Question

I’m new to Ruby, but not to languages that allow lambda’s, such as groovy. So I saw this example:

myArray.product(otherArray).reject{|i,j| i > j}

in a ruby code block, and I hadn’t seen this block take 2 arguments before, but when I went to look at the documentation I can only see the documentation that says that it takes 1 argument. I looked at the same for the enumerable class, but that doc only shows 1 argument also.

I understand that it works, I guess I was hoping that there was an easier way to determine how many arguments it takes other then a guess and test method. How can I tell how many arguments a block takes in Ruby?

Answer 1

This works because Ruby supports destructuring.

Destructuring allows you to bind a set of variables to a corresponding set of values anywhere that you can normally bind a value to a single variable.

This allows the following to hold true:

arr = [1, 2]
x = arr
x == [1, 2] # true

y, z = arr
y == 1 # true
z == 2 # true

You can see from the following code that destructuring in arguments to blocks isn’t unique to the built-in methods that take a block:

def my_method(arr)
  yield arr
end

my_method([1, 2, 3]) {|x| puts x.inspect }
# => [1, 2, 3]
my_method([1, 2, 3]) {|x, y, z| puts x.inspect }
# => 1

Check out Destructuring with Ruby for more information.