Question

This answer on another question says that

array.map(&:to_s)

is faster than

array.map { |n| n.to_s }

In the first example, & turns :to_s into a Proc. The second example uses a block.

Why might a Proc be faster than a block in that benchmark? Is there some optimization that this technique allows the interpreter to do?

Answer 1

As others have been said this is specifically about Symbol#to_proc rather than procs in general and it is almost certainly ruby implementation dependant. Before Symbol#to_proc was in ruby itself, the pure ruby implementations of it were definitely slower the the equivalent block.

For a real answer you’d want to profile ruby while you’re executing such a benchmark.

My reading of the ruby source code is that when you call Symbol#to_proc the proc you get is a bit special: The body of the proc is just a C api call (rb_funcall_passing_block), whereas in the other cases it’s actual ruby code which takes a little longer to execute.