method_missing with unquoted string arguments in Ruby - possible?

Question

I'm learning Ruby and want to be able to do this:

Printer.hi there

and have Ruby output

"hi there"

So far I have the following implementation

class Printer
  def method_missing(name, *args)
     puts "#{name} #{args.join(',')}"
  end
end

But this only lets me do

Printer.hi "there"

If I attempt

Printer.hi there

I get a

NameError: undefined local variable or method `there' for main:Object

which makes sense as I haven't ever defined 'there'. Is there a way to make this work though?

If it were possible and you did it, then it will mess up the whole code. There is no practical point in doing such thing. — sawa, May 03 '13 at 18:01
No, there isn't, unless you figure out a way to automatically create variables that aren't defined. IMO, however, it's not a good syntax anyway since it's counter-intuitive. Better to create a mini DSL. — Dave Newton, May 03 '13 at 18:36
While I have answered as best I could, please note that your title is incorrect. There is no such things as "unquoted strings" in Ruby. Those are either local variables or methods, as the NameError reveals. — Chris, May 04 '13 at 02:02

score 3 · Answer 1 · answered May 04 '13 at 01:48

No, this is not possible in the form given (as far as I know).

You aren't looking for method missing, you are looking for the equivalent in the Ruby interpreter to capture when it cannot find a given symbol. So while you cannot intercept it there, you can do it inside of a block:

def hi(&block)
  begin
    yield
  rescue NameError => e
    e.message =~ /variable or method `(.+)'/
    puts "hi #{$1}"
  end
end

hi { there } # => hi there

Please note that I feel like a terrible world citizen for showing you this. Please don't use it anywhere, ever.

Michael Durrant · Answer 2 · 2013-05-03T20:29:48.277

0

No because strings need to be quoted, so they are not seen as variables.

Otherwise variables such as there would need some special sort of character to indicate that it is a string. However this still wouldn't work well as spaces would then need to be dealt with.

Use single or double quotes.
It's how the language works. accept this and move on to the next challenge :)

edited May 03 '13 at 20:29

answered May 03 '13 at 17:37

Michael Durrant

93,410
97
333
497

There is nothing wrong written here, but I don't see how this answers the question. – sawa May 03 '13 at 18:09
@boulder I think the answer is "yes, it is possible", and that might be debatable, but Michael's answer is contradictory in saying "yes", and then writing things that point to the opposite direction. – sawa May 03 '13 at 18:58
programmers over-analysis, ok so I changed it to "No because strings need to be quoted, so they are not seen as variables." instead of "yes, as strings need to be quoted..." I hope everyone is happier with that :) – Michael Durrant May 03 '13 at 20:30

sawa · Answer 3 · 2013-05-03T18:19:21.947

0

Yes, there is a way. When you write there without an explicit receiver, the receiver is the self object of that scope. In this case, it is main. Define methods_missing in the main context.

def method_missing(name, *args)
  puts "#{name} was called with arguments: #{args.join(',')}"
end

But if you do so, that would mess up the rest of your code, perhaps. I see not point in doing this.

Since the return value of puts is nil, if you do Printer.hi there, it will evaluate to Printer.hi(nil). So in order for it to output "hi there", you need to define:

class Printer
  def self.hi _; puts "hi there" end
end

edited May 03 '13 at 18:19

answered May 03 '13 at 18:04

sawa

165,429
45
277
381

This doesn't seem like it works... Calling Print.hi with one argument does make it print "hi there", but you still can't say Print.hi there and have it work. – Robin Clowers May 04 '13 at 08:00
That was not asked in the question. – sawa May 04 '13 at 08:04
The question is how can I have the code Printer.hi there result in printing the string "hi there". So yes, it was in the question. – Robin Clowers May 04 '13 at 08:10
I mistook your first comment. Actually, it does not make sense to me. `Print.hi there` works. – sawa May 04 '13 at 09:46

Robin Clowers · Answer 4 · 2013-05-04T08:12:11.573

Interestingly you can do this in ruby 1.8.7 with just this code:

def method_missing(*args)
  puts args.join ' '
end

I learned about this from Gary Bernhardt's talk, Wat. In 1.9 this gives you a stack level too deep error unless you do it inside a class. Google lead me to this post on Aurthur's tech blog thing, which claims you can do something similar in JRuby 1.9 mode:

def method_missing(*args)
  puts [method.to_s, args].flatten.join ' '
end

However when I tried this on MRI 1.9.3 it did not work either. So in 1.9 you can't quite do what you want. Here is the closest I could come:

class Printer
  def self.hi(message)
    puts "hi #{message}"
  end
  def self.method_missing(m, *args)
    [m.to_s, args].flatten.join ' '
  end

  def self.bare
    hi there
  end
end

Printer.bare

method_missing with unquoted string arguments in Ruby - possible?

4 Answers4