How to understand this ruby code?

Question

I read following ruby code:

def callbacks(procs)
  procs[:starting].call     # line 1
  puts "Still going"
  procs[:finishing].call    # line 2
end

callbacks(:starting => Proc.new { puts "Starting" },   # line 3
          :finishing => Proc.new { puts "Finishing" }) # line 4

I can guess what it dose. But I don't know what :starting and :finishing in line 1 & 2 mean, and what :starting=> in line 3 and :finishing=> in line 4 mean. It's even hard to find a keyword to google.

Could anybody explain line 1,2,3,4 to me? If you may refer some official doc, that will be even better.

Stefan · Accepted Answer · 2013-08-20T08:27:10.333

3

procs is a Hash, procs[:starting] refers to the value for key :starting, which is set by:

:starting => Proc.new { puts "Starting" }

The method call is equivalent to:

h = {}
h[:starting] = Proc.new { puts "Starting" }
h[:finishing] = Proc.new { puts "Finishing" }
callbacks(h)

edited Aug 20 '13 at 08:27

answered Aug 20 '13 at 08:20

Stefan

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Is :string equivalent to "starting"? – TieDad Aug 20 '13 at 08:30
@EvanLi `:string` is symbol and `"starting"` is string. – Arup Rakshit Aug 20 '13 at 08:32
Why use symbol as hash key rather than string? – TieDad Aug 20 '13 at 08:36
@EvanLi symbols are preferred in this case, but you can use any object. However, they are not the same - if the method expects symbols, passing strings won't work. – Stefan Aug 20 '13 at 08:38
@EvanLi Today also asked [**`What is the use of a hash key with “:”`**](http://stackoverflow.com/questions/18328503/what-is-the-use-of-a-hash-key-with) – Arup Rakshit Aug 20 '13 at 08:42
@Babai Please bare my ugly question. I just started learning Ruby a few days ago. – TieDad Aug 20 '13 at 08:44
@EvanLi no problem..I started with much more uglier than you..so np..go on.. :) – Arup Rakshit Aug 20 '13 at 08:46

Arup Rakshit · Answer 2 · 2013-08-20T08:29:26.453

1

But I don't know what :starting and :finishing in line 1 & 2 mean

:starting and :finishing are the hash keys of the hash procs.

what :starting=> in line 3 and :finishing=> in line 4 mean.

You are sending a Hash object as an argument via the method callbacks.

Read here Hash.

edited Aug 20 '13 at 08:29

answered Aug 20 '13 at 08:19

Arup Rakshit

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How to understand this ruby code?

2 Answers2