Create simple Post scaffolding with string category field.
Use has_scope to allow filtering /posts route ?by_category=
Attempt to reuse has_scope filtering with hardcoded route params.

Notice the benign point of entry of a symbol value in our params
hash in config/routes.rb#4: you can create routes with hardcoded
parameters, as we try to do with featured categories.
However, visiting /posts/tech will result in a 500 RuntimeError:

Expected type :default in params[:by_category], got Symbol

This could occur, as it did here, if we want to expose parameterless
paths to a subset of preferred filterings. It could also occur
easily if you're getting clever with generating routes off STI.

In general, this feature of the routing engine is little known,
rarely used, and an edge case--but one worth considering,
nonetheless.

Incidentally, I believe that the :format key of params also is a
symbol, but one would hope that never made it into has_scope.
A simple fix, but one otherwise uneeded and unexpected in the otherwise elegant routing engine DSL.