Question

This question is closely related to this one but i think is more general.

Recently i try to create type “instances” on the fly with multimethods (or with a unique function constructor if possible), based in a metadata tag. I linked a type (a java class under the hood) with this tag and then i didnt know how to continue in a elegant way (without eval or java reflection and strings); new is a special form and it’s difficult to handle although you use macros.

In seudo-code it would be:

(def my-tagged-data (with-meta my-data {:my-type-tag my-ns.My-Type}))
(def factory-function [tagged-data] 
     (create (:my-type-tag (meta tagged-data)) tagged-data))

Answer 1

You can provide factory functions for your types.

(def my-tagged-data (with-meta my-data {:my-type-factory `my-type-factory}))

(defn factory-function
  [tagged-data] 
  (@(resolve (:my-type-factory (meta tagged-data))) tagged-data))

This may or may not be feasible.