Encontra a menor lista de uma lista de listas de maneira 'lazy'

gistfile1.hs

import Data.List
import Data.Function

--comparing length lazily
bigger::[a] -> [a] -> Ordering
bigger [] [] = EQ
bigger (_:_) [] = GT
bigger [] (_:_) = LT
bigger (_:as) (_:bs) = bigger as bs

smallest::[[a]] -> [a]
smallest = reverse.fst.smallest'.(map ((,) []))
    where 
        smallest' a = case find cmp a of
                          Just l -> l
                          Nothing -> smallest' $ map helper a
                          
        cmp (_ , []) = True
        cmp _ = False
        helper::([a], [a]) -> ([a], [a])    
        helper (as, (b:bs)) = (b:as, bs)

list = [[1..], [1..], [1,2,3], [1,2], [1..], [1..]]

main = do
    -- print $ minimumBy (compare `on` length) list
    -- bug. motivo: length de uma lista infinita não retornar
    
    -- print $ minimumBy (bigger) list
    -- bug. motivo: minumumBy compara 2 a 2
    
    print $ smallest list