If this feature is added, for obvious reasons it should mandate that the first argument to fmt is a string literal, not some other expression that may return a string, not an r.args expression or other expression.

Thank you for the suggestion @marshall007 . This looks useful.
I don't think we'll get to it before our 2.0 release, but we'll keep it on our list.

very nice!

Here's a quick and dirty client-side implementation I've been using in case anyone else finds it useful:

r.fmt = function (template, params) {
  var parts = template.split(/(\{\w+\})/g).map(function (part) {
    if (part.indexOf('{') === 0) {
      var name = part.slice(1, -1);
      return params(name).coerceTo('string');
    }
    return part;
  });

  return r.add(r.args(parts));
};

Example usage:

r.fmt('http://example.com/foos/{id}/bars?type={type}', r.expr({
  id: 123,
  type: 'tag'
}))
// => "http://example.com/foos/123/bars?type=tag"

@marshall007 That's a very neat work-around!