inicio mail me! sindicaci;ón

Using throw and catch to tidy up our code

Let’s say we want a simple controller action that checks, if a given code is valid.
It should return true and false and, if the code is invalid, give the reason (whether that is because it is unknown or because it has been used already). The action could look like this:

def valid
  if code = Code.find_by_value(params[:id])
    unless code.used?
      respond_with :valid => true
    else
      respond_with :valid => false, :reason => 'used'
    end 
  else
    respond_with :valid => false, :reason => 'unknown'
  end 
end 

Now this deep nesting effectively hides the underlying algorithm, a simple one in this case.
Expressing this using throw and catch straightens the code a bit:

def valid
  failure = catch :fail do
    code = Code.find_by_value(params[:id]) or throw(:fail, 'unknown')
    code.unused?                           or throw(:fail, 'used')
    nil
  end
  respond_with({:valid => !failure}.merge(failure ? {:reason => failure}: {}))
end

We are down to one respond_with line but the has merging and the throws at the beginning of the line are not particularly pretty.

Let’s introduce a little Ruby mixin for the Hash class that provides it with a compact method that its friend Array has had all along:

module HashExtensions
  def compact
    self.reject{|key, value| value.nil? }
  end
end

class Hash
  include HashExtensions
end

Now using this to clean up the response hash and moving the throw statements to the end so the steps of the algorithm are visible we have our final version of the action:

def valid
  failure = catch :fail do
    code = Code.find_by_value(params[:id]) or throw :fail, 'unknown'
    code.unused?                           or throw :fail, 'used'
    nil 
  end 
  respond_with({:valid => !failure, :reason => failure}.compact)
end 

Schreibe einen Kommentar