Persistent maps and explicit lookup failure

The idea

Data.Map provides an ordered keyed structure rather than a list scanned for every lookup. Lookup returns Maybe, making missing keys visible without a partial indexing exception. Persistent updates return a new map while the old value remains available, so callers can reason about before and after states without mutation. Keys require an ordering instance consistent with their intended identity.

When several records share a key, choose whether later values replace earlier ones or combine them. InsertWith takes a combining function for the new and old values; argument order matters for noncommutative operations. Use qualified imports to keep map operations distinct from list functions. Test duplicate keys and missing keys, and do not confuse key-sorted traversal with first-occurrence insertion order.

import qualified Data.Map.Strict as M
main = do
  let old = M.fromList [("a",2 :: Int)]
      new = M.insertWith (+) "a" 3 old
  print (M.lookup "a" new)
  print (M.lookup "a" old)
  print (M.lookup "x" new)

Just 5
Just 2
Nothing

The new map combines three with two, while the persistent old map retains its original binding.