eatman> Ahah, no need for this State thing...
14:37 <eatman> As usual it's easier than it looks.
14:37 <Akii> I'm not getting it
14:37 <Akii> :D
14:38 <eatman> To conclude my Haskell week : I still can't find any use for the Reader/State monads.
14:39 <glguy> you never "need" either of those types, they can just be useful if you're reimplementing what they do inline
14:39 <Akii> eatman: I use the State Monad to add the effect of emitting domain events to my functions
14:41 <glguy> they're most useful in isolated bits or when implementing some other opaque type
14:41 <eatman> That, can also be what limitates my understanding of the whole thing : I've no idea what "add the effect of emitting domain events".
14:42 <eatman> *means.
14:45 <Akii> gimmeh a sec, writing an example
14:45 <Akii> but it'll be a boring business example
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15:03 <Akii> eatman: http://lpaste.net/354801
15:03 <Akii> boring basket events hype
15:03 <ski> eatman : a toy example. given `data Tree a = Tip | Node a (Tree a) (Tree a)', write `label :: Tree a -> Tree (Integer,a)' which labels the elements in a pre-order traversal, counting up from `0'
15:04 <ski> then, if you want to, write `normalize :: Fractional a => Tree a -> Tree a' which subtracts the mean of the elements, from each element, with a single tree traversal
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15:27 <eatman> Akii, ski: thanks.
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15:38 <eatman> ski: Ok, got it, not useless, only that I wasn't using it purposely.
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16:43 <saylu> Hey folks!
16:43 <saylu> I’m processing some data and wonder if you can recommend the correct way to structure it.
16:43 <saylu> I’m combining several CSVs together, with a ton of data about a specific URL that stays constant. So one URL will be associated with 20 “columns” of data.
16:44 <saylu> I’ll then be aggregating data and making calculations from it and so on. For example, one column is “Category”, and many URLs can share the same category. So I might want to find the average of a column for that category.
16:44 <saylu> Does a record type seem the right fit, or Data.Map?
16:44 <Akii> Map URL [Column]
16:44 <Akii> could work
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16:48 <Akii> well map reduce err foldl the thing
16:50 <saylu> Foldl+
16:50 <saylu> *foldl?
16:50 <Akii> :t foldl
16:50 <lambdabot> Foldable t => (b -> a -> b) -> b -> t a -> b
16:50 <saylu> Oh, sorry, just mean -- what for?
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16:50 <saylu> Oh, for aggregations?
16:50 <Akii> yup
16:50 <saylu> Right. Yea, that makes sense to do it that way.
16:50 <Akii> well, that's so generic
16:51 <Akii> have you looked at pipes?
16:51 <Akii> like the library
16:51 <saylu> I haven't, no
16:52 <Akii> that's basically stream processing
16:52 <saylu> Checking it now
16:52 <Akii> depending on your level of Haskell which is unknown to me that might be a bit much
16:54 <saylu> I'm getting there, slowly but surely :) Finished most of the haskell book, with exercises, up to the monad transformers chapter I'm in now
16:54 <saylu> So this would be in order to stream data around rather than load big chunks of it into memory then process it?
16:55 <saylu> Fortunately these csvs won't have more than 100,000 rows at any point
16:55 <Akii> well ye, pipes are probably overkill
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16:55 <Akii> I was thinking you could define pipes that transform your data
16:55 <Akii> but well, might as well just use functions
16:56 <Akii> with 100k rows you can easily map over them many times
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17:02 <saylu> I think I'll start without pipes, but it looks interesting, and would make useful practice to extend with them.
17:02 <saylu> is pipes the same concept as conduit?
17:02 <MarcelineVQ> conduits, the other main lib that does what pipes does, also has some stuff for csv
17:02 <MarcelineVQ> I've only just glanced at it though so I couldn't say what's a good route
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17:06 <saylu> Oh, very interesting
17:06 <saylu> https://github.com/ozataman/csv-conduit
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17:07 <saylu> I think mine are a bit too small for this use case, but a bit later in the year I'll be working on a few csvs with a few million rows and this will be helpful -- especially over HTTP
17:07 <saylu> thanks for the notes Akii MarcelineVQ
17:08 <MarcelineVQ> I tried using that but it turned out I don't know how to use conduit hehe, I'll let you know if it works well when I get back to it
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17:24 <saylu> In record types, is camelCase or under_score "accepted style"?
17:28 <ski> i think `camelCase' is the more common naming convention for field names
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17:29 <saylu> What about with pre-pending?
17:29 <saylu> say I have pageTitle, wordCount
17:29 <saylu> but I want to prefix it
17:29 <saylu> so, say sf_pageTitle, sf_wordCount
17:29 <saylu> would this still be a time to use camel case?
17:30 <saylu> sfPageTitle, sfWordCount
17:30 <ski> i'd probably capitalize the previous first letter like that, yes
17:30 <ski> not sure how common it'd be
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17:31 <saylu> Sure thing!
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17:32 <geekosaur> there's a quasi-convention that underscores represent a marshaling type and camel case a Haskell type, when they differ
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17:38 <saylu> Oof
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17:39 <saylu> Anyone know how to skip a line in Cassava, to use the _second_ line as the header?
