The special variable “_”

By Adrian Smith30 Apr 2010500 words2 mins to read

Reading this blog post, Destructuring binds in Ruby just now reminded me of a feature I love about Prolog which I wish would make it into other languages.

Firstly, I love assigning a list to a list of lvalues i.e. variables; this is possible in both PHP and Perl which I use regularly; and no doubt many other languages. (But not Java: why not!?)

($a, $b)     = ($b, $a);       // Perl
list($a, $b) = array($b, $a);  // PHP

PHP, as always, wins in inelegance, having the left side syntactically different to the right side. While it's obviously the case that a list of values and a list of lvalues are technically different, I don't think this difference should be expressed in the syntax.

I mean, in most languages you write e.g. $foo=4 and $bar=$foo; in both those cases you write $foo but yet they do something different (lvalue and rvalue); given that you write them the same there i think the same should apply to lists.

But I digress – What I want to mention is using "underscore" to mean "any variable". I first saw this in Prolog.

For example, imagine you have to implement an interface (e.g. in Java), it requires you to write a function taking two parameters, but one of the parameters you don't care about. Wouldn't it be nice to write

interface ExistingApi {
   public void createObject(String name, Object otherInformation);
}

class MyInstanceOfTheApi {
   public void createObject(String name, _) {
      ...
   }
}

i.e. this shows clearly you do not care about the second parameter.

In current Java (and all languages I program including Perl, PHP) you have to give all variables a name even if you don’t use them, either in function definitions or in “assign to a list” scenarios mentioned above. It is then left as an exercise to the reader to determine if these variables are used or not, and indeed an exercise to the writer to name the variable they are never going to use.

I mean yes, technically you can actually just call variables _ (or $_ (except in Perl where $_ already means something)) but that would then be a coding convention as opposed to a language feature, and who knows if the coding convention is actually used correctly by a programmer. (If _ is a variable there’s nothing stopping someone from using its value.)

And then you have the problem if you have two variables you don't care about, you can't call them both _.

By the way, Programming in Prolog is an excellent book.

This article was written by Adrian Smith on 30 Apr 2010

Follow me: Facebook | Twitter | LinkedIn | Email

More on: Language Design