Comment 1 for bug 821935

Can't do it the way you ask. The individual template items are evaluated separately, then combined together. At that point the priority information, which would need to be in the form of a function, would be lost.

I have added a new template function, strcat_max, that does what you want in a different way. The documentation for the new function is:
strcat_max(max, string1, prefix2, string2, ...)`` -- Returns a string formed by concatenating the arguments. The returned value is initialized to string1. `Prefix, string` pairs are added to the end of the value as long as the resulting string length is less than `max`. String1 is returned even if string1 is longer than max. You can pass as many `prefix, string` pairs as you wish

You would use something like:
{:'strcat_max(100, field('title'), ' - ', field('author'), ',', field('pubdate'))'}

You might want to 'shorten' the first argument(title).