[ruby-core:109997] [Ruby master Feature#19015] Language extension by a heredoc

Issue #19015 has been updated by yugui (Yuki Sonoda).


one bikeshedding..

Usually syntax-suger in Ruby are mapped into methods whose names are very consistent with the syntax. e.g. `` ` ``, `[]=`, or `foo=`. Therefore, it is more consistent to map the new syntax into `<<!LANG` method.
It seems possible to make the lexer distinguish such a notation from `<<` symbol.

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Feature #19015: Language extension by a heredoc
https://bugs.ruby-lang.org/issues/19015#change-99248

* Author: ko1 (Koichi Sasada)
* Status: Open
* Priority: Normal
----------------------------------------
This propose new heredoc extension with `<<!LANG` like

```ruby
doc = <<!LANG
  # description written in lang LANG
  foo bar
LANG
```

and it is translated to:

```ruby
doc = heredoc_extension_LANG(heredoc_text, binding)
```

## Example

```ruby
require 'erb'

def heredoc_extension_erb str, b
  ERB.new(str).run(b)
end

name = 'ko1'

html = <<!erb
<div>Hello <%= name %></div>
erb

puts html #=> <div>Hello ko1</div>
```

## Background / considerations

* Sometimes we write Ruby syntax string with `<<RUBY` and this proposal inspired by it.
* it is similar to shebang (`#!LANG` in shell)
* [Elixir's custom sigil](https://elixir-lang.org/getting-started/sigils.html) translates `~u(...)` translates to `sigil_u(...)`. This is why it translated to `heredoc_extension_LANG(...)` private method call.
* JavaScript has JSX but I don't think it is fit to the Ruby language.
* Heredoc is Ruby's chaos part and already confusing a lot. Additional chaos doesn't matter.
* `<<!foo` is valid syntax but now I don't think it is not used. gem codesearch doesn't find the usage.
* Sorry I couldn't wait 1st/Apr.

## Implementation

I attached the experimental implementation which only supports `erb` (because I couldn't find how to get delimiter to determine a method name :p).


---Files--------------------------------
heredoc_extension.patch (2.7 KB)


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