Question

In Ruby, a function can return multiple* values. Is it possible for a Ruby function to determine how many return values its invoking code is expecting?

For instance:

a = f()                  # caller expects one return value
a, b = f()               # caller expects two return values
a, b, c = f()            # caller expects three return values

If we let r be the number of expected return values, is it possible to write a function f such that it can find r?

Specifically, how could we change the definition of rcount, below, such that is passes the following tests:

a, b = rcount()
puts "#{a},#{b}"           # FAIL, DESIRED: "2,1"      ACTUAL: "1,"
a, b, c = rcount()
puts "#{a},#{b},#{c}"      # FAIL, DESIRED: "3,2,1"    ACTUAL: "1,,"
a, b, c, d = rcount()
puts "#{a},#{b},#{c},#{d}" # FAIL, DESIRED: "4,3,2,1"  ACTUAL: "1,,,"

Where rcount is defined like this:

def rcount()
  ret = []
  r = nil                  # <== Q. IS IT POSSIBLE TO GET r PROGRAMMATICALLY?
  r ||= 1
  last = r - 1
  (0..last).each do |i|
    ret[i] = r - i
  end
  ret
end

*Actually multiple return values is syntactic sugar, the real return value is a single array. All Ruby functions return exactly one value.

Answer 1

No. The Ruby environment doesn’t give you that information. You could use an exception and ParseTree to find out what the calling code looks like, but that is overkill 🙂