Merge only the set fields from one record into another.

  operator :~:
    -> from as Address
    if from.street?
      street :=: from.street
    if from.postcode?
      postcode :=: from.postcode

  addr :~: update

defines module qa.operators.mergepartialrecord

  defines class

    Address
      street <- String()
      city <- String()
      postcode <- String()

      Address()
        ->
          street as String
          city as String
          postcode as String
        this.street :=: street
        this.city :=: city
        this.postcode :=: postcode

      //Constructor for partial updates — only postcode set
      Address()
        -> postcode as String
        this.postcode :=: postcode

      //Merge only copies SET fields from source
      operator :~:
        -> from as Address
        if from.street?
          street :=: from.street
        if from.city?
          city :=: from.city
        if from.postcode?
          postcode :=: from.postcode

      default operator ?
      default operator $
      default operator :=:

  defines program

    MergePartialRecordDemo()
      stdout <- Stdout()

      //Existing address with all fields set
      addr <- Address("10 High Street", "London", "SW1A 1AA")
      stdout.println(`Before: ${addr}`)

      //Partial update — only postcode is set via single-arg constructor
      update <- Address("EC2R 8AH")
      stdout.println(`Update: ${update}`)

      //Merge applies only the set postcode, leaves street and city alone
      addr :~: update
      stdout.println(`After merge: ${addr}`)

      addr.merge(update)

      addr :~: update