Share the recursive pair and inspect the predicate once:
partition :: (a -> Bool) -> [a] -> ([a],[a])
partition _ [] = ([],[])
partition p (x:xs)
  | p x       = (x:yes,no)
  | otherwise = (yes,x:no)
  where ~(yes,no) = partition p xs
For a finite list, structural induction establishes that yes and no are the two required filtered subsequences. The guard evaluates p x once and prepends x to exactly one output, preserving input order in both. Fully consuming both results uses predicate applications and additional work. Sharing the recursive result avoids making separate recursive traversals. This is a stable list partition.

Ancestors (12)

  1. e
  2. 3
  3. Functional programming, design and analysis of algorithms (resit)
  4. 2022
  5. Past exam of the year 1 of the computer science course of the University of Oxford
  6. Year 1 of the computer science course of the University of Oxford
  7. Computer science course of the University of Oxford
  8. Undergraduate course of the University of Oxford
  9. Course of the University of Oxford
  10. University of Oxford
  11. List of universities
  12. Home

View article source

Discussion (0)

New discussion

There are no discussions about this article yet.

Articles by others on the same topic (0)

There are currently no matching articles.
See all articles in the same topic Create my own version