Syntax/Feature Need: Passable Blocks (as args / other?)

[NullVoxPopuli]

context: #460 (comment)

The lack of this capability is a HUGE growing pain for the projects I'm working on and causes a lot of cruft and awkward APIs, and having this feature would simplify a ton of things.

For library / ui / design-system authors, having passable blocks would be an IMMENSE lift / relief -- and it feels very surprising that this wasn't talked about more during the original RFC (or maybe it was planned to be added later, and just no one has gotten around to it)

Related thoughts:

• maybe there is a way to:
  • reference anything
  • pass anything

<SubComponent @myBlock={{:myBlock}}>

<:@myBlock>

</:@myBlock>

---

Example:

the first place this came up was our big table component wrapped with all the bells and whistles.
Here is a more generic, yet very related, example.

A table component

• could yield to="header"
• and yield to="body"
• etc

A wrapper component cannot

• optionally yield to="header" and also have that header block be within the table component's header block (and the same for any block).

Like,

<WrappedTable>
  <:header>
     <th></th>
  </:header>
</WrappedTable>
and
<WrappedTable />

are not possible APIs.

You can hack around this, by, in the implementation of WrappedTable, put an if block around the entirety of Table, but that's 1. terrible, and 2. does not scale past 1 named block.

I have 7 named blocks in Table component

In order to provide a wrapper table (with nice defaults, optional blocks, etc), I cannot fathom (other than actually doing the math) the amount of code it would take to if-stamement my way through implementing that.

[wagenet]

@locks status?

[NullVoxPopuli]

Another potential syntax:

<SubComponent @:myBlock={{:myBlock}} />

This implies:

• :myBlock is a value that can be passed
• a new arg syntax @: implies named block values could be accessed from this.args
  • what would this value look like in JS? do we error upon access, since it's meant for the template?
  • this syntax also implies that we may have other @* notations in the future
• setting a value to a @: argument means the same as if the component wrote out:

  <:myBlock>
    {{yield to="somethingElse"}}
  </:myBlock>

  with the exception that if a value passed to @: is falsey, the named block matching the identifier after the : is not present.

Alternatively (or maybe in addition to, because this is about to be way more typing than the above,
we could allow control-flow syntaxes outisde named-block space.

For example:

{{! the contents of "MyDemo" component }}
<ListItem @list={{@list}}>
  {{#if (has-block "media")}}
    <:media>
      {{yield to="media"}}
    </:media>
  {{/if}}
</ListItem>

e.g.: if <MyDemo> is rendered with a <:media> block, pass those contents to <ListItem>, otherwise render <ListItem> as if no :media block were used.