Creating a setter method that takes extra arguments in Ruby

Question

I'm trying to write a method that acts as a setter and takes some extra arguments besides the assigned value. Silly example:

class WordGenerator
  def []=(letter, position, allowed)
    puts "#{letter}#{allowed ? ' now' : ' no longer'} allowed at #{position}"
  end

  def allow=(letter, position, allowed)
    # ...
  end
end

Writing it as an indexer works and I can call it like this:

gen = WordGenerator.new

gen['a', 1] = true
# or explicitly:
gen.[]=('a', 1, true)

But when I try any of the following, the interpreter complains:

gen.allow('a', 1) = false # syntax error
gen.allow=('a', 1, false) # syntax error

Why won't this work, am I missing the obvious?

Answer 1

It doesn't work because the parser doesn't allow it. An equals sign is allowed in expressions of the form identifier = expression, expression.identifier = expression (where identifier is \w+), expression[arguments] = expression and expression.[]= arguments and as part of a string or symbol or character literal (?=). That's it.

gen.send(:allow=, 'a', 1, false) would work, but at that point you could as well just give the method a name that doesn't include a =.