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crystal-lang

Is it possible to extend class for limited types only?

I would like to add normalize method to Arrays of Numbers only (Float & Int) as it doesn't make sense for String. But it won't compile, is that possible?

class Array(Number)
  def normalize
    multiplier = 1 / max
    map{ |v| multiplier * v }
  end
end

p [1, 2].normalize
Solution

  • This is not possible, as I'm sure you've seen. As such, you have two options.

    1) You can extend the class into a new one that just uses the type you want.

    class NumberArray < Array(Number)
  # Do something
end

    The obvious downside being that you can't use an Array(Number) as a NumberArray.

    2) You can write a guard clause around the normalize method that checks the array type and makes sure it's a number array.

    class Array(T)
  def normalize
    {% unless T < Number %}
      {% raise "Expected Number type for Array#normalize, got #{T}" %}
    {% end %}
    multiplier = 1 / max
    map { |v| multiplier * v }
  end
end