Feature #6758: Object#sequence

• Status: Open
• Assignee: -
• Target version: -

[merborne (kyo endo)]

=begin

== Object#sequence

Let me propose a new method ((Object#sequence)).

次のような実装の((Object#sequence))を提案します。

class Object
  def sequence(init=true, &blk)
    x = self
    Enumerator.new do |y|
      y << x if init
      loop { y << (x = yield x) }
    end
  end
end

((sequence)) generate a sequence by applying a block recursively to the receiver object. The result is wrapped with a Enumerator object, thus it is set under lazy evaluation.

((sequence))は、そのレシーバオブジェクトを初期値として、渡されたブロック（漸化式）を繰り返し適用してシーケンスを生成します。適用の結果はEnumeratorオブジェクトでラップされているので、遅延評価されます。

== Usage;

使い方を示します。

1.sequence { |x| x + 2 }.take(10) # => [1, 3, 5, 7, 9, 11, 13, 15, 17, 19]

3.sequence { |x| x * 2 }.take(10) # => [3, 6, 12, 24, 48, 96, 192, 384, 768, 1536]

[0, 1].sequence { |a, b| [b, a + b] }.take(10).map(&:first) # => [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

[0, 1, 1].sequence { |a, b, c| [b, c, a + b + c] }.take(10).map(&:first) # => [0, 1, 1, 2, 4, 7, 13, 24, 44, 81]

# square root 5
a = 5
eps = 0.0001
1.0.sequence { |x| (x + a/x) / 2.0 }
   .each_cons(2)
   .detect { |a, b| (a - b).abs < eps }[1] # => 2.236067977499978

# Excel label
'A'.sequence { |x| x.succ }.take(30) # => ["A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z", "AA", "AB", "AC", "AD"]

# random boolean(avoid true-true sequence)
true.sequence { |prev| prev ? false : [true, false].sample }.take(20) # => [true, false, true, false, true, false, true, false, true, false, true, false, true, false, true, false, false, false, true, false]

== Some background

== 提案の経緯

Let me explain some background of this request.

本件に関しては、ここに至る若干の経緯がありますので、併せて説明します。

1. I introduced this method as ((Object#repeat)) on my Japanese blog.

2. 私のブログにおいて、本メソッドを((Object#repeat))として紹介する記事を公開。

((URL:http://melborne.github.com/2012/07/12/object-repeat-take-care-of-sequence/))

2. Matz tweeted to the article.

   "um.. the feature is attractive, but the name is.."

3. 記事に対してMatzがつぶやく。

   「うーん、昨日としては魅力的だけど、名前がなあ。」

((URL:https://twitter.com/yukihiro_matz/status/223790181113806848))

3. I updated the article to propose ((Object#repeat_apply)) or ((Object#repeat_call)).

4. Object#repeat_apply, Object#repeat_callを提案するべく記事を更新。

5. Matz tweeted to the article.

   @merborne more clear. but combining two verbs is wrong. I understand naming is difficult.

6. 記事に対してMatzがつぶやく。

   @merborne なるほど「repeat_apply」か「repeat_call」ですか。repeat単体よりは誤解を受けにくいとは思いますが、repeatって動詞とapply/callって動詞の組み合わせは感心しませんね。名前って難しい。

((URL:https://twitter.com/yukihiro_matz/status/224105896110866432))

5. Matz tweeted to the article.

   @merborne I suggest some clue lies around a word "series"..

6. 記事に対してMatzがつぶやく。

   @merborne 「級数/series」あたりに名前のヒントがありそうな。

((URL:https://twitter.com/yukihiro_matz/status/224106160591081472))

6. I tweeted to Matz.

7. 私もつぶやく

   @yukihiro_matz you are right.. repeated_apply repeated_call?..

   @yukihiro_matz たしかに.. repeated_apply repeated_callかな..

((URL:https://twitter.com/merborne/status/224108387653259264))

> @yukihiro_matz clue! `series_by` ? 


> @yukihiro_matz ヒント! series_by ? 

((URL:https://twitter.com/merborne/status/224108809948377088))

> @yukihiro_matz `repeated` is adjective..^^; but I don't like `repeatedly_apply`. I thought once `series_by` is good, but it would be better a method is named based on its behavior, not on its function, I think. How about `repeat_by`?


> @yukihiro_matz repeatedは形容詞でしたね^^; するとrepeatedly_applyですが、今ひとつです。series_byもいいと思ったんですが、できれば機能ではなく動作を言いたい思いがあります。そこでrepeat_byというのはどうでしょうか。

((URL:https://twitter.com/merborne/status/224324670764220416))

the conversation closed..

会話終了..

7. Ideas from other Rubyists

Some Japanese Rubyists tweeted or commented me on the name. Candidates are..

7. 他のRubyistの意見

名前に関し何人かのRubyistからアイディアをもらっています。リストアップします。

iterate
recur
recurrence
recur_with
unfold
sequence
seq

Haskell and Scala have the same functionality with a name of ((iterate)).

