Cosmo is a very flexible template engine, and it has evolved quite a lot since the version that shipped along with the first releases of Sputnik. The documentation has grown accordingly, but the newer features that add more expressive power to Cosmo end up buried among the earlier ones, so at first glance Cosmo may not seem as expressive as template engines in other languages. For example, the first version of Cosmo only allowed iteration via special generator functions that called cosmo.yield for each item they wanted to produce. This lead to code such as:
Newer versions of Cosmo, though, automatically iterate over plain lists, so you can write the above as:
For this simple example you do not gain much, because the do_list function can be made generic, but the feature becomes more useful when you have lists inside lists. Another often-neglected feature is arguments. A Cosmo selector can have arguments, and they are passed as a table to the selector (so it must be a function, or at least a __call metamethod). You can easily write “filters” with that:
Combine arguments with multiple subtemplates and you have the cosmo.cif function, which can be set as an $if selector:
You can do a lot more by writing more selector functions. How about an $import selector that looks up a template file in the filesystem and processes it? Or a $markdown filter, as you can have a long string as an argument? Orbit Pages is one example of extendeding Cosmo with several domain-specific selectors. If you have ideas for generic ones please let me know and I can add them to the cosmo module alongside cosmo.cif.