传递一个在Ruby中将参数作为参数的方法

Question

I'd like to calculate the difference for various values inside 2 hashes with the same structure, as concisely as possible. Here's a simplified example of the data I'd like to compare:

hash1 = {"x" => { "y" => 20 } }
hash2 = {"x" => { "y" => 12 } }

I have a very simple method to get the value I want to compare. In reality, the hash can be nested a lot deeper than these examples, so this is mostly to keep the code readable:

def get_y(data)
  data["x"]["y"]
end

I want to create a method that will calculate the difference between the 2 values, and can take a method like get_y as an argument, allowing me to re-use the code for any value in the hash. I'd like to be able to call something like this, and I'm not sure how to write the method get_delta :

get_delta(hash1, hash2, get_y)    # => 8

Answer 1

The "Ruby way" would be to pass a block:

def get_delta_by(obj1, obj2)
  yield(obj1) - yield(obj2)
end

hash1 = {"x" => { "y" => 20 } }
hash2 = {"x" => { "y" => 12 } }

get_delta_by(hash1, hash2) { |h| h["x"]["y"] }
#=> 8

A method could be passed (indirectly) via:

def get_y(data)
  data["x"]["y"]
end

get_delta_by(hash1, hash2, &method(:get_y))
#=> 8

Answer 2

Building on Stefan's response, if you want a more flexible get method you can actually return a lambda from the function and pass arguments for what you want to get. This will let you do error handling nicely:

Starting with the basics from above...

def get_delta_by(obj1, obj2)
  yield(obj1) - yield(obj2)
end

hash1 = {"x" => { "y" => 20 } }
hash2 = {"x" => { "y" => 12 } }

get_delta_by(hash1, hash2) { |h| h["x"]["y"] }

Then we can define a get_something function which takes a list of arguments for the path of the element to get:

def get_something(*args)
  lambda do |data|
    args.each do |arg|
      begin
        data = data.fetch(arg)
      rescue KeyError
        raise RuntimeError, "KeyError for #{arg} on path #{args.join(',')}"
      end
    end
    return data
  end
end

Finally we call the function using the ampersand to pass the lambda as a block:

lambda_getter = get_something("x","y")
get_delta_by(hash1, hash2, &lambda_getter)

That last bit can be a one liner... but wrote it as two for clarity here.

Answer 3

In Ruby 2.3, you can use Hash#dig method, if it meets your needs.

hash1.dig("x", "y") - hash2.dig("x", "y")
#=> 8