Question

While reading the book Programming Ruby, one example shows how blocks can be used as closure:

def nTimes(aThing)
  return proc {|n| aThing * n}
end

p = nTimes("Hello ")

Now if we output the value of p.call(3) , it would be Hello Hello Hello

However, if our code was simply puts 3 * "Hello " , Ruby would complain about incompatible type.

Why? Thanks.

Answer 1

Your problem has nothing to do with closures or blocks. It is related to how operators are handled in Ruby.

On binary operations like * and +, the object to the left of the operand is the receiver of the method. So when you do "hello " * 3 it calls the * method on the class String and passes 3 as a parameter. The definition of String#* takes integers as parameters and returns self repeated that many times, hence the output "hello hello hello ".

But if you phrase it as 3 * "hello ", the * method of the Fixnum class is called, and "hello " is passed as a parameter. Fixnum#* doesn’t know what to do with String parameters so you get an error.