なお、HaskellとScalaには同種の機能が、((iterate))という名前で実装されているそうです。

Also, @makotokuwata-san tweeted that ((Kernel#seq))(function style) or ((Enumerator.seq()))(a Class method) is better.

また、@makotokuwata氏よりKernel#seqのような関数形式か、Enumerator.seq()のようなクラスメソッドのほうがいいとの意見も頂いています。

> @yukihiro_matz @merborne 初期値と漸化式から順列を作る機能なので、"sequence"か"seq"に1票。あとKernel#seq(initial,&blk)のほうが好きです。RT 「級数/series」あたりに名前のヒントがありそうな。 

((URL:https://twitter.com/makotokuwata/status/225806204390227968))

Thank you for your consideration.

以上、ご検討のほどよろしくお願い致します。

=end

[trans (Thomas Sawyer)]

Nice, but why Object method and not Kernel method?

[merborne (kyo endo)]

trans (Thomas Sawyer) wrote:

Nice, but why Object method and not Kernel method?

Thank you.

If you mean Kernel public method, no difference.

If you mean Kernel private method that is, function style, I prefer having a receiver. Two reason. Passing block argument from a receiver object is more natural for me than from argument of the method. Also I prefer placing this method into method chain.

[mrkn (Kenta Murata)]

I prefer Enumerable.seq and Kernel#seq.

[ngoto (Naohisa Goto)]

In biology, the term "sequence" has special meanings for DNA and protein, and I'm already using the method name "sequence" and its abbreviation "seq" in my libraries and tools (e.g. bioruby). So, I have objection to the name Object#sequence and #seq.

According to WikiPedia, "sequence" has several field-specific meanings other than biology.
http://en.wikipedia.org/wiki/Sequence_%28disambiguation%29

I prefer "iterate" or "recurrence".

[alexeymuranov (Alexey Muranov)]

ngoto (Naohisa Goto) wrote:

In biology, the term "sequence" has special meanings for DNA and protein, and I'm already using the method name "sequence" and its abbreviation "seq" in my libraries and tools (e.g. bioruby). So, I have objection to the name Object#sequence and #seq.

According to WikiPedia, "sequence" has several field-specific meanings other than biology.
http://en.wikipedia.org/wiki/Sequence_%28disambiguation%29

I prefer "iterate" or "recurrence".

I think that the use of "sequence" in biology and all other sciences are special cases of the mathematical meaning (the one at the top of the wikipedia page). So i object to this particular objection. "Recurrence" is a more special term than a sequence. In particular, if the generated sequence is random, it is hard to call it a "recurrence". See also http://en.wikipedia.org/wiki/Recurrence_relation .

[trans (Thomas Sawyer)]

@ngoto (Naohisa Goto) You can still use #sequence for your specialized classes, and if you need to use Object#sequence simply alias it.

class DNA
alias :iterate, :sequence

def sequence
  # your code
end

end

However, mathematically speaking, I think the technical name for this is series. So maybe that would be a better choice?

[alexeymuranov (Alexey Muranov)]

trans (Thomas Sawyer) wrote:

However, mathematically speaking, I think the technical name for this is series. So maybe that would be a better choice?

I believe that in mathematical English "series" means an infinite sum, like 1 + 2 + 3 + 4 + ...
For example: Fourier series.

[trans (Thomas Sawyer)]

I believe that in mathematical English "series" means an infinite sum, like 1 + 2 + 3 + 4 + ...
For example: Fourier series.

Oh, yeah. That's right.

[alexeymuranov (Alexey Muranov)]

=begin
ngoto (Naohisa Goto) wrote:

I prefer "iterate" or "recurrence".

Excuse me, Naohisa Goto, after some thought i agree that (({#iterate})) or (({#recurrence})) are good ideas, as the sequence generated in this way is more or less a recurrent sequence ( http://en.wikipedia.org/wiki/Recurrence_relation ).

I would propose a different behavior however, with an arbitrary number of initializing values:

class Object
def recurrence(*args)
Enumerator.new do |y|
v = [self] + args
v.each { |x| y << x }
loop { y << (x = yield(*v)); v.shift; v << x }
end
end
end

fibonacci = 0.recurrence(1) { |a, b| a + b }

fibonacci.take(10) # => [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

What do you think? The result looks nice to me, but my implementation looks weird.
=end

[merborne (kyo endo)]

alexeymuranov (Alexey Muranov) wrote:

fibonacci = 0.recurrence(1) { |a, b| a + b }

I disagree this. because I can't image fibonacci sequence from this code. and two block variables, which are both used as init data comes from different sources(one from a receiver, one from an argument). It's not natural for me.

with the original, you can easily get a sequence, of which a sequence of diffrences becomes a sequence of numbers with common difference as follows;

[1, 1].sequence { |a, b| [a+2, a+b] }.take(20).map(&:last) # => [1, 2, 5, 10, 17, 26, 37, 50, 65, 82, 101, 122, 145, 170, 197, 226, 257, 290, 325, 362]

# a sequence of diffrences of above sequence becomes [1, 3, 5, 7, 9 ..]

[trans (Thomas Sawyer)]

Please don't name the method #recurrence. #sequence was long enough, and thankfully nice obvious shorthand, if we want to use it, with #seq. While #iterate is ok too, but if we are going to talk about it as a "sequence", which everyone seems to be doing, it might as well be named that.

