How to trim rendered html content

Hello guys, I am not sure if this question has been asked before, but I did a search and did not find it.

I am looking to trim the html rendered from a ~F"“” sigil.

 defp produce_trimmed_html(assigns) do 
    html = ~F"""
    <somecomponent />

Here, String.trim is not working because the html is not a string but binary. How can I do this?

Can you show an example of what binary are you getting so we can see why String.trim doesn’t work on it? All other things being normal, it should work on a binary as well.

I am not able to retrieve the binary data.
Even a simple html causes an error.

defp some_method(assigns) do
    item = ~F"""


VSCode reports this:

The function call will not succeed.

  _item :: %Phoenix.LiveView.Rendered{
    :dynamic => (_ -> []),
    :fingerprint => 264_197_832_235_070_763_522_609_757_631_039_213_058,
    :root => true,
    :static => [<<_::96>>, ...]

will never return since the success typing is:
(binary()) :: binary()

The Surface error I am getting on the console is this:

no function clause matching in String.trim/1
    (elixir 1.13.4) lib/string.ex:1241: String.trim(%Phoenix.LiveView.Rendered{dynamic: #Function<0.132818971/1 in MoonWeb.Pages.IconsPage.get_import_text_by_attribute/2>, fingerprint: 264197832235070763522609757631039213058, root: true, static: ["<div></div>\n"]})

It tells me that item is not a string but binary()

As the error message tells you sigil_F doesn’t return a string. It returns a struct, which is required to optimize templates in LV usage (split dynamic vs. static parts, …). You simply cannot run String.trim on that.

Hi @alexisdevtailor!

You can achieve that with something like:

<somecomponent />
|> Phoenix.HTML.Safe.to_iodata()
|> IO.iodata_to_binary()
|> String.trim()

An alternative approach might work, by trimming first and sigiling later. Maybe something like:

defp produce_trimmed_html(assigns) do 
    <somecomponent />
   |> String.trim
   |> sigil_F
1 Like

@msaraiva Thank you for this suggestion. :blush: It works well, the only problem is that Safe is converting the html tags into safe representations. The data is not user supplied and I am pretty sure it is safe, so I don’t want to convert the html result into safe representation. Is there a way to do this?

@christhekeele Thank you for this suggestion. I am trying to this in a live_view and I get an error stating that sigil_F/1 is undefined. Do I need to import something first? :blush:

Note that returning a trimmed string rather than %Phoenix.LiveView.Rendered{} will remove all diffing, so this is almost definitely not what you want to do unless it’s the top-level render of a dead view. Even the the marginal trimming savings is not going to be worth the extra work.


Upon some digging, I’m now quite sure sigil macros are obliged to do all their work at compile-time only, and are not designed to readily accept run-time strings without a bit of hackery. You could always try to craft a runtime-friendly variant of whatever the sigil does under the covers, but as @chrismccord points out there are other reasons to rethink the approach.