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Dhruv Luthra
Dhruv Luthra

Reputation: 31

Specialize Generic Function (Multimethod) on Two Arguments

I am trying to build a binary search tree in common lisp. I have defined the binary search class using the CLOS like this: 

(defclass bst ()
  ((root :type node
         :accessor tree-root
         :initform nil
         :initarg root)))

I am trying to define a generic function that takes in the tree object and a key and returns a boolean true if the tree contains the key and nil if the tree does not contain the key.

Right now I have the following definition of a generic function: 

(defgeneric contains ((tree bst) (key))
   (:documentation "returns boolean of whether the given tree contains a particular key)

I get the following error when I load the file into the REPL (I am using SBCL): 

Required argument is not a symbol: (TREE BST)

Am I misunderstanding how generic functions work? I can't seem to define the function properly.

Upvotes: 2

Views: 225

Answers (1)

Vatine
Vatine

Reputation: 21268

Yes, defgeneric defines a generic function. You can either specify methods in the call to defgeneric or by using defmethod.

You need one of:

(defgeneric contains (tree key)
   (:documentation "returns boolean of whether the given tree contains a particular key")
   (:method ((tree bst) key) ...))

or:

(defgeneric contains (tree key)
   (:documentation "returns boolean if a given tree contains a given key"))

(defmethod contains ((tree bst) key)
  ...)

Upvotes: 9

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