Disabling hanging indentations with fill-paragraph

Question

I want to be able to fill a whole paragraph with fill-paragraph, first line included, between the left-margin and fill-column values (adaptive-fill-mode is disabled).

I am able to do this, but I noticed that if the first line has an hanging indentation (meaning it has a different indentation from the left-margin variable), I am unable to remove it, unless I manually go to the first line, removing it. I want to be able to remove the hanging indentation by just running fill-paragraph.

Example of hanging indentation, considering left-margin equal to 0:

  this is an example
of an hanging indentation
in a paragraph.

What I want to happen when I run fill-paragraph:

this is an example
of an hanging indentation
in a paragraph.

But I am unable to recreate such a behaviour. The documentation on the topic barely mentions hanging indentations, and is not helpful on how to disable them.

Answer 1

Since it seems like there is no option to remove them, I created a hack to work around this.

(defun my-fill-paragraph (&optional justify)
  (interactive "P")
  (backward-paragraph)
  (skip-chars-forward " \n\t")
  (indent-to-left-margin)
  (fill-paragraph))

(setq fill-paragraph-function 'my-fill-paragraph)

What this does is moving at the beginning of the paragraph, skip any whitespace and newline and then indent the first line of the paragraph to left-margin. After that, it just runs fill-paragraph normally with the new indentation.