Question

I just wondered whether there is any good reason for or even an advantage in having to invoke Procs using proc.call(args) in Ruby, which makes higher-order function syntax much more verbose and less intuitive.

Why not just proc(args)? Why draw a distinction between functions, lambdas and blocks? Basically, it’s all the same thing so why this confusing syntax? Or is there any point for it I don’t realize?

Answer 1

You need some way to distinguish between calling the Proc and passing it around.

In Python and ECMAScript, it’s simple: with parentheses it’s a call, without it’s not. In Ruby, leaving off the parentheses is also a call, therefore, there must be some other way to distinguish.

In Ruby 1.8, Proc#call and its alias Proc#[] serve that distinction. As of Ruby 1.9, obj.(arg) is syntactic sugar for obj.call(arg) and Proc#() is also an alias for Proc#call.

So, you can call a Proc like this:

• foo.call(1, 2, 3)
• foo[1, 2, 3]
• foo.(1, 2, 3)

And you can even also define () for your own classes.

BTW: the same problem is also why you have to use the method method to get a hold of a method object.