What is the &: of &:aFunction doing?

Question

I'm reviewing someone's ruby code and in it they've written something similar to:

class Example
  attr_reader :val
  def initialize(val)
    @val = val
  end
end

def trigger
  puts self.val
end

anArray = [Example.new(10), Example.new(21)]
anArray.each(&:trigger)

The :trigger means the symbol is taken and the & transforms it into a proc?

If that's correct, is there any way of passing variables into trigger apart from using self.?

This is related but never answered: http://www.ruby-forum.com/topic/198284#863450

Related question: http://stackoverflow.com/questions/1217088/what-does-mapname-mean-in-ruby — Andrew Grimm, Aug 07 '12 at 23:34
Also related: http://stackoverflow.com/questions/23695653/can-you-supply-arguments-to-the-mapmethod-syntax-in-ruby — Uri Agassi, Jun 03 '14 at 09:18

score 2 · Answer 1 · answered Aug 07 '12 at 13:46

2

Symbol#to_proc is a shortcut for calling methods without parameters. If you need to pass parameters, use full form.

[100, 200, 300].map(&:to_s) # => ["100", "200", "300"]
[100, 200, 300].map {|i| i.to_s(16) } # => ["64", "c8", "12c"]

answered Aug 07 '12 at 13:46

Sergio Tulentsev

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score 2 · Accepted Answer · answered Aug 07 '12 at 13:46

2

is there any way of passing variables into trigger

No.

You're invoking Symbol#to_proc which does not allow you to specify any arguments. This is a convenient bit of sugar Ruby provides specifically for invoking a method with no arguments.

If you want arguments, you'll have to use the full block syntax:

anArray.each do |i|
  i.trigger(arguments...)
end

answered Aug 07 '12 at 13:46

user229044

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I couldn't find this in the docs, how did you learn this / could I have learnt this by myself? Thanks. – AJP Aug 07 '12 at 23:21
Google "symbol to_proc". The second link is to [the documentation](http://www.ruby-doc.org/core-1.9.3/Symbol.html#method-i-to_proc). – user229044 Aug 07 '12 at 23:51
Nice, I was only trying `&:` and thus not finding anything, including the `(&:to_s)` in that documentation. – AJP Aug 08 '12 at 13:54

score 0 · Answer 3 · answered Jun 03 '14 at 09:15

0

This will do exactly what you need:

def trigger(ex)
  puts ex.val
end

anArray = [Example.new(10), Example.new(21)]
anArray.each(&method(:trigger))
# 10
# 21

answered Jun 03 '14 at 09:15

Uri Agassi

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What is the &: of &:aFunction doing?

3 Answers3

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