Recs.
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SpecsUpdate
Pros
Pro Well organized and thorough documentation
Slim's documentation is well organized and detailed, every concept is thoroughly explained and it is very helpful for both advanced users and beginners.
Pro Flexible
Slim doesn't demand that you stick to a fixed folder structure. As long as you load Slim the right way you can do anything from there the way you like it.
Pro Supports tie-ins for Rack-like middleware
Rack is an interface used in Ruby frameworks used to group and order modules, which most of the time are Ruby classes, and specify between them.
Slim uses a simple concept for it's middleware. By wrapping HTTP requests and responses it unifies the middleware into a single method call.
Cons
Con Too minimal
While it's true that Slim is a microframework, it's still too minimal. When used for throwaway projects or simple prototypes, it's perfect. But in the long run, it becomes less and less useful and you end up in implementing a fully custom framework in trying to tackle all the missing features.
So in the end, if the project would go further than a prototyping stage it would be easier to just implement another microframework that has those missing features out of the box.
Con Lack of learning resources for Slim 3
There are very few tutorials and learning resources for Slim 3.