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Feature #17768

closed

Proposal: Downward assignments

Added by mame (Yusuke Endoh) 7 months ago. Updated 7 months ago.

Status:
Rejected
Priority:
Normal
Assignee:
-
Target version:
-
[ruby-core:103135]
Tags:

Description

Rightward assignments have been introduced since 3.0.
To be honest, I'm not a big fan of the syntax because it does not add a new dimension to Ruby.
Why don't we bring Ruby to the next dimension?

Proposal

I propose "downward assignments".

p(2 * 3 * 7)  #=> 42
  ^^^^^var

p var         #=> 6

This new syntax intercepts the intermediate value of a subexpression.
In the above example, the subexpression 2 * 3 is captured to var.

You can capture multiple subexpressions in one line.

puts("Hello" + "World")  #=> HelloWorld
     ^^^^^^^x  ^^^^^^^y

p x  #=> "Hello"
p y  #=> "World"

This proposal solves some long-standing issues in Ruby.

Use case 1

Everyone has written the following code.

while (line = gets) != nil
  p line
end

This code is not so bad, but there's something that has been on my mind: is it really good to put an assignment into a condition expression?
I'm afraid that it makes the loop condition unclear.

Unfortunately, it is difficult to keep the condition clear in Ruby.
If the assignment is removed from the condition, the code becomes even more unclear as follows.

while true
  line = gets
  break if line == nil
  p line
end

By using my proposal, you can make the condition crystal-clear.

while gets != nil
      ^^^^line
  p line
end

Use case 2

Consider that we want to get from an array the last element that meets a condition.

ary = [1, 2, 3, 4, 5]

ary.each {|elem| found = elem if elem.even? }

p found  #=> 4

As you know, this code does not work.
We need to add found = nil to declare the variable "found" in the outer scope.
But this is unarguably dirty.

My proposal allows to make the code very straightforward.

ary = [1, 2, 3, 4, 5]

ary.each {|elem| elem if elem.even? }
                 ^^^^found

p found  #=> 4

Use case 3

When writing a constructor, we need to write each field name whopping three times.

class C
  def initialize(foo, bar)
    @foo = foo
    @bar = bar
  end
end

My proposal mitigates the problem to two times.

class C
  def initialize(foo,    bar)
                 ^^^@foo ^^^@bar
end

Patch

A proof-of-concept is attached.

$ cat test.rb
p(2 * 3 * 7)
  ^^^^^var

p var


while gets != nil
      ^^^^line
  p line
end


ary = [1, 2, 3, 4, 5]

ary.each {|elem| elem if elem.even? }
                 ^^^^found

p found  #=> 4

$ echo -e "foo\nbar" | ./miniruby test.rb
42
6
"foo\n"
"bar\n"
4

Notes:

  • The syntax allows only ASCII characters because "East Asian width" is a hell.
  • My patch does not implement binding a method parameter (Use case 3).
  • There are some known bugs. Look for them.

Compatibility

A line that suddenly starts with ^ is invalid currently.
This is why I chose "downward" since upward assignments are incompatible.

      vvvv line
while gets

When the previous line continues, ^ is appropriately handled as an XOR binary operator.

x = 1

# The following is considered as: y = 2^x
y = 2\
    ^x

p x  #=> 1
p y  #=> 3

So, I think this proposal is 100% compatible.

Discussion

I'm unsure how should we handle this.

p(2 * 3 * 7)
      ^^^^^var

Files

2021-aprilfool.patch (9.07 KB) 2021-aprilfool.patch mame (Yusuke Endoh), 04/01/2021 12:02 AM

Updated by ko1 (Koichi Sasada) 7 months ago

super convenient!!1

Updated by znz (Kazuhiro NISHIYAMA) 7 months ago

I think this syntax is not irb friendly.

Updated by mrkn (Kenta Murata) 7 months ago

Is this intentional?

$ cat test.rb
p(2 * 3 * 7)
  ^^^^^var1
       ^^^var2

p var1
p var2
$ ./miniruby test.rb
test.rb:3: syntax error, unexpected '^', expecting end-of-input
       ^^^var2

Updated by marcandre (Marc-Andre Lafortune) 7 months ago

I'd like a shorthand for cases where we don't really need a variable...

while gets != nil
  p   ^^^^
end

(It took me a while to realize the date in Japan :-) )

Updated by mame (Yusuke Endoh) 7 months ago

marcandre (Marc-Andre Lafortune) wrote in #note-4:

(It took me a while to realize the date in Japan :-) )

I waited until 00:00 UTC 🕛👍

Updated by shevegen (Robert A. Heiler) 7 months ago

plot twist by mame similar to last year:

  • It's actually not a joke. :)

plot twist by matz this year:

  • He actually WILL approve joke suggestions this year.

Updated by Dan0042 (Daniel DeLorme) 7 months ago

What an amazing, beautiful, and revolutionary idea!
But what about using an overbar (U+203E) instead of circumflex? It's not that hard to type and it brings extra clarity and visual pleasantness to the code, while eliminating any possibility of conflict with existing syntax.

p(2 * 3 * 7)  #=> 42
  ‾‾‾‾‾var

p var         #=> 6

puts("Hello" + "World")  #=> HelloWorld
     ‾‾‾‾‾‾‾x  ‾‾‾‾‾‾‾y

p x  #=> "Hello"
p y  #=> "World"

Updated by Nakilon (Victor Maslov) 7 months ago

Bidirectional assignments should be implemented to make swapping values:

var1, var2 = var2, var1

much shorter:

var1
vv^^
var2

Updated by mame (Yusuke Endoh) 7 months ago

  • Status changed from Open to Rejected

I think April 1st ended in the world. I hope you enjoyed it!

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