Feature #15829: Object#then_if(condition){}

• Status: Rejected
• Assignee: -
• Target version: -

[foonlyboy (Eike Dierks)]

I'd like to propose some sugar to Object#then

There should be Object#then_if(condition, &block)

The block should only be applied when the condition is true,
otherwise the object should be returned without applying the block.

Rationale:

I frequently use Object#then with Rails to extend queries like this:

foo.then {|query|
  if(condition)
   query.where(zip:zap)
  else
   query
  end
}

by using the proposed Object#then_if the example could be simplified to:

foo.then_if(condition) {|query|
   query.where(zip:zap)
}

I believe that this also applies to a lot of other use cases,
i.e. only applying some transformation if some condition is true,
but otherwise leaving the result untouched.

[foonlyboy (Eike Dierks)]

class Object
  def then_if(condition, &block)
    if condition
      self.then(&block)
    else
      self
    end
  end
end

[foonlyboy (Eike Dierks)]

A (somehow contrived) example:

[1,2,3,4,5].map{|n| n.then_if(n.even?){'even'}}
=> [1, "even", 3, "even", 5]

[foonlyboy (Eike Dierks)]

A more complex example:

scope :blacklisted_at, -> (seller=nil, flag=true) {
  then_if(! flag.nil?) {
    joins(:disk_seller_maps)
      .then_if(seller) {|q|
      q.where(disk_seller_map:{seller_id:seller})
    }
      .then_if(! flag) {|q|
      all.where.not(id: q)
    }
  }
}

Not really sure if this does what I mean,
but it looks a lot more concise now.

Before that all that cases would have expanded to 6 cases,
but now it looks a lot more concise.

• then_if(! flag.nil?) shields from flag.nil?
• then_if(seller) only applies if a seller was given
• then_if(! flag) negates the query

This brings the code down from 2x3=6 variants,
to only two code blocks (the second one being trivial in this case)

[sawa (Tsuyoshi Sawada)]

Related to https://bugs.ruby-lang.org/issues/15557.

[nobu (Nobuyoshi Nakada)]

• Description updated (diff)

As it seems useful only when condition doesn't use the parameter query, it is questionable to me if it is generic enough to be a language feature.

[sawa (Tsuyoshi Sawada)]

I have exactly the same concern as nobu. And that problem stems from the fact that, in this proposal, the condition is given as an argument of the method, which means that it has to be evaluated independently of the return value that appears in the middle of the method chain.

That should take us back to #15557, where the condition is proposed to be given as a block (or a proc, following a suggestion by nobu).

[ko1 (Koichi Sasada)]

• Status changed from Open to Rejected

It seems difficult to introduce.

if you are interest about this ticket yet, could you reopen it and file on our dev-meeting agenda?
https://bugs.ruby-lang.org/issues/15996

Thanks.

[okuramasafumi (Masafumi OKURA)]

How about calling a condition object if it's callable instead of simply using value as a condition?

class Object
  def then_if(condition, &block)
    if condition
      if (condition.respond_to?(:call) && condition.call(self)) || condition
        self.then(&block)
      end
    else
      self
    end
  end
end

Then we can do something like this:

'str'.then_if(Proc.new {|str| str.downcase == str}) {|str| str.upcase}

I know it's complicated and not beautiful, but it's an idea anyway.