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For key-value lookup, use Data.Map:

import qualified Data.Map.Strict as Map

ages :: Map.Map String Int
ages = Map.fromList [("Alice", 30), ("Bob", 25)]

Map.lookup "Alice" ages   -- Just 30
Map.lookup "Carol" ages   -- Nothing
Map.size ages              -- 2
Map.insert "Carol" 40 ages -- new map with Carol added

The qualified import + as Map is conventional — Data.Map reuses many Prelude names (map, filter, etc.), so qualifying avoids clashes.

Data.Map is immutable — every "modification" returns a new map. Backed by a balanced binary tree, so operations are O(log n).

Data.Set works similarly for set operations:

import qualified Data.Set as Set

let s = Set.fromList [3, 1, 4, 1, 5]   -- {1, 3, 4, 5}
Set.size s                              -- 4 (deduplicates)
Set.member 4 s                          -- True