Handling nil/false attributes and attr defaults consistently

[thiagomajesk]

Typically nil and false will omit the attribute as per the documentation:
https://hexdocs.pm/phoenix_live_view/Phoenix.Component.html#sigil_H/2-syntax

false or nil - if a value is false or nil, the attribute is omitted. Some attributes may be rendered with an empty value, for optimization purposes, if it has the same effect as omitting [...]

So, the following:

<span class={nil}>Hello World</span>
<span class={false}>Hello World</span>

Will output: <span class="">Hello World</span>

And the following:

<span data-class={nil}>Hello World</span>
<span data-class={false}>Hello World</span>

Will output: <span>Hello World</span>

It gets especially weird because this is not how it's handled if you have a function component with global attributes:

attr :rest, :global
slot :inner_block

defp span(assigns), do: ~H"<span {@rest}>{render_slot(@inner_block)}</span>"

The following:

<.span class={nil}>Hello World</.span>
<.span class={false}>Hello World</.span>
<.span data-class={nil}>Hello World</.span>
<.span data-class={false}>Hello World</.span>

Will output: <span data-phx-id="m3-phx-GBS__HJfKXiKyBHD">Hello World</span>

I don't know exactly what optimizations the doc is referring to, but to me, it already feels too implicit/ hard to track.

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And it gets a little trickier because you can bypass the compiler warning with something like the following:

attr :class, :string, default: "foo"
slot :inner_block

defp span(assigns), do: ~H"<span class={@class}>{render_slot(@inner_block)}</span>"

Invoking it like this <.span class={if nil, do: nil}>Hello World</.span> also won't pick up the default value of the attr.

So, "nillish" values are handled in 3 different ways. Given that conditional logic with attrs is pretty common, should we try to make this more consistent for better DX?

• Always empty attributes with nil unless it's a boolean attribute!?
• Prevent compiler type checking attr when nil (unless attr is required)!?
• Empty attr assign / use default value when nil is given to attr