Ruby "in" method

Do you ever find yourself writing:

if foo == x or foo == y
  ...
end

And wishing you could write:

if foo == x or y
  ...
end

Of course, that won't work, even though it reads nicely as English ("if foo is x or y"), because what it's actually saying is "if foo is x, or failing that, if y is neither false nor nil".

The first alternative does do what we want, but it doesn't read as nicely in English ("if foo is x or foo is y"); no native English speaker would ever use such a repetitive construction.

Of course, programming is not about speaking English, although it is sometimes tempting to make our code "read nicely" when spoken out loud.

So perhaps you consider this alternative to get rid of the reptition:

if [x, y].include? foo
  ...
end

That gets rid of the repetition by it still doesn't sound so great when spoken ("if 'x and y' include foo"). More "rubyesque" seeing as it uses a method defined on an enumerable; but also smells of unjustified complexity because in reality all we want to do is check a pair of expressions, not enumerate over a sizeable collection.

Next try: let's add a new method to Object to put things back in a more natural order:

class Object
  def in ary
    ary.include? self
  end
end

# now we can do:
if foo.in [x, y]
 ....
end

So that's a little better but still doesn't sound perfect when read out loud ("if foo is in 'x and y'"), and we still have that "unjustified complexity" smell; it's just that it's hidden now one level down inside a method on Object. (Does burying it like that make the smell better or worse?)

At this point I'm about ready to give up and just keep on writing foo == x or foo == y.

Update

Sho Fukamachi mailed me reminding me of this alternative:

case foo
when x, y, z
  ...
end

As he says, "I find that nice and neat, obviously it's a line longer than a simple if but can look very nice and clean if you got a number of them to do".

Doesn't address the "reads nicely out loud" criterion but it does avoid the smell of using a temporary array. The Ruby parser will also accept it without the extra newline anyway, so it is not necessarily longer:

case foo when x, y, z
   ...
end

But still, doesn't help much in the case where you want the if at the end of the line so that you can put the entire thing on one line:

... if foo == x or foo == y

Update 2: 26 January 2009

Another suggestion from Sho:

def in *arr
  arr.include? self
end

Then you can do things like:

>> 1.in 1, 2
=> true
>> 1.in 2, 3
=> false