Filter function

The Filter function is a HOF that takes another HOF, namely ByMake, and performs a data transformation.

func (cars Collection) Filter(fn FilterFunc) Collection {
   filteredCars := make(Collection, 0)
   for _, car := range cars {
      if fn(car) {
         filteredCars = append(filteredCars, car)
      }
   }
   return filteredCars
}