CODEWITHSUNDEEP
haskell
zer3f
zer3f

Reputation: 37

How to match one argument in a data that has multiple arguments with other data?

I have a created a data similarly like this

data Set = Set Integer [Char]

set1 :: [Set]
set1 = [ Set 1 ['a','f','k']
         ,Set 2 ['p','q','s']
       ]

If a given char is in the set1 function and it returns element in set1 or Nothing

checkChar :: Char -> [Set] -> Maybe Set

if i given 'p' to checkChar function with set1 as another parameter,then it should return Set 2 ['p','q','s']

what i tried

checkChar x [] =  Nothing
checkChar x (Set y [z] : Set ys [zs])   --Line 1
   | x `elem` [z] = Just $ Set y [z]
   | otherwise = checkChar x (Set ys [zs])  --Line 2

I got following errors for Line 1 and 2

Couldn't match expected type ‘[Set]’ with actual type ‘Set’

How to solve this?

How to match data that consists multiple arguments but want to check with one of the its argument with other data?

Upvotes: 1

Views: 126

Answers (1)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 476729

You can not write (… : Set ys zs). The (:) data constructor expects as second item a list of Sets not a Set object.

You can match this with:

checkChar :: Char -> [Set] -> Maybe Set
checkChar x [] = Nothing
checkChar x (Set y z : xs)
   | x `elem` z = Just (Set y z)
   | otherwise = checkChar x xs

Here we use xs as tail, which is a (possibly) empty list of Sets. Furthermore by using as pattern Set y z, this would only match lists with exactly one item. By using z, z will be assigned the entire list of items of the Set object.

Upvotes: 2

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