Interpolating boolean attributes in HEEx

What is the proper way to interpolate boolean attributes like checked in an input in the new HEEx engine?

For example, in LEEx if I wanted to include it based on some condition, I would do this:

<input type="checkbox" name="web_pages[]" value="<%= %>" <%= if in @selected, do: "checked" %> />

Which would generate HTML like this:

<input type="checkbox" name="web_pages[]" value="1" />

<input type="checkbox" name="web_pages[]" value="2" checked />

Now in HEEx I do this

<input type="checkbox" name="web_pages[]" value={} checked={if in @selected, do: "true", else: nil} />

Which also works, if a nil value is passed then the checked attribute won’t be passed at all, but if I pass true then it will generate this:

<input type="checkbox" name="web_pages[]" value="2" checked="true" />

Which is correct, but longer and a bit uglier. I would have expected that this would work:

<input type="checkbox" name="web_pages[]" value={} {if in @selected, do: "checked"} />

But it doesn’t, it fails with protocol Enumerable not implemented for nil of type Atom. It looks like interpolation must always be bound to an attribute name in the HTML tag, right?

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It is OK to assign to multiple dynamic attributes at once, using the syntax of your last example.

However, the result is expected to be a keyword-list or a map. Currently you are returning a single string.

Instead, you can do, for instance:

<input type="checkbox" name="web_pages[]" value={} {if in @selected, do: %{checked: true}} />

However, in neither case will the result be a valueless attribute.
I don’t think this is a bad thing. Instead, not supporting this special case makes it easier for the HEEx parser to always generate correct HTML.

<input type="checkbox" checked={@boolean}> should just work.


Makes sense, I would prefer to be able to generate valueless attributes like checked instead of checked=true but it’s not that important, just checking if there was a way to do that.


I’ve no heex project at hand to check, but I’m quite confident it just generates a valueless attribute or no attribute.

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Ah! It does! The following works as expected, generating a valueless attribute for true or no attribute for false.

<input type="checkbox" name="web_pages[]" value={} checked={ in @selected} />

Awesome, thanks!


Here is a Surface reference on boolean attributes. Is there corresponding documentation in Phoenix Liveview on this subject that we can reference?

Surface (