From a colleague:

  GPT Pretty smart(?!).

But this is of course simple, and searchable.

Still…

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GPT-4 when prompted:

Write binary search in Haskel. Print results using putStrLn. Ensure bug-free code by specifying preconditions, postconditions, invariants, variants, internal states.

gives the following response:

-- Binary search in Haskell
-- Precondition: the list is sorted in ascending order
-- Postcondition: the function returns the index of the target element if found, or Nothing otherwise
-- Invariant: the search space is always within the bounds of the list
-- Variant: the search space decreases by half at each recursive call
-- Internal state: the lower and upper bounds of the search space

binarySearch :: Ord a => [a] -> a -> Maybe Int
binarySearch xs x = go xs x 0 (length xs - 1)
  where
    go xs x lo hi
      | lo > hi = Nothing -- base case: search space is empty
      | x == midElem = Just mid -- base case: target element is found
      | x < midElem = go xs x lo (mid - 1) -- recursive case: search in the left half
      | otherwise = go xs x (mid + 1) hi -- recursive case: search in the right half
      where
        mid = (lo + hi) `div` 2 -- calculate the middle index
        midElem = xs !! mid -- get the middle element

-- Print results using putStrLn
main :: IO ()
main = do
  let xs = [1, 3, 5, 7, 9] -- a sorted list of integers
  let x = 5 -- a target element to search for
  let result = binarySearch xs x -- call the binary search function
  case result of -- pattern match on the result
    Nothing -> putStrLn "Element not found" -- print a message if Nothing is returned
    Just i -> putStrLn $ "Element found at index " ++ show i -- print a message with the index if Just i is returned

The above code runs:

https://onecompiler.com/cpp/3z48yt2pp