Crate monad

gistfile1.clj

(ns pallet.crate
  "The crate monad.

   There are two issues with crates as monads.

     i) need to reimplement crate functions to return functions of one
     argument [state], so that they have a common signature for manipulation
     as monadic functions.

     ii) to achieve pluggable error handling, etc, via monads, requires
     runtime use of monads, which isn't possible with c.c.monads."
  (:require
   [clojure.contrib.monads :as monad]))

(def ^{:doc "The crate monad is an identity monad"}
  crate-m monad/identity-m)

(defmacro cratefn
  "Create an anonymous crate function"
  [[request & args] & body]
  `(fn [~@args] (fn [~'request] ~@body)))


(def a (cratefn [request a] (assoc request :a a)))
(def bc (cratefn [request b c] (assoc request :b b :c c)))

(use 'clojure.test)

(deftest cratefn-test
  (is (= {:a 1} ((a 1) {:a 2})))
  (is (= {:a 1 :b 2 :c 3} ((bc 2 3) {:a 1}))))


(deftest basic-test
  (is (= {:a 1} (monad/with-monad crate-m (m-bind {:a 1} identity))))
  (is (= {:a 1 :b 2}
         (monad/with-monad crate-m
           ((monad/m-chain [ #(assoc % :a 1) #(assoc % :b 2)]) {}))))
  (is (= {:a 1 :b 2 :c 3}
         (monad/with-monad crate-m
           ((monad/m-chain [(a 1) (bc 2 3)]) {}))))
  (is (= {:a 1 :b 2 :c 3}
         (monad/with-monad crate-m
           ((monad/m-chain [(a 4) (bc 2 3) (a 1)]) {})))))