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; "Solving" the expression problem with classical multiple dispatch. apply-op is
; a single multimethod that dispatches on an operation / type pair (the
; operation is just a keyword).
; This approach has more theoretical than practical value - demonstrating how
; the problem maps to multiple dispatch. For realistic scenarios the
; expression.multimethod and expression.protocols approaches are preferable.
; Eli Bendersky []
; This code is in the public domain.
(ns expression.multidispatch)
(defrecord Constant [value])
(defrecord BinaryPlus [lhs rhs])
(defmulti apply-op
(fn [op ty] [op (class ty)]))
(defmethod apply-op [:evaluate Constant]
[_ c] (:value c))
(defmethod apply-op [:evaluate BinaryPlus]
[op bp] (+ (apply-op op (:lhs bp)) (apply-op op (:rhs bp))))
(defmethod apply-op [:stringify Constant]
[_ c] (str (:value c)))
(defmethod apply-op [:stringify BinaryPlus]
[op bp]
(clojure.string/join " + " [(apply-op op (:lhs bp))
(apply-op op (:rhs bp))]))
; Testing
(def c1 (Constant. 1.1))
(def c2 (Constant. 2.2))
(def p1 (BinaryPlus. c1 c2))
(def p2 (BinaryPlus. p1 c2))
(apply-op :evaluate c1)
(apply-op :evaluate p1)
(apply-op :evaluate p2)
(apply-op :stringify p1)
(apply-op :stringify p2)