Why does () returns nil in Ruby?

Question

In Ruby, when you run:

()
=> nil

The output is nil. I don't understand which Ruby mechanism this is using.

I thought it was calling self(), but self() returns syntax error, unexpected '(', expecting end-of-input.

Why does this return nil, and which language feature is this using?

Answer 1

"No value" is treated as nil in very many places in Ruby:

-> { break }.()
#⇒ nil

42 if false
#⇒ nil

The same is here: parentheses are redundant but they maintain the code block, the empty one, hence it’s treated as nil.

---

With Ruby 2.6+ you might check the AST yourself:

main > RubyVM::AbstractSyntaxTree.parse('()')
#⇒ (SCOPE@1:0-1:2 tbl: [] args: nil body: (BEGIN@1:1-1:1 nil))