What is the &: of &:aFunction doing? [duplicate]

StackOverflow https://stackoverflow.com/questions/11847336

  •  25-06-2021
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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

Was it helpful?

Solution

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

OTHER TIPS

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"]

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
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