[ruby-core:103152] [Ruby master Feature#17768] Proposal: Downward assignments

[nakilon@gmail.com]

Issue #17768 has been updated by Nakilon (Victor Maslov).


Bidirectional assignments should be implemented to make swapping values:

```
var1, var2 = var2, var1
```

much shorter:

```
var1
vv^^
var2
```

----------------------------------------
Feature #17768: Proposal: Downward assignments
https://bugs.ruby-lang.org/issues/17768#change-91225

* Author: mame (Yusuke Endoh)
* Status: Open
* Priority: Normal
----------------------------------------
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"](http://www.unicode.org/reports/tr11/) 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)


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