modifyFirstOrSnoc.purs

modifyFirstOrSnoc ∷ ∀ a. (a → a → Maybe a) → Array a → a → Array a
modifyFirstOrSnoc g xs x =
  tailRec
    (\{ acc, xs' } →
       maybe
         (Done (acc `Array.snoc` x))
         (\{ head, tail } →
            maybe
              (Loop { acc: acc `Array.snoc` head, xs': tail })
              (Done ∘ (append acc) ∘ (flip Array.cons tail))
              (g head x))
       (Array.uncons xs'))
    {xs': xs, acc: [] }