How to forward blocks in Ruby

2014/6/30

TL;DR

Use Proc.new

Calling Enumerators - normal use

You're writing some code which calls an Enumerator - a function that makes repeated calls to the block of code that you provide.

def yield_me_2_things
  yield 'Thing 1'
  yield 'Thing 2'
end

yield_me_2_things { |x| puts x }

This will print:

Thing 1
Thing 2

The values are supplied by yield_me_2_things and the printing is done in the block, { |x| puts }, that is passed to that method.

Generalize

I can now make a generalized method, to handle any number of things:

def yield_me_n_things(n)
  1.upto(n) do |i|
    thing = "Thing #{i}"
    yield thing
  end
end

yield_me_n_things(2) { |x| puts x }

...the output is the same.

An alternative: use a block

I could equally have implemented the method using a `&block` parameter - for the caller, it makes no difference:

def call_this_block_with_n_things(n, &block)
  1.upto(n) do |i|
    thing = "Thing #{i}"
    block.call thing
  end
end

call_this_block_with_n_things(2) { |x| puts x }

...the output is the same.

The problem

What if I want one Enumerator to call another?

What if I want to keep the specific version (yield_me_2_things) but just make it call the generalized method?

def enumerate_n_things(n) # How do I receive the block?
  1.upto(n) do |i|
    thing = "Thing #{i}"
    # How do I call the block?
  end
end

def enumerate_2_things
  enumerate_n_things(2) # How do I forward the block?
end

enumerate_2_things { |x| puts x }
enumerate_2_things(2) { |x| puts x }

How should I write the two methods, while keeping both usable independently?

Attempt 1: Forward using yield

With `yield`, you don't explicitly receive the block, you just call it.

Does that work across two levels? I.e., does the block get passed to method I call?

def enumerate_n_things(n)
  1.upto(n) do |i|
    thing = "Thing #{i}"
    yield thing
  end
end

def enumerate_2_things
  enumerate_n_things(2)
end

enumerate_2_things { |x| puts x }

No, doesn't work, enumerate_n_things doesn't receive a block.

I get this error:

no block given (yield) (LocalJumpError)

Attempt 2: Forward using a block

def enumerate_n_things(n, block) # Note: no '&'
  1.upto(n) do |i|
    thing = "Thing #{i}"
    block.call thing
  end
end

def enumerate_2_things(&block)
  enumerate_n_things(2, block)
end

enumerate_2_things { |x| puts x }

Prints:

Thing 1
Thing 2

But we can no longer pass a block to the generalized method:

enumerate_n_things(2) { |x| puts x }

enumerate_n_things now expects the block as a normal parameter.

I get this error:

wrong number of arguments (1 for 2)

Solution: Proc.new

def enumerate_n_things(n, block = Proc.new)
  1.upto(n) do |i|
    thing = "Thing #{i}"
    block.call thing
  end
end

def enumerate_2_things(block = Proc.new)
  enumerate_n_things(2, block)
end

enumerate_2_things { |x| puts x }
enumerate_n_things(2) { |x| puts x }

Both calls now work!

Proc.new transforms any block passed to a method into a Proc.

If we use that as the default value for a block parameter we can call methods directly with blocks, or forward blocks between enumerators.