Proposal: Interfaces for more generic code

[funny-falcon]

Crystal has interfaces in Java sense:

Crystal has Interfaces in Java sense

During discussion we’ve discovered Crystal has interfaces in Java sense. Module can have abstract method, and its inclusion demands implementation: https://play.crystal-lang.org/#/r/e1jl module Moo abstract def size : Int end struct Fix include Moo end # Error: abstract `def Moo#size()` must be implemented by Fix Containers could be specialized to modules, function/method could have argument restricted to module, and their call is correctly polymorphic: https://play.crystal-lang.org/#…