[ruby-core:101806] [Ruby master Feature#17471] send_if method for improved conditional chaining

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


osyo (manga osyo) wrote in #note-1:
```ruby
puts number.tap { break _1 * 2 if _1 > 5 }.send(:-, 1)
puts %w(Merry Christmas).tap { break _1.map(&:upcase) if answer == 'y' }.join(' ')
```

Hello and thanks for sharing this Ruby idiom, wouldn't have thought of it myself and I like it a lot! 

I guess there are a couple of reasons I would still prefer an explicit `send_if` method:
1. Because the `*_if` family of method already exists and their usage is predictable
2. Because the control flow with `send_if` would be a bit more explicit. I guess less skilled Rubyists would take a while to figure out tap + break, because it's a control flow disruption

> FYI : Proposal of [[Feature #15829] Object#then_if(condition){}](https://bugs.ruby-lang.org/issues/15829)

I've seen that proposal and get the general sense of it, however I don't like the condition being the argument because it's inconsistent with `keep_if` and `delete_if` from e.g. `Array`.

Now, #15557 would be an alternative to my proposal. If I understood it correctly, that way you could write my `send_if` code as:

```ruby
# This time I'll too use numbered parameters :)

# Proposal
puts number.send_if(:*, 2) { _1 > 5 }.send(:-, 1)
puts %w(Merry Christmas).send_if(:map, proc: :upcase ) { answer == 'y' }.join(' ')

# With #15557
puts number.when { _1 > 5 }.then { _1 * 2 }.send(:-, 1)
puts %w(Merry Christmas).when { answer == 'y' }.then { _1.map(&:upcase) }.join(' ')
```

The only thing which I still prefer about `send_if` is that it requires no mutable state internally, however given it's a single assignment I would gladly accept it for the sake of readability.

I don't think the two proposals are incompatible though, they could coexist and they are both predictable. `send_if` would sometimes result in more concise code but at the moment, given that Ruby doesn't support multiple blocks, I believe what @sawa proposed implements the same style of programming in a more elegant way.

----------------------------------------
Feature #17471: send_if method for improved conditional chaining
https://bugs.ruby-lang.org/issues/17471#change-89633

* 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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