This article is part of the “Meta Advent 2019” series. I’ve committed to writing a new blog post here every day until Christmas.

It’s finally time for another “Weird Ruby” installment after a long hiatus! Today’s topic will be invoking (calling) procs/lambdas and it’s fairly short. For the sake of brevity I’ll be using only the term proc going forward, but everything in this article applies to both procs and lambdas.

How would you invoke a proc object like this one?

add_five = ->(x) { x + 5}

Usually you’d do this by using the call method.

add_five.call(10) # => 15

Simple enough, right? Ruby, however, has always been known for providing numerous ways to do the same thing and for a tendency to use a lot of operator overloading. That’s why it shouldn’t be a surprise that in practice you might find more people doing proc invocation like this:

add_five.(10) # => 15

This syntax is rather elegant, but it certainly looks a bit weird, as it deviates from the common practice to have some method name after the method invocation operator . - e.g. object.method_name(arg1, arg2). Still, that’s not weird enough to justify an entire article. Well, it’s good that we can do even better, as there’s a third way to invoke a proc:

add_five[10] # => 15

This looks a lot like some element access in a data structure, but it’s actually a proc invocation. The reason for this is that procs override the [] operator. That particular usage is a remnant of the older days of Ruby and you’d do well to avoid this syntax, as the resulting code is quite confusing. Using call or .() will certainly result in clearer code. Generally I think it’s a very bad idea to override [] for anything that doesn’t map somehow to accessing an element in an underlying data structure.

But, wait, there’s more!

add_five.yield(10) # => 15
add_five === 10 # => 15

I think the last example is particularly weird, even if it will work only for a single parameter scenario (or a no parameters scenario if you pass nil to ===). There’s more than one way to invoke a proc indeed!

That’s all I have for you today! Hopefully it was both weird and useful! See you tomorrow!

Articles in the Series