Missing Integer

Haskell Missing Integer

import qualified Data.Set as Set

missingInteger :: [Int] -> Int
missingInteger a = go 1 (Set.toAscList (Set.fromList (filter (> 0) a)))
  where
    go m []       = m
    go m (v : vs)
      | m /= v    = m
      | otherwise = go (m + 1) vs

This records the positive numbers that exist, then returns the smallest positive value that is still missing.