Question

If you have a list such as this in Haskell:

data TestType = A | B | C deriving (Ord, Eq, Show) 

List1 :: [TestType]
List1 = [A,B,C,B,C,A,B,C,C,C]

Is it possible to write a function to determin which element is represented the least in a list (so in this case ‘A’)

My initial thought was to write a helper function such as this but now I am not sure if this is the right approach:

appears :: TestType -> [TestType] -> Int
appears _ [] = 0
appears x (y:ys) | x==y = 1 + (appears x ys)
                 | otherwise = appears x ys

I am still fairly new to Haskell, so apologies for the potentially silly question.

Many thanks

Answer 1

Slightly alternative version to Matt’s approach

import Data.List
import Data.Ord

leastFrequent :: Ord a => [a] -> a
leastFrequent = head . minimumBy (comparing length) . group . sort