Cro Template Syntax

Generalities §

A template starts out in content mode, meaning that a template file consisting of plain HTML:

<h1>Oh, hello there</h1>
<p>I've been expecting you...</p>

Will result in that HTML being produced.

Syntax significant to the templating engine consists of a HTML-like tag that begins with a non-alphabetic character. Some stand alone, such as <.foo> and <$foo>, while others have a closer, like <@foo>...</@> The closers do not require one to write out the full opener again, just to match the "sigil". One may repeat the opening alphabetic characters of an opener in the closer if desired, however (so <@foo> could be closed with </@foo>).

The topic variable §

As in Raku, there is a notion of current topic - the $_ variable - and some conveniences for working with it. The data that is passed to the template to render is placed into the topic, and for simple templates one can access properties from that. More complex templates can instead use the parts mechanism, described later.

Indexing hash, array, and object data §

The <.name> form can be used to access object properties of the current topic. If the current topic does the Associative role, then this form will prefer to take the value under the name hash key, falling back to looking for a method name if there is no such key. Failure to find the method is a soft failure in the case of an Associative (it produces Nil), and an exception otherwise.

For example, given a template greet.crotmp:

<p>Hello, <.name>. The weather today is <.weather>.</p>

Rendered with a hash:

template 'greet.crotmp', {
    name => 'Dave',
    weather => 'rain'
}

The result will be:

<p>Hello, Dave. The weather today is rain.</p>

The hash fallback is to ease the transition from using a Hash at first, and then refactoring towards a model object later on.

Various other forms are available:

<.elems()> with parentheses will always be a method call, even if used on an Associative (so can be used to overcome the key fallback)
<.<elems>> will always be a hash index
<.[0]> indexes the array element 0
<.{$key}> can be used to do indirect hash indexing
<.[$idx]> can be used to do indirect array indexing

These can all be chained, thus allowing for things like <.foo.bar.baz> for digging into objects/hashes. When using the indexer forms, then only the leading . is required, thus <.<foo>.<bar>> could be written instead as <.<foo><bar>>.

The result of the indexing or method call will be strigified, and then HTML encoded for insertion into the document.

Variables §

Various Cro template constructs introduce variables. These include iteration, subroutines, macros, and parts. Note that variables that are in scope in the route block at the location template is called are not in scope in the template; only variables explicitly introduced inside of the template can be referenced.

The <$name> syntax is used to refer to a variable. It will be stringified, HTML encoded, and inserted into the document. It is a template compilation time error to refer to a variable that does not exist. The current topic can be accessed as <$_>, and this is the only variable that is in scope at the start of a template.

It is allowed to follow the variable with any of the syntax allowed in a <.foo> tag, for example <$product.name> or <$product<name>>. For example, assuming we were inside a construct that defined the variables $person and $weather, for example:

<:sub greeting($person, $weather)>
  <p>Hello, <$person.name>. The weather is <$weather.description>, with a low of
    <$weather.low>C and a high of <$weather.high>C.</p>
</:>

Then calling the template sub would render something like:

<p>Hello, Daria. The weather is sunny, with a low of
  14C and a high of 25C.</p>

Iteration §

The @ tag sigil is used for iteration. It may be used with any Iterable source of data, and must have a closing tag </@>. The region between the two will be evaluated for each value in the iteration, and by default the current target will be set to the current value.

For example, given the template:

<select name="country">
  <@countries>
    <option value="<.alpha2>"><.name></option>
  </@>
</select>

And the data:

{
    countries => [
        { name => 'Argentina', alpha2 => 'AR' },
        { name => 'Bhutan', alpha2 => 'BT' },
        { name => 'Czech Republic', alpha2 => 'CZ' },
    ]
}

The result would be:

<select name="country">
    <option value="AR">Argentina</option>
    <option value="BT">Bhutan</option>
    <option value="CZ">Czech Republic</option>
</select>

It is possible to avoid repetition and risk by using the "structured tag" syntax. This allows the previous example:

<select name="country">
  <@countries>
    <option value="<.alpha2>"><.name></option>
  </@>
</select>

To be abbreviated as:

<select name="country">
  <@countries option value="<.alpha2>"><.name></@>
</select>

That is, the option opening tag will be emitted, and a matching closing </option> will be produced as if written prior to the </@>

The <@foo> syntax is short for <@.foo>, and follows the same rules as <.foo> for resolution. It is also possible to write <@$foo> to iterate over a variable, and to index properties, for example <@$band.rockstars>.

To specify a variable to declare and populate with the current iteration value instead, place a : afterward the iteration target and name the variable. For example, the earlier template could be written as:

<select name="country">
  <@countries : $c>
    <option value="<$c.alpha2>"><$c.name></option>
  </@>
</select>

Which leaves the current topic in place. This can also be used with the structured tag syntax:

<select name="country">
  <@countries : $c option value="<$c.alpha2>"><$c.name></@>
</select>

If the opening and closing iteration tags are the only thing on the line, then no output will be generated for those lines, making the output more pleasant.

