@EricShulman ‘s response to an earlier query. Regarding the display of captions instead of titles raised something that has been on my mind for some time. His example of a view widget inside a transclusion widget in an ingenious way to have a if..then condition.
so, what is this called? What do I look up to learn about it?
In particular, what widgets support this, how many levels of Nesting are supported?
Bobj
There are many widgets that use their contents as a normal part of their operation. For example:
$let, $vars, $set and $wikify each define variables that are then available for reference within their content$list evaluates a filter and renders content for each matching tiddler$action-confirm displays an “ok/cancel” message and, if “ok” is clicked, performs whatever action widgets are within it contentsThere are also some widgets that completely ignore their contents, such as $text, $edit-text, $count, $browse, and most (but not all) $action- widgets.
However, there are very few widgets that use their content for “fallback” handling. These include: $transclude and $view (as previously noted), and also $macrocall (which SAYS it’s contents are ignored, but actually works as a fallback if the $name=... parameter references a non-existent macro). There may be other widgets that do this as well, but the documentation on this is spotty at best.
-e
Long story short: If a widget by default is “self closing”. So by default it has no body. It’s called a fallback body
Long story 
As Eric pointed out for many widgets it’s common to have a “content”. The area between widget start-marker and the end-marker is also called body. eg: $let
The let-widget always “renders” it’s body.
<$let test-a="abc">
<<test-a>>
</$let>
Some widgets have a “fallback” value that is used, if a filter or a variable is empty. eg: list-widget. The emptyMessage parameter is used if the list-widget filter produces no result. eg:
<$list filter="[tag[non-existant]]" emptyMessage="n/a">
<$link/>
</$list>
So the emptyMessage contains a so called fallback value
Some widgets by default “ignore” their body. $view and $transclude, because they have parameters, that are used to show the content of something. $view and $transclude have a tiddler-parameter. Those widgets by default are self closing. They have /> at the end. They have no body. eg:
<$transclude $tiddler="non-existant"/>
Since the “non-existant” tiddler does not exist this transclusion would show nothing. Which may or may not be what a user wants. If we always need “output” there needs to be a mechanism allows us to do so. That’s the “fallback body”.
This fallback body is “prominently” used with the caption field in lists and the TOC macros. Which does this:
\define toc-caption()
\whitespace trim
<span class="tc-toc-caption tc-tiny-gap-left">
<$set name="tv-wikilinks" value="no">
<$transclude field="caption">
<$view field="title"/>
</$transclude>
</$set>
</span>
\end
By default the transclusion does not show anything, if it’s parameter does not exist. So if there is no caption-field, it would be empty. Since we do not want that, the transclusion widget uses a fallback body. …
It shows the title instead. Since the title field is the only field a tidder must have we can be sure there is something. Even if it is $:/very/long/title/like/this. May be not what we want to see, but at least there is something.
This mechanism is also used by the tabs-macro and probably many other places.
Hope that makes sense
-mario
Perfect sense @pmario , thank you for taking the time to explain.
One question though , can such a fallback facility be nested, ie. can the fallback widget itself have a fallback widget?
Bobj
Yes. For example, suppose you have a custom field named alt-title, you could write something like this:
<$transclude field="alt-title">
<$transclude field="caption">
<$view field="title"/>
</$transclude>
</$transclude>
This would let you have an alt-title that supersedes the caption, with a final fallback to title if neither an alt-title nor caption are defined.
-e
Great @EricShulman thanks for the clarification, by you both. Be nice to have this in the documentation but I guess that’ll not happen unless it is and I haven’t come across it.
bobj
other related fall backs are;
<$view> widget is displayed if the field or property is missing or empty.
ts-missing to specify the content to be rendered if the transclusion target is not defined (i.e. a missing tiddler or a missing field).We can also mention the fact that if we transclude a transclusion, transclusion “attempts to wikify” the transclusion, and one part of wikification is transclusion, and this will keep occurring to an arbitrary depth.