So I have code that looks similar to this toy example in a Common Lisp project:
(defgeneric to-list (data) (:documentation "Convert given data type to a proper list")) (defmethod to-list ((data list)) list) (defmethod to-list ((data vector)) (coerce data 'list)) (defmethod to-list ((data binary-tree)) (traverse #'(lambda (x) x) data 'in-order)) ;;; ... So on for other collection datatypes ... This way I can write a function to work on multiple collection types without having to define multiple methods to specialize on each. In addition it allows me to define TO-LIST method for any future types I would to use those functions on. From what I understand this would be done similarly in Haskell with a type-class: class Listable a where toList :: a -> [b] But I am not a Haskell expert, and my code doesn't literally have TO-LIST. I see that Racket has generic functions, but they see to work a bit differently that CLOS-style generics. Further, like Haskell's type-classes I would like to be able to provide a contract for functions that take a Listable type: (provide (contract-out <...> [foo (-> listable? bar?)] <...>)) I can do this in CL using a :BEFORE method (defmethod foo :before ((collection t) <...>) (unless (find-method #'to-list () (list (class-of collection)) nil) ; ensure collection object has TO-LIST method ; otherwise raise error condition) <...>) What would be the appropriate Racket construct or idiom to get sort of "type-class" or "interface" that can be enforced through contracts? Thanks, Helmut
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