17:40 <Akii> saylu: don't know but maybe you can skip the line before giving it to Cassava
17:41 <Akii> if that helps
17:42 <saylu> Hmm
17:43 <saylu> I suppose I read it in as a ByteString, so if I can throw away the first "row" of the bytestring then I'd be good to go
17:43 <saylu> But since Cassava is already there to parse the CSV, seems it would be more convenient to use that directly
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17:45 <saylu> unfortunately the first row of my CSV is wasted on the text "internal_html" in the first column and nothing else
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17:46 <Akii> why tho
17:46 <Akii> I'd just throw that line away
17:46 <saylu> Ha! Just what the tool that generates these outputs
17:47 <saylu> *adds
17:47 <Akii> that's just the same with the JSON the other day
17:47 <Akii> where you have some strange "__embedded" things
17:52 <evilmaid> sounds absurd =^.^=
17:52 <Akii> nobody would ever do that
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17:56 <USBoss> When I run stack setup, GHC fails to compile a sanity check because the sanity check file is not present in my temp folder (Windows 10). Can anyone tell me why it would not be there?
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17:58 <USBoss> \join #haskell
17:59 <Akii> it's `/join #haskell`
17:59 <evilmaid> or turn that \ upside down
17:59 <Akii> flip \
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18:01 <USBoss> always forget which slash it is
18:08 <ski> \ɾoı̣u #ɥɐsʞǝןן
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18:09 <Akii> `:t (/join) :: IO ()` tho
18:09 <xunien> aha seriously
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19:31 <evilmaid> nice little anecdote "thinking in types" https://robots.thoughtbot.com/thinking-in-types nice example of embracing haskells type system to work around heterogenous lists
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19:49 <abhiroop> I want to use the `modify` function of State monad inside a let binding which resides inside a do notation. Something like this: http://lpaste.net/354812
19:49 <abhiroop> How do i go about it
19:51 <geekosaur> what the paste shows looks reasonable; do you have an actual failing case, and if so how does it fail?
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20:03 <abhiroop> geekosaur: What I actually have is this: http://lpaste.net/354813
20:04 <abhiroop> I cant write modify inside the let clause
20:04 <abhiroop> nor in the in clause
20:04 <geekosaur> yes, becuase let doesn;t work that way, you want to write the modify in the bindings
20:04 <geekosaur> which makes no sense
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20:05 <geekosaur> s/you want to/you are trying to/
20:06 <geekosaur> possibly you intended a do in there, but I can't quite tell from that little information
20:07 <geekosaur> alternately you wanted let ... in do ...
20:07 <geekosaur> but then you can just write the let in the do-specific format without the in
20:08 <abhiroop> Yeah I wanted to do something like let .. in do
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21:17 <saylu> Hmm -- I need to consolidate several csvs keyed by the first column, URL. If I'm going to be doing a ton of lookups (they aren't in the same order) does it make sense to use a map of 12-tuples rather than a list of record types (with 12 fields)?
21:19 <saylu> Parse each csv into a map of tuples keyed by URL or into a list of record types with a "URL" field, and then combine into a final map of tuples or record type with the consolidated data
21:19 <saylu> Kind of like doing lookups in Excel from multiple sheets
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21:21 <monochrom> Yes, I would use a dictionary, Data.Map or Data.Hashmap.
21:22 <monochrom> In fact, I did.
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21:25 <saylu> Oh -- well, I suppose a map of records would work fine
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23:12 <reptar_> how would i go about splitting a list at a specific element?
23:14 <reptar_> like ["frog", "frog", "fish", "frog", "fish", "frog", "frog", "frog", "frog"] -> [["frog", "frog"], ["frog"],, ["frog", "frog", "frog", "frog"]]
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23:27 <thang1> Haskell doesn't have a built in solution for this. Look up "take" and "drop"
23:28 <thang1> nvm thought you wanted a list slice. You want something different
23:29 <thang1> reptar_: check out Data.List.Split. It has functions splitOn, splitOneOf, splitWhen, and so on
23:30 <reptar_> thang1: will do, thanks!
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23:32 <thang1> In particular, splitAt "fish" [list....] should do what you want
23:33 <thang1> (btw: I got this by googling "split list on predicate in haskell")
23:33 <thang1> > splitOn "fish" ["frog","frog","fish","frog","fish","frog","frog","frog","frog"]
23:33 <lambdabot> error:
23:33 <lambdabot> • Couldn't match expected type ‘Char’ with actual type ‘[Char]’
23:33 <lambdabot> • In the expression: "frog"
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23:37 <thang1> Ahh whoops, strings are lists of char so it's a list of lists. It'll choke on that a little
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23:40 <thang1> >split (startsWith "fish") ["frog","frog","fish","frog","fish","frog","frog","frog","frog"] -- This shouldn't work either
23:40 <thang1> > split (startsWith "fish") ["frog","frog","fish","frog","fish","frog","frog","frog","frog"] -- This shouldn't work either... even when typed correctly
23:40 <reptar_> thang1: it's ok, i can use getContents, then splitOn, then split the frogs
23:40 <lambdabot> error:
23:40 <lambdabot> • Couldn't match expected type ‘Char’ with actual type ‘[Char]’
23:40 <lambdabot> • In the expression: "frog"
23:41 <thang1> no problem, glad I could help. The split library has combinators so that it's possible to create some fancier stuff if you get so inclined
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