[alexeymuranov (Alexey Muranov)]

merborne (kyo endo) wrote:

alexeymuranov (Alexey Muranov) wrote:

fibonacci = 0.recurrence(1) { |a, b| a + b }

I disagree this. because I can't image fibonacci sequence from this code. and two block variables, which are both used as init data comes from different sources(one from a receiver, one from an argument). It's not natural for me.

with the original, you can easily get a sequence, of which a sequence of diffrences becomes a sequence of numbers with common difference as follows;

[1, 1].sequence { |a, b| [a+2, a+b] }.take(20).map(&:last) # => [1, 2, 5, 10, 17, 26, 37, 50, 65, 82, 101, 122, 145, 170, 197, 226, 257, 290, 325, 362]

# a sequence of diffrences of above sequence becomes [1, 3, 5, 7, 9 ..]

Ok, i agree, my proposed behavior was not intuitive and not completely general.

However, the original proposal looks to me rather restrictive and not very efficient. Between

[0, 1].sequence { |a, b| [b, a + b] }.take(10).map(&:first)

and

Enumerator.new { |y| a, b = 0, 1; loop { y << a; a, b = b, a + b } }.take(10)

i think i would choose the second, even though it is longer.

[merborne (kyo endo)]

alexeymuranov (Alexey Muranov) wrote:

However, the original proposal looks to me rather restrictive and not very efficient. Between

[0, 1].sequence { |a, b| [b, a + b] }.take(10).map(&:first)

and

Enumerator.new { |y| a, b = 0, 1; loop { y << a; a, b = b, a + b } }.take(10)

i think i would choose the second, even though it is longer.

how about this?

b = 1
0.sequence { |a| a, b = b, a + b; a}.take(10)

[alexeymuranov (Alexey Muranov)]

merborne (kyo endo) wrote:

how about this?

b = 1
0.sequence { |a| a, b = b, a + b; a}.take(10)

Well, in my opinion this is not too different from

a, b, s = 0, 1, []
10.times { s << a; a, b = b, a + b }
s

because it requires an external local variable.

[merborne (kyo endo)]

alexeymuranov (Alexey Muranov) wrote:

merborne (kyo endo) wrote:

how about this?

b = 1
0.sequence { |a| a, b = b, a + b; a}.take(10)

Well, in my opinion this is not too different from

a, b, s = 0, 1, []
10.times { s << a; a, b = b, a + b }
s

because it requires an external local variable.

#sequence returns an enumerator object. It means it can be evaluated later.

class Enumerator
  def lazy(&blk)
    Enumerator.new do |y|
      each { |e| yield(y, e) }
    end
  end
end

# even fibonacci sequence
b = 1
0.sequence { |a| a, b = b, a + b; a }
 .lazy { |y, fib| y << fib if fib.even? }.take(10) # => [0, 2, 8, 34, 144, 610, 2584, 10946, 46368, 196418]

Enumerator#lazy will be introduced in Ruby 2.0.

When #sequence is called from an array object, it is natural for me to get an arrays, each of which is same size of receivers, as its returning sequence.

[0, 1].sequence { |a, b| [b, a + b] }.take(10) # => [[0, 1], [1, 1], [1, 2], [2, 3], [3, 5], [5, 8], [8, 13], [13, 21], [21, 34], [34, 55]]

# create two different sequences at once.
[1, 1].sequence { |a, b| [a+2, b*2] }.take(10) # => [[1, 1], [3, 2], [5, 4], [7, 8], [9, 16], [11, 32], [13, 64], [15, 128], [17, 256], [19, 512]]

so, the first fib example is acceptable for me :)

[yhara (Yutaka HARA)]

• Category set to core
• Target version set to 2.6

The method seems useful, but adding it to Object or Kernel would be feature creep. I still remember how I was memorizing basic Ruby classes. The number of features in them is already such, that adding new features raises the bar for novices more than linearly.

I suggest to make this a public class method of Enumerator, which it constructs. As for the name, I do not think that "sequence" is optimal. Better name would be "produce", similar sounding but not same as "reduce". Like this:

Enumerator.produce( 1 ) { |a| a + 2 }

I would definitely leave it up to the user alone to add the shortcut method to Object class, if ze wants.

[naruse (Yui NARUSE)]

• Target version deleted (2.6)