[ruby-core:101705] [Ruby master Feature#17471] send_if method for improved conditional chaining

Issue #17471 has been reported by ozu (Fabio Pesari).

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Feature #17471: send_if method for improved conditional chaining
https://bugs.ruby-lang.org/issues/17471

* Author: ozu (Fabio Pesari)
* Status: Open
* Priority: Normal
----------------------------------------
# Background

Method chaining is very important to many Ruby users, since everything in Ruby is an object.

It also allows easier functional programming, because it implements a pipeline where each step can happen without mutation.

Conditional chaining allows an even more declarative style of programming. Right now, it is possible to conditionally chain methods to a degree but in some cases it is a bit verbose.

# Proposal

I propose that a `send_if` method is added, which works roughly like this:

``` ruby
# Internal condition
puts 'If you give me a number larger than 5, I will double it. I will subtract 1 in any case.'
number = gets.chomp.to_i
# An implementation without send_if
puts (number > 5 ? number.send(:*, 2) : number).send(:-, 1)
# Implementation with send_if [1]
puts number.send_if(:*, 2) {|obj| obj > 5}.send(:-, 1)

# External condition
puts 'Do you want a loud Merry Christmas? (y or I take it as a no)'
answer = gets.chomp
# An implementation without send_if
puts %w(Merry Christmas).send(:map, &->(e) {answer == 'y' ? e.upcase : e}).join(' ')
# Implementation with send_if [2]
puts %w(Merry Christmas).send_if(:map, proc: :upcase ) { answer == 'y' }.join(' ')
```
# Implementation

Here is a Ruby implementation (obviously, everything is released under the same license terms as Ruby itself):

```ruby
class Object
  def send_if(method, *args, proc: nil)
     yield(self) ? self.send(method, *args, &proc) : self
  end
end
```

This implementation works as intended with both examples I posted above.


# Evaluation

I don't believe `send_if` brings significant performance penalties, compared to the alternatives.

I am not 100% satisfied with my implementation in terms of usability, for two reasons:
1. I did not find any stdlib methods which are consistent with the function signature I've specified. More specifically, I don't like the named `proc:` parameter I used, but I couldn't think of a better alternative. Please, tasukete!
2. Ruby does not support multiple blocks, which would be required for my ideal implementation (short of [3], see later):

```ruby
puts %w(Merry Christmas).send_if(:map, &:upcase) { answer == 'y' }.join(' ')
```

# Discussion

I know for sure there are more skilled Rubyists than myself here who can come up with nicer alternatives to my `send_if` examples, but I think `send_if` would be nice to have because:
* The `*_if` family of methods is a staple of the stdlib (e.g. `receive_if`, `delete_if`, `keep_if`, etc.)
* In some cases, it decreases the amount of code needed

I know my examples could be written without ever using `send` but `send` makes it possible to use any Ruby method (rather than write specific methods like `map_if`, etc.).

In the future, some syntactic sugar could be built so that method chaining is even more fluid, without any need for `send`. An example using an `.?{}` operator I just made up:

```ruby
# Syntax-level conditional chaining [3]
puts %w(Merry Christmas).?{answer == 'y'}map(&:upcase).join(' ')
```

Of course, `{answer == 'y'}` would be a block and this would be equivalent to my example above [2], but without any need for a `send` method (since this operator would apply to all methods).

If someone is interested, I can make a separate proposal for this operator, but perhaps it's asking too much :)

I'd be happy to discover more elegant solutions and critiques!

Merry Christmas to everybody and thanks for reading!



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