A Confusing Rubyism

Ben Scofield, Development Director, July 12, 2008 5

The following code has the potential to be terribly confusing:

if x = some_method
  monkify
else
  rhinofy
end

In Ruby, the return value of an assignment statement is the value assigned (run a = 1 in irb and you'll see it returns 1), so this code can be understood as:

Evaluate some_method and assign the value to x
If the new value of x evaluates to true, run monkify
If the new value of x evaluates to false, run rhinofy

Unfortunately, however, most developers are much more accustomed to seeing the equality operator == in conditionals, which means that often we'll misinterpret that code as:

Evaluate some_method and compare the result to x
If they are the same, run monkify
If they are different, run rhinofy

In the interest of clarity, then, it's probably better to avoid using an assignment statement in a conditional. Of course, other concerns may override the need for clarity - but it's something to keep in mind.

Posted in Tips and Tricks on 07/12/08 5 comments

Morgan Roderick said on 07/15 at 12:36 PM

Interesting.

I’ve been (mis)using similar behaviour in javascript for years.




var i = 0, car;


while( car = cars[i++]){


// do stuff


}

I suppose that the reason it works is exactly the same reason it works in Ruby ... but in this case I think the code is actually very easy to read.

TJ Stankus said on 07/24 at 11:02 AM

Good point Ben.

It makes it slightly less confusing if the method in the conditional is named some_method? and written like this:
x = if some_method? monkify else rhinofy end
Or, even better:
x = some_method? ? monkify : rhinofy
(Hopefully the code formats okay, that’s part of my point.)

TJ Stankus said on 07/24 at 11:04 AM

Well, the indention/whitespace didn’t work out in my comment above. I would typically indent the lines in the first example to line up with the if statement.

Ben said on 07/25 at 06:21 PM

Those are easier to read, TJ, but they don’t quite preserve the original intent of the code. In the example I provided, x is assigned the value from some_method, while in your rewrites it is assigned either the value from monkify or rhinofy, depending on the value returned from some_method.

TJ Stankus said on 07/26 at 01:32 AM

I should read the post more carefully before commenting. :) But, I guess that further proves your point - it is confusing! I agree, the better practice is to not use assignment in conditionals.

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