Existence Check - inverse of Inverted Section

[clach04]

Option 3 in #157 in Stache for Python.

Copy/paste extract from readme:

Existence Check {{?exists}}{{/}}

See #157

Forces a check of the tag name, rather than imply that it is a section block. This
is useful for check if an array has members rather than iterate over the members

{{?array}}
stuff\n
{{/}}

with {array: [1, 2, 3, 4]} results in:

stuff

as opposed to

{{#array}}
stuff\n
{{/}}

which would render

stuff
stuff
stuff
stuff

I can't claim any ownership of this, this was implemented in https://github.com/hyperturtle/Stache by hyperturtle, more about my fork #172

[clach04]

I make extensive use of this, and I've seen a number of requests (see #157 (comment) for links to some requests) for this functionality (even if not this syntax). Typically used to avoid including html content for a section or even boiler plate for a table so as to avoid an empty table body. My primary use case is omitting entire sections, not just the table boilerplate.

Whilst I like this syntax, I'm not wedded to it. I could happily use an alternative syntax based on census in #157

[determin1st]

[[^array]]
  is empty
[[|]]
  is not empty
[[/]]

[[#array]]
  [[.]]
[[|]]
  is empty
[[/]]

[jgonggrijp]

That is also an interesting solution I had not thought of. Shall I add it to the table in #157?

[determin1st]

as you wish

[jgonggrijp]

Done.

[clach04]

Here are 3 uses that I've cobbled together from some real use cases I have at work:

variables

{
    "code": "void main()\n{\n}\n",
    "crimes": [
        "like country music",
        "leave the toliet seat up"
    ],
    "crime_details": [
        {
            "crime": "like country music",
            "url": "http://somewhere/1"
        },
        {
            "crime": "leave the toliet seat up",
            "url": "http://somewhere/2"
        }
    ]
}

Template

<h1>Demo not exists use cases</h1>

<h2>Code block tags</h2>

{{?code}}
<pre><code>
{{code}}
</code></pre>
{{/code}}

<h2>Simple Value Lists(i.e. not hash/dictionary)</h2>

{{! Only show crimes text and list if there is one. NOTE Stache only feature }}
<!-- crimes -->
{{?crimes}}
<p>You are a criminal and a generally awful person in general. These are the crimes you are guilty of:
</p>

<ol>
{{/crimes}}

{{#crimes}}
    <li>{{.}}</li>
{{/crimes}}

{{?crimes}}
</ol>
{{/crimes}}

<h2>Hash/Dictionary Lists</h2>

{{! Only show crimes text and list if there is one. NOTE Stache only feature }}
<!-- crime_details -->
{{?crime_details}}
<p><p>You are a criminal and a generally awful person in general. These are the crimes you are guilty of:
</p>

<ol>
</p>
{{/crime_details}}

{{#crime_details}}
    <li><a href="{{{url}}}">{{crime}}</a></li>
{{/crime_details}}

{{?crimes}}
</ol>
{{/crimes}}

Rendered Output

<h1>Demo not exists use cases</h1>

<h2>Code block tags</h2>

<pre><code>
void main()
{
}

</code></pre>

<h2>Simple Value Lists(i.e. not hash/dictionary)</h2>

<!-- crimes -->
<p>You are a criminal and a generally awful person in general. These are the crimes you are guilty of:
</p>

<ol>

    <li>like country music</li>
    <li>leave the toliet seat up</li>

</ol>

<h2>Hash/Dictionary Lists</h2>

<!-- crime_details -->
<p><p>You are a criminal and a generally awful person in general. These are the crimes you are guilty of:
</p>

<ol>
</p>

    <li><a href="http://somewhere/1">like country music</a></li>
    <li><a href="http://somewhere/2">leave the toliet seat up</a></li>

</ol>



<h1>Demo not exists use cases</h1>

<h2>Code block tags</h2>

<pre><code>
void main()
{
}

</code></pre>

<h2>Simple Value Lists(i.e. not hash/dictionary)</h2>

<!-- crimes -->
<p>You are a criminal and a generally awful person in general. These are the crimes you are guilty of:
</p>

<ol>

    <li>like country music</li>
    <li>leave the toliet seat up</li>

</ol>

<h2>Hash/Dictionary Lists</h2>

<!-- crime_details -->
<p><p>You are a criminal and a generally awful person in general. These are the crimes you are guilty of:
</p>

<ol>
</p>

    <li><a href="http://somewhere/1">like country music</a></li>
    <li><a href="http://somewhere/2">leave the toliet seat up</a></li>

</ol>

I do like the ^ | syntax BUT for my selfish use cases (above), I'd have to use something like:

{{^variable}}{{|}}
stuff here
{{/variable}}

I'd rather the proposed ^^ instead, but again I do like the ^ | syntax and would likely have some uses cases for it.

I feel more experienced with Mustache when I first wrote that code sample above so I'd likely not write a template in that style BUT for the list examples I do need a plain not exists (not falsey) check.

[jgonggrijp]

The code example would work with a regular section, but otherwise, I agree there is a need for something like a list nonemptiness check. Your examples are pretty typical and unsurprising for this use case. I don't mean that as a bad thing, by the way.

While there are some people who think no new solution is required, I expect most of the controversy to be about how to solve it if you do. We already have #157 for that discussion, so I will not go into it very deeply over here.

My objection to the current proposal, a dedicated sigil {{?, is that we would be introducing a change to the language just for this single use case. {{^^ has the same problem, except that it also has far-reaching implications for the grammar of the language: sigils suddenly behaving like unary operators that can be combined OR sigils suddenly being multi-character.

If we introduce some kind of new feature in Mustache that solves the list nonemptiness problem (and again, I agree we should), then please let it be something that also solves a whole range of other problems. @determin1st's {{|}} already does much better in that regard, but I personally still prefer filters.

[determin1st]

{{?crimes}}
  <ol>
  {{#crimes}}
    <li>{{.}}</li>
  {{/}}
  </ol>
{{/}}

VS

{{^crimes}}
{{|}}
  <ol>
  {{#crimes}}
    <li>{{.}}</li>
  {{/}}
  </ol>
{{/}}

VS

{{#crimes-boolean-flag-maybe-from-the-helper-you-push-along-the-data}}
  <ol>
  {{#crimes}}
    <li>{{.}}</li>
  {{/}}
  </ol>
{{/}}

imo, the third is the best one.
using lambdas with auto-ejecting helpers allows to split big templates into parts, for example

...
{{put crimes}}
...

where put is a lambda (a function) which will check whether crimes is non-empty and optionally create and push crimes helper onto the stack

[jgonggrijp]

What are "auto-ejecting helpers"?

[determin1st]

those helpers that are pushed by lambda (inside the function's body).

ejection procedure must differ for a variable and a block. this improvement to lambdas will allow cleaner solution where previously data enrichment/mixin took place.