Ruby Method Chaining intermediary value

Question

irb(main):001:0> def foo(x)
irb(main):002:1> x * 10
irb(main):003:1> end
=> nil
irb(main):004:0> def bar(y)
irb(main):005:1> y + 3
irb(main):006:1> end
=> nil
irb(main):007:0> foo(10).tap{|x| bar(x)}
=> 100

I was hoping this method would allow for the chaining of methods without assigning local variables, i.e. to return 103 instead of 100. What's going on here?

Answer 1

Your bar method takes y and returns y+3 but it doesn't attempt to modify y. For bar to have the effect you want it to have, it needs to modify its argument. Sadly, numbers are immutable in ruby. You can assign a different number to a variable (i.e., a different object (i.e., with a different object_id)), but you can't change the original object. If you use mutable object such as strings and let bar do something that mutates the string that gets passed in, then you can use your tap construct:

def foo(x)
  x*10
end

def bar(y)
  y << '3'
end

foo('a').tap{|x| bar(x)} #=> "aaaaaaaaaa3"

For objects like numbers, you can use classical procedural chaining [bar(foo(10))].

Answer 2

Perhaps the tap documentation can clear it up?

Yields x to the block, and then returns x. The primary purpose of this method is to “tap into” a method chain, in order to perform operations on intermediate results within the chain.

You want it to return y, but it doesn't do that, it returns x back to the chain.

It's effectively doing:

def tap(x)
  yield x
  x
end