semantic/src/Patch.hs

module Patch where

-- | An operation to replace, insert, or delete an item.
data Patch a =
  Replace a a
  | Insert a
  | Delete a
  deriving (Functor, Show, Eq)

-- | Return the item from the after side of the patch.
after :: Patch a -> Maybe a
after (Replace _ a) = Just a
after (Insert a) = Just a
after _ = Nothing

-- | Return the item from the before side of the patch.
before :: Patch a -> Maybe a
before (Replace a _) = Just a
before (Delete a) = Just a
before _ = Nothing

-- | Calculate the cost of the patch given a function to compute the cost of a item.
patchSum :: (a -> Integer) -> Patch a -> Integer
patchSum termCost patch = (maybe 0 termCost $ before patch) + (maybe 0 termCost $ after patch)