[ruby-core:90798] [Ruby trunk Bug#15483] Proc or Method combination with Symbol

[aycabta@gmail.com]

Issue #15483 has been reported by aycabta (aycabta .).

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

* Author: aycabta (aycabta .)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
In [Feature #6284](is titled "Add composition for procs"), 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"]
```




-- 
https://bugs.ruby-lang.org/