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How to type polymorphic variants with open generic functions?

I'd like to implement a language with polymorphic variants, but instead of using pattern matching, I'd like to use Common Lisp-like generic functions that are defined with define-generic, and to which cases can be added with define-method.

Where in pseudo-ML I'd say

let length list =
  case list of 
    `Nil -> 0
  | `Cons _ cdr -> 1 + length cdr

in my language I'd say

(define-generic length)
(define-method (length `Nil) 0) 
(define-method (length (`Cons _ cdr)) (+ 1 (length cdr))) 

I'd like to able to add methods in different compilation units, and only type-check generic functions at the end of compile-time, once all methods are known. Another possibility might be to defer type checking of generic functions to link time.

Any ideas or pointers to things I should check out? I'm currently reading Simple Type Inference for Structural Polymorphism, and TAPL is lying on the coffee table.

solution to the ever narrowing posts problem?

are there any drupal hackers who could somehow fix the ever narrowing text problem? :-)