path: root/src/clojure/contrib/monads/examples.clj

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;; Monad application examples
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(ns clojure.contrib.monads.examples
  (:use clojure.contrib.monads)
  (:require (clojure.contrib [accumulators :as accu])))

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;;  Sequence manipulations with the sequence monad
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; Note: in the Haskell world, this monad is called the list monad.
; The Clojure equivalent to Haskell's lists are (possibly lazy)
; sequences. This is why I call this monad "sequence". All sequences
; created by sequence monad operations are lazy.

; Monad comprehensions in the sequence monad work exactly the same
; as Clojure's 'for' construct, except that :while clauses are not
; available.
(domonad sequence
   [x (range 5)
    y (range 3)]
    (+ x y))

; Inside a with-monad block, domonad is used without the monad name.
(with-monad sequence
  (domonad
     [x (range 5)
      y (range 3)]
     (+ x y)))

(domonad sequence
   [x  (range 5)
    y  (range (+ 1 x))
    :when  (= (+ x y) 2)]
   (list x y))

; An example of a sequence function defined in terms of a lift operation.
; We use m-lift2 because we have to lift a function of two arguments.
(with-monad sequence
   (defn pairs [xs]
      ((m-lift 2 #(list %1 %2)) xs xs)))

(pairs (range 5))

; Another way to define pairs is through the m-seq operation. It takes
; a sequence of monadic values and returns a monadic value containing
; the sequence of the underlying values, obtained from chaining together
; from left to right the monadic values in the sequence.
(with-monad sequence
   (defn pairs [xs]
      (m-seq (list xs xs))))

(pairs (range 5))

; This definition suggests a generalization:
(with-monad sequence
   (defn ntuples [n xs]
      (m-seq (replicate n xs))))

(ntuples 2 (range 5))
(ntuples 3 (range 5))

; Lift operations can also be used inside a monad comprehension:
(domonad sequence
   [x  ((m-lift 1 (partial * 2)) (range 5))
    y  (range 2)]
    [x y])

; The m-plus operation does concatenation in the sequence monad.
(domonad sequence
   [x  ((m-lift 2 +) (range 5) (range 3))
    y  (m-plus (range 2) '(10 11))]
   [x y])


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;; Handling failures with the maybe monad
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; Maybe monad versions of basic arithmetic
(with-monad maybe
   (def m+ (m-lift 2 +))
   (def m- (m-lift 2 -))
   (def m* (m-lift 2 *)))

; Division is special for two reasons: we can't call it m/ because that's
; not a legal Clojure symbol, and we want it to fail if a division by zero
; is attempted. It is best defined by a monad comprehension with a
; :when clause:
(defn safe-div [x y]
  (domonad maybe
     [a x
      b y
      :when (not (zero? b))]
     (/ a b)))

; Now do some non-trivial computation with division
; It fails for (1) x = 0, (2) y = 0 or (3) y = -x.
(with-monad maybe
   (defn some-function [x y]
      (let [one (m-result 1)]
 	   (safe-div one (m+ (safe-div one (m-result x))
			     (safe-div one (m-result y)))))))

; An example that doesn't fail:
(some-function 2 3)
; And two that do fail, at different places:
(some-function 2 0)
(some-function 2 -2)

; In the maybe monad, m-plus selects the first monadic value that
; holds a valid value.
(with-monad maybe
   (m-plus (some-function 2 0) (some-function 2 -2) (some-function 2 3)))

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;;  Random numbers with the state monad
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; A state monad item represents a computation that changes a state and
; returns a value. Its structure is a function that takes a state argument
; and returns a two-item list containing the value and the updated state.
; It is important to realize that everything you put into a state monad
; expression is a state monad item (thus a function), and everything you
; get out as well. A state monad does not perform a calculation, it
; constructs a function that does the computation when called.

; First, we define a simple random number generator with explicit state.
; rng is a function of its state (an integer) that returns the
; pseudo-random value derived from this state and the updated state 
; for the next iteration. This is exactly the structure of a state
;  monad item.
(defn rng [seed]
  (let [m      259200
	value  (/ (float seed) (float m))
	next   (rem (+ 54773 (* 7141 seed)) m)]
    (list value next)))

; We define a convenience function that creates an infinite lazy seq
; of values obtained from iteratively applying a state monad value.
(defn value-seq [f seed]
  (let [[value next] (f seed)]
    (lazy-cons value (value-seq f next))))

; Next, we define basic statistics functions to check our random numbers
(defn sum [xs]  (apply + xs))
(defn mean [xs]  (/ (sum xs) (count xs)))
(defn variance [xs]
  (let [m (mean xs)
	sq #(* % %)]
    (mean (for [x xs] (sq (- x m))))))

