CODEWITHSUNDEEP
rubyvariablesblockproc
Lucy Bain
Lucy Bain

Reputation: 2616

How can I pass a block/proc and variable directly to a function?

I'm trying to learn about blocks and procs and I feel like this should be doable, but so far I haven't been successful.

What I'd like:

def main_action(generic_variable, block)
 generic_variable += 1 # I do more interesting stuff than this in my version
 yield(generic_variable)
end

main_action(3, {|num| num + 5 })

Or something similar.

The closest I found was http://mudge.name/2011/01/26/passing-blocks-in-ruby-without-block.html which seemed to require creating a passing method to send the blocks around:

class Monkey

  # Monkey.tell_ape { "ook!" }
  # ape: ook!
  #  => nil
  def self.tell_ape(&block)
    tell("ape", &block)
  end

  def self.tell(name, &block)
    puts "#{name}: #{block.call}"
  end
end

If I use the same passing method set up in my code I can get it to work, but it seems like unnecessary code. I tried calling Monkey.tell("ape", { "ook!" }) directly, but got syntax error, unexpected '}', expecting tASSOC

Is it possible to send a variable and a block (or proc, I'm not picky) to a function in one call? Also, if you have time, what's going on above? Why doesn't that work?

Upvotes: 2

Views: 1919

Answers (2)

Lucy Bain
Lucy Bain

Reputation: 2616

Based on sawa's answer I was able to recreate the function in the question (Monkey.tell) using a block and proc. Here they are in case anyone else finds them useful!

def tell_proc(name, proc)
  puts "#{name}: #{proc.call('ook')}"
end
tell_proc('ape', ->(sound) { sound + "!" })

def tell_block(name, &block)
  puts "#{name}: #{block.call('ook')}"
end
tell_block('ape') {|sound| sound + "!" }

Upvotes: 2

sawa
sawa

Reputation: 168071

You seem to be confusing blocks and procs. A block is not a ruby object. You cannot pass it like

foo(args, block)

The only way to pass it is foo(args){...} or foo(args) do ... end. However, in method definition, you can receive a block, convert it into a proc, and accept it as one of its arguments like this:

def foo(args, &block)
  ...
end

On the other hand, You can pass a proc object as an argument like

foo(args, proc)

There are several ways to create a proc object, and a literal that is close to the block literal is the lambda: ->{...}, but it is different from the block syntax {...}. Using this, you can do

foo(args, ->{...})

but not

foo(args, {...})

unless {...} is a hash.

Upvotes: 5

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