Updated: Breaking changes for boolean attributes in HAML 6

Changes

-Haml 6 introduced [breaking changes](https://github.com/haml/haml/blob/main/CHANGELOG.md#600) regarding the definition of ["boolean attributes"](https://github.com/haml/haml/issues/1091). Only `aria-`, `data-` and a list of [select attributes](https://github.com/haml/haml/blob/main/CHANGELOG.md#600) (e.g. `hidden`, `disabled`) are considered "boolean". ["Boolean attributes" express meaning by their _presence_](https://haml.info/docs/yardoc/file.REFERENCE.html#boolean-attributes).
+Haml 6 introduced [breaking changes](https://github.com/haml/haml/blob/main/CHANGELOG.md#600) regarding the definition of ["boolean attributes"](https://github.com/haml/haml/issues/1091). Only `aria-`, `data-` and a list of [select attributes](https://github.com/haml/haml/blob/main/CHANGELOG.md#600) (e.g. `hidden`, `disabled`) are considered "boolean". ["Boolean attributes" express meaning by their _presence_](https://haml.info/docs/yardoc/file.REFERENCE.html#boolean-attributes).
**What actually changes is the handling non-boolean attributes** (see examples below). Haml will continue to render or drop boolean attributes depending on their value – but it will always render non-boolean attributes, even when given a value of `false`.
### Interpretation of "boolean" attributes does not change
- No Haml value => no HTML value
- Boolean Haml value => attribute is rendered depending on that value
- All other values => rendered verbatim
| Haml                           | HTML generated by Haml 5 & 6 |
|----------------------------|-----------------------|
| %button(disabled)          | `<button disabled></button>`          |
| %button(disabled=true)     | `<button disabled></button>`          |
| %button(disabled=false)    | `<button></button>`                      |
| %button(disabled='true')   | `<button disabled="true"></button>`      |
| %button(disabled='false')  | `<button disabled="false"></button>`     |
| %button(disabled='')       | `<button disabled=''></button>`          |
| %button(data-test)         | `<button data-test></button>`         |
| %button(data-test=true)    | `<button data-test></button>`         |
| %button(data-test=false)   | `<button></button>`                      |
| %button(data-test='true')  | `<button data-test="true"></button>`     |
| %button(data-test='false') | `<button data-test="false"></button>`    |
| %button(data-test='')      | `<button data-test=''></button>`         |
### Other attributes will be handled differently by Haml 6
New: custom attributes will always render an HTML value. They are not toggled by their value. To render a custom attribute without a value, you can either
- Assign an empty string (`custom-attribute=''`)
- Use a `data-` attribute (`data-custom-attribute`) – recommended for attributes that actually convey data
- Or add the custom attribute to the list of boolean attributes (see below)
| Haml                           | HTML generated by Haml 5 | HTML generated by Haml 6 |
|----------------------------|-----------|-----------|
| **%button(up-test)**           | `<button up-test></button>`          | `<button up-test="true"></button>`          |
| **%button(up-test=true)**      | `<button up-test></button>`          | `<button up-test="true"></button>`          |
| **%button(up-test=false)**     | `<button></button>`          | `<button up-test="false"></button>`          |
| %button(up-test='true')    | `<button up-test="true"></button>`          | same          |
| %button(up-test='false')    | `<button up-test="false"></button>`          | same          |
| %button(up-test='')    | `<button up-test=''></button>`          | same          |
## Extending the list of attributes considered "boolean"
You can add to the list of "boolean attributes". However, please do sparingly and prefer the alternatives suggested above.
```ruby
# config/initializers/haml.rb
Haml::BOOLEAN_ATTRIBUTES.push(/^app-/)
Haml::BOOLEAN_ATTRIBUTES.push(*%w[one another])
```
## Boolean [Unpoly](https://unpoly.com) attributes
Be very selective about patching `up-` attributes so you can still use expressions like `[up-follow=false]` or `[up-history=false]`.
Starting with [Unpoly 3.8](https://github.com/unpoly/unpoly/discussions/646), most Unpoly attributes can be enabled with a value `"true"` and be disabled with a value `"false"`. This pairs well with the default (unpatched) Haml 6 behavior.
A different approach is to create attributes from a ruby hash:
```haml
%tr{ { 'up-expand': ('' if superuser?) }.compact }
```
This will provide you with `<tr up-expand="">` for a superuser and `<tr>` otherwise.