; rng implements a uniform distribution in the interval [0., 1.), so
; ideally, the mean would be 1/2 (0.5) and the variance 1/12 (0.8333).
(mean (take 1000 (value-seq rng 1)))
(variance (take 1000 (value-seq rng 1)))

; We make use of the state monad to implement a simple (but often sufficient)
; approximation to a Gaussian distribution: the sum of 12 random numbers
; from rng's distribution, shifted by -6, has a distribution that is
; approximately Gaussian with 0 mean and variance 1, by virtue of the central
; limit theorem.
; In the first version, we call rng 12 times explicitly and calculate the
; shifted sum in a monad comprehension:
(def gaussian1
   (domonad state
      [x1  rng
       x2  rng
       x3  rng
       x4  rng
       x5  rng
       x6  rng
       x7  rng
       x8  rng
       x9  rng
       x10 rng
       x11 rng
       x12 rng]
      (- (+ x1 x2 x3 x4 x5 x6 x7 x8 x9 x10 x11 x12) 6.)))

; Let's test it:
(mean (take 1000 (value-seq gaussian1 1)))
(variance (take 1000 (value-seq gaussian1 1)))

; Of course, we'd rather have a loop construct for creating the 12
; random numbers. This would be easy if we could define a summation
; operation on random-number generators, which would then be used in
; combination with reduce. The lift operation gives us exactly that.
; More precisely, we need (m-lift 2 +), because we want both arguments
; of + to be lifted to the state monad:
(def gaussian2
   (domonad state
      [sum12 (reduce (m-lift 2 +) (replicate 12 rng))]
      (- sum12 6.)))

; The statistics should be strictly the same as above, as long as
; we use the same seed:
(mean (take 1000 (value-seq gaussian2 1)))
(variance (take 1000 (value-seq gaussian2 1)))

; We can also do the subtraction of 6 in a lifted function, and get rid
; of the monad comprehension altogether:
(with-monad state
   (def gaussian3
        ((m-lift 1 #(- % 6.))
           (reduce (m-lift 2 +) (replicate 12 rng)))))

; Again, the statistics are the same:
(mean (take 1000 (value-seq gaussian3 1)))
(variance (take 1000 (value-seq gaussian3 1)))

; For a random point in two dimensions, we'd like a random number generator
; that yields a list of two random numbers. The m-seq operation can easily
; provide it:
(with-monad state
   (def rng2 (m-seq (list rng rng))))

; Let's test it:
(rng2 1)

; fetch-state and get-state can be used to save the seed of the random
; number generator and go back to that saved seed later on:
(def identical-random-seqs
  (domonad state
    [seed (fetch-state)
     x1   rng
     x2   rng
     _    (set-state seed)
     y1   rng
     y2   rng]
    (list [x1 x2] [y1 y2])))

(identical-random-seqs 1)

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;;  Logging with the writer monad
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; A basic logging example
(domonad (writer accu/empty-string)
  [x (m-result 1)
   _ (write "first step\n")
   y (m-result 2)
   _ (write "second step\n")]
  (+ x y))

; For a more elaborate application, let's trace the recursive calls of
; a naive implementation of a Fibonacci function. The starting point is:
(defn fib [n]
  (if (< n 2)
    n
    (let [n1 (dec n)
	  n2 (dec n1)]
      (+ (fib n1) (fib n2)))))

; First we rewrite it to make every computational step explicit
; in a let expression:
(defn fib [n]
  (if (< n 2)
    n
    (let [n1 (dec n)
	  n2 (dec n1)
	  f1 (fib n1)
	  f2 (fib n2)]
      (+ f1 f2))))

; Next, we replace the let by a domonad in a writer monad that uses a
; vector accumulator. We can then place calls to write in between the
; steps, and obtain as a result both the return value of the function
; and the accumulated trace values.
(with-monad (writer accu/empty-vector)

  (defn fib-trace [n]
    (if (< n 2)
      (m-result n)
      (domonad
        [n1 (m-result (dec n))
	 n2 (m-result (dec n1))
	 f1 (fib-trace n1)
	 _  (write [n1 f1])
	 f2 (fib-trace n2)
	 _  (write [n2 f2])
	 ]
	(+ f1 f2))))

)

(fib-trace 5)

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;; Sequences with undefined value: the maybe-t monad transformer
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; A monad transformer is a function that takes a monad argument and
; returns a monad as its result. The resulting monad adds some
; specific behaviour aspect to the input monad.

; The simplest monad transformer is maybe-t. It adds the functionality
; of the maybe monad (handling failures or undefined values) to any other
; monad. We illustrate this by applying m