Issue #15538 has been updated by kke (Kimmo Lehto).

Description updated
Subject changed from Erb indenting / unindenting trim mode to Erb indenting / unindenting 

Perhaps `<%~` would be good as it resembles the squiggly heredoc `<<~EOB`

I'll try to improve on your PoC.

It seems to only work when the `<%|` is in the beginning of the line, for anything else it fails. I have yet to find a way to find the index of `stag`, but I have a hunch it has to be done in the line scanner and yielded there.

Also, something like this should work:

```
<%= "abcd" %> <%~ [1.2.3].each do |num| -%>
                <%= num %>
              <%~ end -%>
```

which would produce:

```
abcd 1
     2
     3
 ```


----------------------------------------
Feature #15538: Erb indenting / unindenting 
https://bugs.ruby-lang.org/issues/15538#change-76808

* Author: kke (Kimmo Lehto)
* Status: Third Party's Issue
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
In `Erb`, would it be possible to add a new tag that would indent the following content to match the depth of the tag? The tag could be `<%~` (to resemble the `<<~EOS` squiggy heredoc).

## Reason

Something like this would be easy to follow:

``` ruby
1
<%- [2, 3, 4].each do |num| -%>
  <%- unless num == 3 -%>
    <%= num %>
  <%- end -%>
<%- end -%>
5
```

But unfortunately it will render with "extra" indentation:

```
1
    2
    4
5
```

Currently, to avoid this, you have to write your template using either no indentation:

```
1
<%- [2, 3, 4].each do |num| -%>
<%- unless num == 3 -%>
<%= num %>
<%- end -%>
<%- end -%>
5
```

Or a weird jumpy indentation:

```
1
<%- [2, 3, 4].each do |num| -%>
  <%- unless num == 3 -%>
<%= num %>
  <%- end -%>
<%- end -%>
5
```

With the `<%~` it could be written as:

```ruby
1
<%~ [2, 3, 4].each do |num| -%>
  <%- unless num == 3 -%>
    <%= num %>
  <%- end -%>
<%~ end -%>
5
```

And it would output as desired without the "extra" indentation:

```
1
2
4
5
```

Another example:

```
<%= "abcd" %> <%~ [1.2.3].each do |num| -%>
                <%= num %>
              <%~ end -%>
```

would produce:

```
abcd 1
     2
     3
 ```


## Using with `=`

It would also be handy if the `~` could be used in `<%=` statements, perhaps as `<%~=`. This would be excellent for example when templating YAML's:

```yaml
<%- bars = %w(abc def)" -%>
foo:
  bar: 
    <%~= bars.map { |bar| "- #{bar}\n" } %>
```

Which would reindent the statement outcome to produce something like:

```yaml
foo:
  bar:
    - abc
    - def
```

This would require these new tags:

1. `<%~` begin a code block and begin or end reindentation mode. content produced inside the block will be reindented to the depth of the `<` character in `<%~`. If the indentation mode was already active due to a previous `<%~`, it ends the indentation mode.
2. `<%~=` like regular `<%=` but multiline strings will be reindented to the column of the `<` character





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