Dictionary+merge.swift

extension Dictionary {
  mutating func merge(other: [Key: Value], by combine: (Value, Value) -> Value) {
    for (key, newValue) in other {
      if let oldValue = self[key] {
        self[key] = combine(oldValue, newValue)
      } else {
        self[key] = newValue
      }
    }
  }

  func merged(other: [Key: Value], by combine: (Value, Value) -> Value) -> [Key: Value] {
    var copy = self
    copy.merge(other, by: combine)
    return copy
  }
}