What is `-@` in ruby?

Question

I ran into this code snippet in a codeWars solution.

def invert(list)
  list.map(&:-@)
end

I've never seen the @ before in ruby. It looks like it lets you specify an operation on the iterator in a &: anonymous function call.

I'm trying to work on my ruby terminology. What is the @ called in Ruby and when was it introduced?

It *looks* like `:-` is the binary `a - b` operator and `:-@` is the unary `-a` operator. — Ry-, Oct 23 '18 at 22:54
ampersand colon would call the meth foo as a proc https://stackoverflow.com/questions/1961030/ruby-ampersand-colon-shortcut — xander-miller, Oct 23 '18 at 23:02
You can check with any number as `23.methods => [:to_s, :inspect, :-@, ..` It is mandatory for list to have all objects which supports method '-@' — ray, Oct 24 '18 at 10:11
@xander-miller: So, if `list.map(&:foo)` calls a method named `foo` on every element, then what does `list.map(&:@-)` do? — Jörg W Mittag, Oct 26 '18 at 20:32

score 0 · Answer 1 · answered Oct 23 '18 at 22:58

0

Here's an example of how you can use it

[1,2].map(&:-@) #=> [-1, -2]

answered Oct 23 '18 at 22:58

lacostenycoder

1

I found my answer in the comments on codeWars. `-@` is actually a method on integer. If you call `1.methods` you can see it. The method inverts the value of the integer. – xander-miller Oct 23 '18 at 23:08
yes that was clearly explained in the link I commented on your question but this clearly illustrates a use case IMHO. – lacostenycoder Oct 23 '18 at 23:11
2

@xander-miller It is not specific to Integer, it is merely a syntax for defining unary operators, not just `-`. Think of a vector class, you would likely define unary negation, and they are not integers. It is just another method name, with some different syntax on how to clarify it from a binary operator. – ForeverZer0 Oct 23 '18 at 23:31

1 Answers1