Multiple recursion functions in haskell

Question

I need to recurse through two lists in a function and I can't see a way to do it.

siteRating6OrHigher :: [Film] -> [String]
siteRating6OrHigher [] = []
siteRating6OrHigher ((Film title _ _ ((_, rating):ratings)):restOfFilms)
    | rating >= 6 = [title] ++ siteRating6OrHigher restOfFilms
    | otherwise = siteRating6OrHigher restOfFilms

My data is formatted: testDatabase = [Film "Blade Runner" "Ridley Scott" 1982 [("Amy",6), ("Bill",9), ("Ian",7), ("Kevin",9), ("Emma",4), ("Sam",5), ("Megan",4)], etc

So currently I am recursing through the 'restOfFilms' but i also need to recurse through the list of ratings that are in the Film type. Is there a way to do this so it recurses through each set of ratings for each film?

Thanks

data structure:

data Film = Film String String Int [(String, Int)]
        deriving (Eq,Ord,Show,Read)

Can you provide the data structures you're working with so the example is self-contained? — Stephen Diehl
– Stephen Diehl, Commented Feb 24, 2014 at 18:12
check if any rating in the list of ratings for a film is above six. My current method only checks the first one in the list — user3347868
– user3347868, Commented Feb 24, 2014 at 18:20

Mark H · Accepted Answer · 2014-02-24 18:47:26Z

3

I would fmap over the list, converting any ratings > 6 into boolean True, then use foldr to go through the list with (||) and default value False, which should reduce the list to a single True if any are above 6 and False otherwise.

foldr (||) False (fmap ((> 6) . snd) ratings)

EDIT: Simpler, as per Rhymoid's suggestion

EDIT 2: The whole solution.

siteRating6OrHigher :: [Film] -> [String]
siteRating6OrHigher [] = []
siteRating6OrHigher ((Film title _ _ ratings):restOfFilms)
    | above60 ratings = [title] ++ siteRating6OrHigher restOfFilms
    | otherwise = siteRating6OrHigher restOfFilms
  where above60 = any ((> 6) . snd)

edited Feb 24, 2014 at 18:47

answered Feb 24, 2014 at 18:30

Mark H

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6 Comments

user824425 Over a year ago

Note that any p = foldr (||) False . map p is provided by the Prelude: you can just use any (> 6) instead.

user3347868 Over a year ago

That compiles fine but is still only returning the films that have a rating of greater than 6 in the first one, it doesnt seem to be recursing the other ones

user3347868 Over a year ago

is it possible to do the any .. as a list comprehension

bheklilr Over a year ago

@user3347868 You could do any [r > 6 | (_, r) <- ratings], but I wouldn't say it's more readable than any ((> 6) . snd)

user3347868 Over a year ago

@bheklilr tried

siteRating6OrHigher :: [Film] -> [String] siteRating6OrHigher [] = [] siteRating6OrHigher ((Film title _ _ ratings):restOfFilms)     | (any [rating >= 6 | (_, rating) <- ratings ]) == True = [title] ++ siteRating6OrHigher restOfFilms     | otherwise = siteRating6OrHigher restOfFilms

but it doesnt work

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