Sometimes one wants to emit a separator between values (but that should not be repeated after the final value). Such a separator can be specified using the <:separator>...</:> tag directly within the body of the iteration:

<@news>
  <h3><.headline></h3>
  <p><.body></p>
  <:separator>
    <hr/>
  </:>
</@>

Conditionals §

The ? and ! tag sigils are used for conditionals. They may be followed by either a . and then a topic access (for example, <?.is-admin>...</?>) or by a variable (<!$user.is-admin>...</!>). For example:

<?.is-admin>
  <p>You are an admin! Much wow!</p>
</?>
<!$basket.products>
  <span class="panic">Your basket is empty. Quick, buy something!</span>
</!>

The structured tag form could be used instead, whereby the tag that should be opened and later closed is specified after the condition. The prior example could thus be written more compactly as:

<?.is-admin p>
  You are an admin! Much wow!
</?>
<!$basket.products span class="panic">
  Your basket is empty. Quick, buy something!
</!>

For more complex conditions, a subset of Raku expressions is accepted, using the syntax <?{ $a eq $b }>...</?>. The only thing notably different from Raku is that <?{ .answer == 42 }>...</?> will have the same hash/object semantics as in <.answer>, for consistency with the rest of the templating language.

The following constructs are allowed:

Variables ($foo)
Use of the topic, method calls, and indexing, to the degree supported by the <.foo> tag syntax
Parentheses for grouping
The comparison operations ==, !=, <, <=, >=, >, eq, ne, lt, gt, ===, and !===
The &&, ||, and and or short-circuit logic operators
The +, -, *, /, and % math operations
The ~ and x string operations
Numeric literals (integer, floating point, and rational)
String literals (single quoted, without interpolation)

Those wishing for more are encouraged to consider writing their logic outside of the template.

An else-clause can be expressed using <!>...</!>:

<?$user.logged-in>
  Logged in as <$user.name>
</?>
<!>
  <a href="/lgoin">Log in</a>
</!>

While <!?cond>...</?> can be used for an elsif clause:

<?{.x > 5}>
  over five
</?>
<!?{.x > 0}>
  over zero
</?>
<!>
  zero or less
</!>

These all work with the structured tag forms:

<?{.x > 5} div>
  over five
</?>
<!?{.x > 0} p>
  over zero
</?>
<! span>
  zero or less
</!>

If the opening and closing condition tags are the only thing on the line, then no output will be generated for those lines, making the output more pleasant.

Subroutines §

It is possible to declare template subroutines that may be re-used, in order to factor out common elements.

A simple template subroutine declaration looks like this:

<:sub header>
  <header>
    <nav>
      blah blabh
    </nav>
  </header>
</:>

It can then be called as follows:

<&header>

A template sub may take parameters:

<:sub select($options, $name)>
  <select name="<$name>">
    <@$options>
      <option value="<.value>"><.text></option>
    </@>
  </select>
</:>

And called with arguments:

<&select(.countries, 'country')>

The arguments may be an expression as valid in a <?{ ... }> condition - that is, literals, variable access, dereferences, and some basic operators are allowed.

As in Raku, you can have named - optional - arguments as well:

<:sub haz(:$name)>
  I can haz <$name>!
</:>

<&haz(:name('named arguments'))>

Defaults can also be set (and implicitly make positional parameters optional too):

<:sub result($value = 0, :$unit = 'kg')>
  <$value> <$unit>
</:>

Macros §

A template macro works somewhat like a template subroutine, except that the usage of it has a body. This body is passed as a thunk, meaning that the macro can choose to render it 0 or more times), optionally setting a new default target. For example, a macro wrapping some content up in a Bootstrap card might look like:

<:macro bs-card($title)>
  <div class="card" style="width: 18rem;">
    <div class="card-body">
      <h5 class="card-title"><$title></h5>
      <:body>
    </div>
  </div>
</:>

Where <:body> marks the point for the body to be rendered. This macro could be used as:

<|bs-card('My Stuff')>
  It's my stuff, in a BS card!
</|>

To set the current target for the body in a macro, use <:body $target>.

Inserting HTML and JavaScript §

Everything is HTML escaped by default. However, sometimes it is required to place a blob of pre-rendered HTML into the template output. There are two ways to achieve this.

The HTML built-in function, called as <&HTML(.stuff)>, first checks that there is no script tag or attribute starting with javascript:; if there are any, it will consider this as an XSS attack attempt and throw an exception.
The HTML-AND-JAVASCRIPT built-in function does not attempt any XSS protection, and simply inserts whatever it is given without any kind of escaping.

Note that the HTML function does not promise completely foolproof XSS protection. Use both of these functions very carefully.

Comments §

While standard HTML comment syntax may be used inside of templates, they will be passed straight along into the rendered output. An alternative syntax is available for template comments, which are discarded at the point the template is being parsed, and so never make it into the output.

<p>This is rendered!</p>
<!-- And this comment goes to the client too -->
<#>But this is not</#>

Template comments may span multiple lines and contain tags (both HTML ones and template syntax):

<h2>Offers</h2>
<p>All of our offers are currently unavailable!</p>
<#>
<ul>
  <@offers li><$_></@>
</ul>
</#>

Contents

Generalities
The topic variable
Indexing hash, array, and object data
Variables
Iteration
Conditionals
Subroutines
Macros
Inserting HTML and JavaScript
Comments