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copied!<p>Yes, following Luis Casillas answer, here is an implementation in clojure of the <code>Free</code> monad in clojure.</p> <pre><code> (use 'clojure.algo.monads) ;; data Free f r = Free (f (Free f r)) | Pure r (defn pure [v] {:t :pure :v v}) (defn impure [v] {:t :impure :v v }) (defn free-monad [fmap] (letfn [ (fm-result [v] (pure v)) (fm-bind [mv mf] (if (= :pure (:t mv)) (mf (:v mv)) ;; Pure a >>= f = f a (impure ;; Free fa >>= f = Free (fmap (>>=f) fa) ((fmap (fn [lmv] (fm-bind lmv mf))) (:v mv))))) ] { :m-result fm-result :m-bind fm-bind :m-zero ::undefined :m-plus ::undefined } ) ) </code></pre> <p>And an example from <a href="http://www.haskellforall.com/2012/06/you-could-have-invented-free-monads.html" rel="nofollow">Why free monads matter</a>:</p> <p>Definition of toy language.</p> <pre><code> ;; Toy language ;; (defn output [c n] [{:t :output :v c}, n]) (defn bell [n] [{:t :bell}, n]) (defn done [] [{:t :done}, nil]) (defn toy-fmap [f] (fn [[e c]] (if (= :done (:t e)) [e c] [e (f c)] )) ) </code></pre> <p>Definition of the monad for the toy language + helper functions</p> <pre><code> ;; (def tt-monad (free-monad toy-fmap)) (defn liftF [toy] (impure ((toy-fmap (fn [c] (pure c))) toy)) ) (defn m-output [x] (liftF (output x nil))) (defn m-bell [] (liftF (bell nil))) (defn m-done [] (liftF (done))) (defn f-m-done [_] (m-done)) </code></pre> <p>And checking some rules :</p> <pre><code> ;; return "a" >>= output is Free(Output "a", ()) ;; {:t :impure, :v [{:t :output, :v \a} {:t :pure, :v nil}]} (with-monad tt-monad (m-bind (m-result \a) m-output) ) ;; output "a" >>= return is Free(Output "a", ()) ;; {:t :impure, :v [{:t :output, :v \a} {:t :pure, :v nil}]} (with-monad tt-monad (m-bind (m-output \a) m-result) ) ;; ((output 'A' >> done) >> output 'C') (with-monad tt-monad (m-bind (m-bind (m-output \a) f-m-done) (fn [_] (m-output \c)))) ;;(output 'A' >> (done >> output 'C')) is output 'A' Done (with-monad tt-monad (m-bind (m-output \a) (fn [x] (m-bind (m-done) (fn [_] (m-output \c)))))) </code></pre> <p>This could be improved quite a lot in terms of readability of the data structure. Comments and improvements most welcome.</p>

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