Combining multiple lists with strings in haskell

Question

For an assignment im trying to combine 4 lists of scraped data into 1. All 4 of them are ordered correctly and shown below.

["Een gezonde samenleving? Het belang van sporten wordt onderschat","Zo vader, zo dochter","Milieuvriendelijk vervoer met waterstof","\"Ik heb zin in wat nog komen gaat\"","Oorlog in Oekraïne"]
["Teamsport","Carsten en Kirsten","Kennisclip","Master Mind","Statement van het CvB"]
["16 maart 2022","10 maart 2022","09 maart 2022","08 maart 2022","07 maart 2022"]
["Directie","Bot","CB","Moniek","Christian"]

My desired output would be like this

[["Een gezonde samenleving? Het belang van sporten wordt onderschat", "Teamsport", "16 maar 2022", "Directie"], [...], [...], [...], [...]]

I've tried some of the solutions found on the internet but i don't understand some of them and most of them are about 2 lists or give errors when i try to implement them.

For more reference, my code looks like this:

urlString :: String
urlString = "https://www.example.com"

--Main function in which we call the other functions
main :: IO()
main = do
    resultTitle <- scrapeURL urlString scrapeHANTitle
    resultSubtitle <- scrapeURL urlString scrapeHANSubtitle
    resultDate <- scrapeURL urlString scrapeHANDate
    resultAuthor <- scrapeURL urlString scrapeHANAuthor
    print resultTitle
    print resultSubtitle
    print resultDate
    print resultAuthor

scrapeHANTitle :: Scraper String [String]
scrapeHANTitle =
    chroots ("div" @: [hasClass "card-news__body"]) scrapeTitle

scrapeHANSubtitle :: Scraper String [String]
scrapeHANSubtitle =
    chroots ("div" @: [hasClass "card-news__body"]) scrapeSubTitle

scrapeHANDate :: Scraper String [String]
scrapeHANDate = 
    chroots ("div" @: [hasClass "card-article__meta__body"]) scrapeDate

scrapeHANAuthor :: Scraper String [String]
scrapeHANAuthor =
    chroots ("div" @: [hasClass "card-article__meta__body"]) scrapeAuthor

-- gets the title of news items
-- https://www.utf8-chartable.de/unicode-utf8-table.pl?start=8192&number=128&utf8=dec
-- some titles contain special characters so use this utf8 table to add conversion
scrapeTitle :: Scraper String String
scrapeTitle = do
    text $ "a" @: [hasClass "card-news__body__title"]

-- gets the subtitle of news items
scrapeSubTitle :: Scraper String String
scrapeSubTitle = do
    text $ "span" @: [hasClass "card-news__body__eyebrow"]

--gets the date on which the news item was posted
scrapeDate :: Scraper String String 
scrapeDate = do
    text $ "div" @: [hasClass "card-news__footer__body__date"]

--gets the author of the news item
scrapeAuthor :: Scraper String String 
scrapeAuthor = do
    text $ "div" @: [hasClass "card-news__footer__body__author"]

I also tried the following below but it gave me a bunch of type errors.

mergeLists :: Maybe [String] -> Maybe [String] ->Maybe [String] -> Maybe [String] -> Maybe [String]
mergeLists = \s1 -> \s2 -> \s3 -> \s4 ->s1 ++ s2 ++ s3 ++ s4

Why are there four empty lists at the end?

willeM_ Van Onsem
– willeM_ Van Onsem

2022-03-23 20:00:07 +00:00
Commented Mar 23, 2022 at 20:00 — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Mar 23, 2022 at 20:00

willeM_ Van Onsem · Accepted Answer · 2022-03-23 20:25:15Z

1

You can make use of the Monoid instance and work with:

mergeLists :: Maybe [String] -> Maybe [String] ->Maybe [String] -> Maybe [String] -> Maybe [String]
mergeLists s1 s2 s3 s4 = s1 <> s2 <> s3 <> s4

Here you are however scraping the same page, so you can combine the data from the scraper with:

myScraper :: Scraper String [String]
myScraper = do
    da <- scrapeHANTitle
    db <- scrapeHANSubtitle
    dc <- scrapeHANDate
    dd <- scrapeHANAuthor
    return da ++ db ++ dc ++ dd

and then run this with:

main :: IO()
main = do
    result <- scrapeURL urlString myScraper
    print result

or shorter:

main :: IO()
main = scrapeURL urlString myScraper >>= print

edited Mar 23, 2022 at 20:25

answered Mar 23, 2022 at 20:05

willeM_ Van Onsem

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4 Comments

Cent Meister Over a year ago

I don't know if i'm stupid or not but when i try to apply your solution like this in the do loop, it only prints s1. i call it print (mergeLists resultSubtitle resultTitle resultDate resultAuthor)

willeM_ Van Onsem Over a year ago

@CentMeister: yes, I made a mistake. See edit.

Cent Meister Over a year ago

I have it semi-working right now. I changed return (da ++ db ++ dc ++ dd) Now however i get a list of [da, db, dc, dd] while my intention was

[[da(first element), db(first element), dc(first element), dd(first element)], [da(second element), db(second element), dc(second element), dd(second element)], [...], [...]]

willeM_ Van Onsem Over a year ago

@CentMeister: then you use return (transpose [da, db, dc, dd]). (with transpose from Data.List).

Francis King · Accepted Answer · 2022-03-24 09:52:11Z

0

You can combine four lists using zip4 from Data.List.

import Data.List

list1 = ["Een gezonde samenleving? Het belang van sporten wordt onderschat","Zo vader, zo dochter","Milieuvriendelijk vervoer met waterstof","\"Ik heb zin in wat nog komen gaat\"","Oorlog in Oekraïne"]
list2 = ["Teamsport","Carsten en Kirsten","Kennisclip","Master Mind","Statement van het CvB"]
list3 = ["16 maart 2022","10 maart 2022","09 maart 2022","08 maart 2022","07 maart 2022"]
list4 = ["Directie","Bot","CB","Moniek","Christian"]

result = zip4 list1 list2 list3 list4

result2 = [[x1,x2,x3,x4] | (x1,x2,x3,x4) <- zip4 list1 list2 list3 list4]

The two results differ slightly. Result result creates a list of tuples. Result result2 creates a list of lists, as requested. A list of tuples is probably better, because:

The list can contain any number of values, all of the same type (Haskell lists are homogenous)
Tuples can contain any types, so more flexibility
Tuples with two values are a different type than tuples with three values, so if you want collections of four values using tuples stops the user squeezing in a collection of three values or five values

answered Mar 24, 2022 at 9:52

Francis King

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2 Comments

Cent Meister Over a year ago

Thank you for your reply. In my case the lists are of equal length so that is not a problem. However is it right to assume that if i want to specify Types like type Title = String and use those for the scraping, that a list of tuples could be used?

Francis King Over a year ago

If you use zip, or a variant, the algorithm will run until one list runs out. Also, zip will take any types. :t zip4 -- zip4 :: [a] -> [b] -> [c] -> [d] -> [(a, b, c, d)] where a, b, c, d are types.

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