Diff.hs

-- | This module contains a utility for diffing two records.
--
-- @since 0.0.1.0
module Prairie.Diff
    ( module Prairie.Diff
    , module Prairie.Update
    ) where

import Prairie.Class
import Prairie.Update

-- | Given two 'Record's, this function produces a list of 'Update's that
-- can be performed on the first record such that it will equal the second.
--
-- @
-- 'updateRecord' ('diffRecord' old new) old == new
-- @
--
-- A @['Update' rec]@ can be serialized with 'ToJSON', sent over the wire, and
-- parsed with 'FromJSON', so you can efficiently and easily represent
-- patches to 'Record's.
--
-- @since 0.0.1.0
diffRecord
    :: (Record rec, FieldDict Eq rec)
    => rec
    -- ^ The old record.
    -> rec
    -- ^ The new record.
    -> [Update rec]
diffRecord old new = foldMap go allFieldsList
  where
    go (SomeField f) =
        withFieldDict @Eq f $
            let
                newVal = getRecordField f new
             in
                if getRecordField f old /= newVal
                    then [SetField f newVal]
                    else []