Add fallback to ease showing default content

[kaspth]

Just a quick idea to provide ease providing fallback content, in case a section is optional.

Before:

<% if partial.header? %>
  <%= partial.header %>
<% else %>
  <h1>Default Header</h1>
<% end %>

After:

<%= partial.header.fallback do %>
  <h1>Default Header</h1>
<% end %>

Or the one liner in this simple case <%= partial.header.fallback tag.h1("Default Header") %>.

I haven't wrapped this up and I'm gonna sleep on this if any knits or issues pop up.

[kaspth]

Technically, we already support this. Just for reference:

<% if header = partial.header.presence %>
  <%= header %>
<% else %>
  <h1>Default Header</h1>
<% end %>

[kaspth]

Suppose we could also support <%= partial.header.fallback.h1 "Default Header" %> by doing something like this:

def fallback(content = nil, &block)
  presence or begin
    case
    when content.present? then content
    when block_given?     then @view_context.capture(&block)
    else
      @view_context.tag # Return the plain tag context so people can call `h1` etc on it.
    end
  end
end

Though I don't know if that's worth it really.

[domchristie]

I quite like the Rails render collection fallback API, so I wonder if there's some inspiration to be taken from this?

<%= render(partial: "ad", collection: @advertisements) || "There's no ad to be displayed" %>

[kaspth]

@domchristie yeah, that works because render executes immediately whereas we defer until ERB calls to_s, so we could support the same with:

<%= partial.header.presence || "Default Header" %> <%# This already works today %>
<%= partial.header.presence or capture do %> <%# Here's the issue, that users need to call `capture` when they use a block fallback. %>

Reading this, I think I'm going to change it so that we alias presence to call to trim this a bit.

[kaspth]

@domchristie I ended up fiddling with something in the same ballpark in #84. And looking at this again the fallback does seem a bit fiddly.