CODEWITHSUNDEEP
common-lispsdl-2sbclclos
ZhangBing
ZhangBing

Reputation: 45

Generic Function cannot find applicable method

When I call (draw win) I get an error the "generic function can't find applicable method. I have just gotten into CLOS and I am using sdl2kit to simply render a window.

(defclass game-window (kit.sdl2:window)
  ((rotation :initform 0.0)))

(defmethod render ((win game-window))
 (with-slots (rotation) win
 (gl:load-identity)
 (gl:rotate rotation 0 0 1)
 (gl:clear-color 0.0 0.0 1.0 1.0)
 (gl:clear :color-buffer)
 (gl:begin :triangles)
 (gl:color 1.0 0.0 0.0)
 (gl:vertex 0.0 1.0)
 (gl:vertex -1.0 -1.0)
 (gl:vertex 1.0 -1.0)
 (gl:end)))
(defgeneric draw(win)
 (:documentation "draw window"))
(defun main ()
 (let (( win (make-instance 'game-window)))
  (draw win)))

Upvotes: 0

Views: 1106

Answers (1)

Dan Robertson
Dan Robertson

Reputation: 4360

So there are two issues. One is an English language issue (you have to be super careful because every English word is overloaded with multiple programming meanings. Eg when you write “call” it isn’t clear whether you mean “name” or “invoke/apply”.) but that doesn’t really matter

The other issue is that you don’t know what a generic function is. Here is a quick explainer.

A generic function has (roughly) three parts:

  1. A lambda list
  2. A method combination
  3. A set of methods

A method has three parts

  1. A specialised lambda list
  2. Some code
  3. Something related to the method combination (typically nil)

When you call a generic function this happens:

  1. It looks at the types of the arguments
  2. Various rules are applied to order the methods by specificity
  3. The method combination says how to combine these into a function
  4. The code for the combined function is generated and compiled
  5. The generated function is called with the arguments.

Steps 2-4 are typically cached.

Step 3 comes in roughly two forms:

  1. Standard method combination:
    1. All around methods are called
    2. Then all before methods
    3. Most specific method is called (with an appropriate chain of methods set up for call-next-method)
    4. Then all after methods
    5. Result is returned through all the around methods
  2. Other method combinations typically apply all the methods’ results to some operator like and or progn or append.

To add a method to the set of methods of a generic function one typically uses defmethod with the same name as the generic function

Upvotes: 1

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