Feature #6721: Object#yield_self

• Status: Closed
• Assignee: matz (Yukihiro Matsumoto)
• Target version: -

[alexeymuranov (Alexey Muranov)]

I think the following method is missing from Ruby:

 class Object
   def yield_self(*args)
     yield(self, *args)
   end
 end

I do not know a good use case, but it looks very natural to me. It can be used in method chains.

What do you think? Is there an alternative?

[jballanc (Joshua Ballanco)]

How is this significantly different than Object#tap?

[alexeymuranov (Alexey Muranov)]

It executes the block and returns its output. For example:

2.yield_self { |x| x*x } # => 4

[alexeymuranov (Alexey Muranov)]

I've come up with some use case for illustration. I have also looked into the Ruby on Rails Object#try method because it can serve a similar purpose. I think yield_self is more basic than try.

Here are two examples of a use case:

attr = object.associated_object.yield_self { |o| o.attribute unless o.nil? }

mailing_address = { :name   => person[:name],
                    :street => person[:address].yield_self { |a| a[:street] if a.is_a?(Hash) }
                  }

Here is for comparison the implementation of Object#try in Ruby on Rails:

def try(*a, &b)
  if a.empty? && block_given?
    yield self
  else
    __send__(*a, &b)
  end
end

[nobu (Nobuyoshi Nakada)]

I'm not against the feature itself, but don't like the name.

[alexeymuranov (Alexey Muranov)]

nobu (Nobuyoshi Nakada) wrote:

I'm not against the feature itself, but don't like the name.

#yield_to, #submit_to, #surrender, #capitulate ? :)

Or otherwise, #apply:

2.apply { |x| x*x }  # => 4

[drbrain (Eric Hodel)]

Your current names are less clear than using a local variable. Using a local variable reveals your intentions very clearly:

o = object.associated_object
attr = o.attribute if o

It's obvious that attr is only set if the associated object exists.

For your second example there's just too much going on to clearly see what the intention is. By first separating data gathering from creating of the mailing_address Hash things become much clearer:

address = person[:address]
street = address[:street] if address.is_a?(Hash)

mailing_address = {
  :name   => person[:name],
  :street => street,
}

As in the first example, your current names don't reveal what yield_self is supposed to do in a way that's clearer than using local variables for construction of mailing_address

[alexeymuranov (Alexey Muranov)]

drbrain (Eric Hodel) wrote:

Your current names are less clear than using a local variable. Using a local variable reveals your intentions very clearly:

Well, using method chains with blocks is always less clear than using local variables, i think.

[trans (Thomas Sawyer)]

This is basically #ergo in Ruby Facets. Essentially:

  def ergo
    return yield(self) if block_given?
    self
  end

[yhara (Yutaka HARA)]

• Category set to core
• Target version set to 2.6

[alexeymuranov (Alexey Muranov)]

After commenting on #6284, i have a new proposition for this method's name: Object#^. Also, i suggest to allow it to take a block, a proc, a lambda, or a symbol. I think this will not conflict with existing uses of #^, however the classes that implement it for certain argument types should not forget to call super if the argument type is not recognized by them.

For example:

# Formatting a string:
format_as_title = lambda { |str| "Title: #{ str.strip.capitalize }" }
title = "something to be a title" ^ format_as_title     # instead of  `format_as_title["something to be a title"]`

# Squaring the 2:
four = 2 ^ { |x| x*x }                                  # instead of `four = 2 * 2`

# Converting a string to an integer:
five = "5" ^ :to_i                                      # instead of `five = "5".to_i`

This is consistent with a rare mathematical notation for function application: sometimes instead of "f(x)", the "exponential" notation "x^f" is used.

This would also open a door to compose lambdas from left to right, if the majority decides so (this is being discussed in #6284)

#ergo is a well-thought method name, I like it better than all others.

[headius (Charles Nutter)]

It occurs to me #apply is used in some other languages to refer to the elements of a collection rather than to the collection itself.

[1,2,3].apply {|n| puts n}

Did we ever decide if the #self method would be added? If it were, it would be simple to have it take a block:

four = 2.self {|n| n * n}

That would make #self basically be #ergo as defined by Facets.

Worth noting that you can get nearly as concise syntax today, albeit in reverse order:

four = ->{|n| n * n}.(2)

[alexeymuranov (Alexey Muranov)]

Here is a "real life" use case. It again has to do with formatting strings.

I want to have a list of conference participants in the form:
Full Name (Affiliation, academic position)
but without empty parentheses or trailing comma if the person has not
provided the affiliation or the position. So i did like this:

class Participant
  def full_name_with_affiliation_and_position
    full_name +
      lambda { |x| x.empty? ? '' : " (#{ x })" }[[affiliation, academic_position].compact.join(', ')]
  end
end

(I will appreciate any more elegant solution.)

