Question

I understand import Data.List.

But what does qualified mean in the statement import qualified Data.List?

Answer 1

A qualified import makes the imported entities available only in qualified form, e.g.

import qualified Data.List

result :: [Int]
result = Data.List.sort [3,1,2,4]

With just import Data.List, the entities are available in qualified form and in unqualified form. Usually, just doing a qualified import leads to too long names, so you

import qualified Data.List as L

result :: [Int]
result = L.sort [3,1,2,4]

A qualified import allows using functions with the same name imported from several modules, e.g. map from the Prelude and map from Data.Map.