await...then shorthand

[Rich-Harris]

Follow-up to #952. This...

{{#await promise then foo}}
  <p>{{foo}}</p>
{{/await}}

...would be a nice shorthand for this:

{{#await promise}}
  <!-- no need to show a pending state -->
{{then foo}}
  <p>{{foo}}</p>
{{catch error}}
  <!-- promise is guaranteed to fulfil -->
{{/await}}

[antony]

I'd recommend an implicit variable as an option, since seeing await and then together seems strange.

Coming from a groovy background we always have it:

Explicit:

def myVariable = 'foo'
myVariable.with { myVariable ->
  myVariable == 'foo'
}

Implicit:

def myVariable = 'foo'
myVariable.with {
  it == 'foo'
}

So this would mean you could either have the implicit:

{{#await foo}}
  {{it}} equals foo
{{/await}}

Or the explicit:

{{#await foo then quux}}
  {{quux}} equals foo
{{/await}}

Sorry about the atrocious code examples, it's been a long day!

[Rich-Harris]

Thanks for the idea. To be honest I think that's a bit more confusing — it's not at all clear at first glance that the {{it}} equals foo fragment is the then block as opposed to the pending block, whereas requiring the then keyword makes it unambiguous and avoids introducing any magic variables (which then have to be deconflicted against any existing uses of it, etc. I've released this as 1.46.1