With #yield_self (or any other name for it), i would have written:

class Participant
  def full_name_with_affiliation_and_position
    full_name +
      [affiliation, academic_position].compact.join(', ').yield_self { |x| x.empty? ? '' : " (#{ x })" }
  end
end

This would be a bit more readable for me.

Edited 2013-02-09.

Why you can't simply do the following?

def full_name_with_affiliation_and_position
  a_ap = " (#{a_ap})" unless (a_ap = [affiliation, academic_position].compact.join ', ').empty?
  "#{full_name}#{a_ap}"
end

[alexeymuranov (Alexey Muranov)]

Anonymous wrote:

Why you can't simply do the following?

def full_name_with_affiliation_and_position
  a_ap = " (#{a_ap})" unless (a_ap = [affiliation, academic_position].compact.join ', ').empty?
  "#{full_name}#{a_ap}"
end

I can, but i guess i want it to look more like declarative programming, than like imperative.

[ko1 (Koichi Sasada)]

• Assignee set to matz (Yukihiro Matsumoto)

[aleph1 (Elias Levy)]

nobu (Nobuyoshi Nakada) wrote:

I'm not against the feature itself, but don't like the name.

At its core this feature relates to method chaining and transforming the object, something that cannot be done with Object#tap.

Some suggested names then: transform, alter, mutate, map, morph.

map may be the best choice, as its already used in enumerables and this is a natural equivalent for single objects. That said, it may lead to unnoticed bugs if someone thinks they are applying a map operation on an enumerable but for some reason they do so against some other object. So maybe one of the other names is better to ensure such cases fail.

[nobu (Nobuyoshi Nakada)]

(13/05/17 17:01), aleph1 (Elias Levy) wrote:

map may be the best choice, as its already used in enumerables and this is a natural equivalent for single objects. That said, it may lead to unnoticed bugs if someone thinks they are applying a map operation on an enumerable but for some reason they do so against some other object. So maybe one of the other names is better to ensure such cases fail.

If it were Kernel#map, which would you expect by {foo: 42}.map {...} ?

nobu (Nobuyoshi Nakada) wrote:

I'm not against the feature itself, but don't like the name.

+1 to this opinion

[alexeymuranov (Alexey Muranov)]

I have checked if by any chance Haskell had it, apparently it doesn't: http://stackoverflow.com/questions/4090168/is-there-an-inverse-of-the-haskell-operator

I have found that in Alonzo Church's "The calculi of lambda-conversion", he uses "[M^N]" (with superscript) as an alternative notation for the "application" of a term N to a term M (in addition to the basic lambda-calculus notation "(NM)").

This operation would roughly correspond to apply in Scheme with reverse order of arguments. If i understand correctly, the apply in Scheme roughly corresponds to call in Ruby, so maybe reverse_call?

Edited 2013-08-31.

[alexeymuranov (Alexey Muranov)]

Another idea for the name: Object#cast.

class Object
  def cast(*args)
    if block_given?
      yield(self, *args)
    else
      p = args.pop
      unless p.respond_to?(:call)
        raise ArgumentError, 'the last argument should be callable when no block is given'
      end
      p.call(self, *args)
    end
  end
end

2.cast {|x| x**3 } # => 8

p = proc {|x, y| x**y }
3.cast(2, p) # => 9

Another option: Object#toss.

Edited 2013-09-01.

[abinoam (Abinoam P. Marques Jr.)]

May I give a name suggestion?
Does "tap!" make sense in english?

2.tap {|x| x*2 } # => 2

2.tap! {|x| x*2 } # => 4

The exclamation mark alerts that the return value is being changed.

[nobu (Nobuyoshi Nakada)]

• Related to Feature #6688: Object#replace added

[nobu (Nobuyoshi Nakada)]

• Related to deleted (Feature #6688: Object#replace)

[nobu (Nobuyoshi Nakada)]

• Related to Feature #6684: Object#do added

[nobu (Nobuyoshi Nakada)]

• Related to Feature #10095: Object#as added

[nobu (Nobuyoshi Nakada)]

• Has duplicate Feature #11717: Object#trap -- pass object to block and return result added

[nobu (Nobuyoshi Nakada)]

• Related to deleted (Feature #10095: Object#as)

[nobu (Nobuyoshi Nakada)]

• Has duplicate Feature #10095: Object#as added

[nobu (Nobuyoshi Nakada)]

• Description updated (diff)

[nobu (Nobuyoshi Nakada)]

• Has duplicate Feature #12760: Optional block argument for `itself` added

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Closed

Applied in changeset trunk|r58528.

---

object.c: Kernel#yield_self

• object.c (rb_obj_yield_self): new method which yields the
  receiver and returns the result.
  [ruby-core:46320] [Feature #6721]