Allow expressions as custom tag parameters

[smarter]

In liqp 0.6 it was possible to use a custom tag foo like this:

{% foo a.b.c %}

But since #46 the parameter is always parsed as the string "a.b.c", and there seems to be no built-in way to parse and evaluate the expression in this string. The following does not work either (parsing fails with a NoViableAltException):

{% foo {{ a.b.c }} %}

Is there an alternative to this? The documentation generator of the next-gen Scala compiler currently relies on this: scala/scala3#3859

[bkiers]

Yeah, that is unfortunate that the specs say (all!) parameters provided to a custom tag are teated as a single string.

You could evaluate the parameter-string(s) as an expression in your custom tag. Perhaps a bit of a hack, but do-able:

import liqp.nodes.LNode;
import liqp.tags.Tag;

public class Test {

  public static void main(String[] args) {

    String source = "{% dummy a.b.c %}{{ a.b.c }}{% enddummy %}";

    String jsonVariables = "{ \"a\": { \"b\": { \"c\": 42 }}}";

    String rendered = Template
        .parse(source)
        .with(new Dummy())
        .render(jsonVariables);

    System.out.println(rendered);
  }
}

class Dummy extends Tag {

  Dummy() {
    super("dummy");
  }

  @Override
  public Object render(TemplateContext context, LNode... nodes) {

    String parameters = nodes[0].render(context).toString();

    Object evaluated = Template
        .parse(String.format("{{%s}}", parameters))
        .render(context.getVariables());

    System.out.println("parameters -> " + parameters);
    System.out.println("evaluated  -> " + evaluated);

    return null;
  }
}

will print:

parameters -> a.b.c
evaluated  -> 42