Issue #15483 has been updated by osyo (manga osyo).


I am thinking like this.

NOTE: Here we define it as follows.

* functional object
  * defined `#call` (and `#<<` `#>>`) object
  * e.g. `Proc` `Method`
* blockable object
  * defined `#to_proc` object
  * e.g. `Symbol` `Hash`


## Current

* `Proc#<<` and `Proc#>>` arguments is functional object
  * call `#call`.
* `Proc#<<` and `Proc#>>` is not call `#to_proc`
* `Proc#<<` and `Proc#>>` is not accept block argument


## Composite function in Ruby

* Composite function is functional object and functional object
* `functional object >> functional object # => OK
* `functional object >> other object` # => NG
* `other object >> functional object` # => NG


## `Symbol` is functional object

* `Symbol` is blockable object
* `Symbol` is not functional object
* Handling `Symbol` with compositing functions is incorrect
* What about other blockable objects?
  * e.g. `Hash`
  * `Hash` is functional object?


## `Proc#<<` is call `#to_proc` ?

* It should be explicitly converted to `Proc` (functional object) with` # to_proc`
  * `proc << :hoge` => NG: `:hoge` is not `Proc`
  * `proc << :hoge.to_proc` => OK : Explicitly convert `:hoge` to `Proc`
* Same as not handling `"42"` as a `Integer`
  * `1 + "42"` =>  NG : `"42"` is not `Integer`
  * `1 + "42".to_i` => OK : Explicitly convert `"42"` to `Proc`


## Proposal1 : `Symbol` to functional object

* define `Symbol#>>` `Symbol#<<` `Symbol#call`
* What about other blockable objects?
  * `Hash` is functional object?
* Is it really necessary for `Symbol` ?
* Is `Symbol` really a "functinal object" ?

```ruby
# Symbol to functional object
class Symbol
	def call(*args, &block)
		to_proc.call(*args, &block)
	end

	def <<(other)
		to_proc << other
	end

	def >>(other)
		to_proc >> other
	end
end

p %w{72 101 108 108 111}.map(&(:to_i >> :chr))
# => ["H", "e", "l", "l", "o"]
```


## Proposal2 : `Symbol` to functional object

* `Proc#<<(other)` to `Proc#<<(other, &block)`
* Prioritize `other` ?

```ruby
class Proc
	prepend Module.new {
		def <<(other = nil, &block)
			# other or block?
			super(other || block)
		end

		def >>(other = nil, &block)
			# other or block?
			super(other || block)
		end
	}
end

# :to_i convert to Proc
# must be `.>>`
p %w{72 101 108 108 111}.map(&(:to_i.to_proc.>> &:chr))
# => ["H", "e", "l", "l", "o"]
```


## Proposal3 : Define syntax sugar for `#to_proc`

* For example, define `#to_proc` to `@~`.
  * or other Unary operator
  * `@+` `@-` `@!` `&` ?
* Do not change current specifications
* I think this is good

```ruby
# Add ~@
class Object
	# ~ is to_proc
	# ~ or other unary operator?
	def ~@
		to_proc
	end
end

# Use Symbol#to_proc
p %w{72 101 108 108 111}.map(&(:to_i.to_proc >> :chr.to_proc))

# alias ~ is to_proc
p %w{72 101 108 108 111}.map(&~:to_i >> ~:chr)
```

Thank you :)

[Japanese](https://gist.github.com/osyo-manga/1725a4a670aac54452eca92269a3822b)


----------------------------------------
Feature #15483: Proc or Method combination with Symbol
https://bugs.ruby-lang.org/issues/15483#change-76128

* Author: aycabta (aycabta .)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
In [Feature #6284], Matz said

> We need more discussion if we would add combination methods to the Symbol class.

Right, let's get started to discuss.

For your information, recent a few months I'm discussing this with @osyo .

## This is a discussion of "design"

I understand that all features of this issue have both merits and demerits, but I guess that language design is most important. All features of this issue related to each other.

## Abstract

At present, you can use `Proc#>>` or `Proc#<<` with `Symbol#to_proc`.

```ruby
%w{72 101 108 108 111}.map(&(:to_i.to_proc >> :chr.to_proc))
# => ["H", "e", "l", "l", "o"]
```

This is convenient but methods that take block can take a proc with `&` syntax sugar instead of `#to_proc` by right, like `[1, 2, 3].map(&:to_s)`. So `Symbol#to_proc` looks like too long for `Proc#>>` or `Proc#<<`. Therefore, you need new syntax sugar.

## Receiver

### `Symbol#>>` and `Symbol#<<`

`Symbol#>>` and `Symbol#<<` will be considered, but this means that `Symbol` is treated as `Proc` partially. The `[1, 2, 3].map(&:to_s)` treats `Symbol` as `Proc` partially too, but it's with pre-positioned `&`.

```ruby
%w{72 101 108 108 111}.map(&(:to_i >> :chr.to_proc))
# => ["H", "e", "l", "l", "o"]
```

I can't come up with other ideas for the `Symbol` receiver.

### New `&:symbol_name` syntax sugar for `:symbol_name.to_proc`

```ruby
%w{72 101 108 108 111}.map(&(&:to_i >> :chr.to_proc)))
# => ["H", "e", "l", "l", "o"]
```

## Argument

### Calls `#to_proc` by `Proc#>>` or `Proc#<<` internally as a duck typing

```ruby
%w{72 101 108 108 111}.map(&(:to_i.to_proc >> :chr))
# => ["H", "e", "l", "l", "o"]
```

In this case, `Proc#>>`(`:to_i.to_proc >>`) calls `Symbol#to_proc`(for `:chr`) inside.

This is useful to use with `Hash#to_proc`:

```ruby
h = { Alice: 30, Bob: 60, Cris: 90 }
%w{Alice Bob Cris}.map(&(:to_sym.to_proc >> h))
# => [30, 60, 90]
```

### `Proc#>>` and `Proc#<<` take block as an argument

```ruby
%w{72 101 108 108 111}.map(&(:to_i.to_proc >> &:chr))
```

## Combination of receiver and argument

`Symbol#>>` and calling `#to_proc` internally:

```ruby
%w{72 101 108 108 111}.map(&(:to_i >> :chr))
# => ["H", "e", "l", "l", "o"]
```

`&:symbol_name` syntax sugar for `:symbol_name.to_proc` and `Symbol#>>` and taking block:

```ruby
%w{72 101 108 108 111}.map(&(&:to_i >> &:chr))
# => ["H", "e", "l", "l", "o"]
```




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