Monad transformer that transforms a monad m into a monad in which the base values can be invalid (represented by nothing, which defaults to nil). The third argument chooses if m-zero and m-plus are inherited from the base monad (use :m-plus-from-base) or adopt maybe-like behaviour (use :m-plus-from-transformer). The default is :m-plus-from-base if the base monad m has a definition for m-plus, and :m-plus-from-transformer otherwise.
; clojure/contrib/monads.clj:455 (defn maybe-t ([m] (maybe-t m nil :m-plus-default)) ([m nothing] (maybe-t m nothing :m-plus-default)) ([m nothing which-m-plus] (monad-transformer m which-m-plus [m-result (with-monad m m-result) m-bind (with-monad m (fn m-bind-maybe-t [mv f] (m-bind mv (fn [x] (if (identical? x nothing) (m-result nothing) (f x)))))) m-zero (with-monad m (m-result nothing)) m-plus (with-monad m (fn m-plus-maybe-t [& mvs] (if (empty? mvs) (m-result nothing) (m-bind (first mvs) (fn [v] (if (= v nothing) (apply m-plus-maybe-t (rest mvs)) (m-result v))))))) ])))
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