Simple templating using HTML5 as the template language. Templates are specified by adding special attributes to tags. During substitution, these attributes are stripped from the HTML. The following attributes are recognized:

• insert="identifier" - replace the tag's contents with the value bound to identifier in the substitution context.
• replace="identifier" - replace the whole tag and its contents with the value bound to identifier in the substitution context.
• when="identifier" - only render this tag if identifier is set to true in the substitution context.
• unless="identifier" - the dual of when; only render this tag if identifier is set to false in the substitution context.
• forall="identifier" - render this tag and its contents once for each element in the list bound to identifier in the substitution context. The contents of the element may refer to the current iteration's value of identifier by that same name.

Substitution can also be performed on the attributes of tags. The following attribute substitutions are recognized:

• when:identifier:attr="value" - only include attr is identifier is set to true in the substitution context.
• unless:identifier:attr="value" - only include attr is identifier is set to false in the substitution context.
• insert:identifier:attr="value" - overwrite the value of attr with whatever identifier is bound to in the substitution context.

Contexts can be nested, in which case nested keys are separated by periods, as in parent.child.grandchild. Keys may be prefixed with a question mark, in which case they are considered to be "weak keys". If a weak key does not exist in the context, it will be replaced by a sensible default value instead of causing an error. The defaults for the different value types are as follows:

• bool: false
• string: ""
• array: []
• object: {}

As numbers are treated just like strings, they have an empty string as their default value as well.

In general, values used as text must be declared text by the context and so on, but the following coercions are permitted:

• bool to string
• array to bool

Coercion of array to bool, with the empty list being considered false and all other list considered true, is permitted to allow templates to take special action in the case of an empty list.

Contexts may be constructed programatically using the provided combinators, converted from JSON objects or lists of key-value pairs, or parsed from a YAML-formatted string using parseContext.