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Thursday, October 1, 2020

Contract Programmer Seeks Job in Cambridge (£500 reward)

Anyone in Cambridge need a programmer? I'll give you £500 if you can find me a job that I take.

CV at http://www.aspden.com

I make my usual promise, which I have paid out on several times:

If, within the next six months, I take a job which lasts longer than one month, and that is not obtained through an agency, then on the day the first cheque from that job cashes, I'll give £500 to the person who provided the crucial introduction.

If there are a number of people involved somehow, then I'll apportion it fairly between them. And if the timing conditions above are not quite met, or someone points me at a shorter contract which the £500 penalty makes not worth taking, then I'll do something fair and proportional anyway.

And this offer applies even to personal friends, and to old contacts whom I have not got round to calling yet, and to people who are themselves offering work, because why wouldn't it?

And obviously if I find one through my own efforts then I'll keep the money. But my word is generally thought to be good, and I have made a public promise on my own blog to this effect, so if I cheat you you can blacken my name and ruin my reputation for honesty, which is worth much more to me than £500.

And I also make the following boast:

I know all styles of programming and many languages, and can use any computer language you're likely to use as it was intended to be used.

I have a particular facility with mathematical concepts and algorithms of all kinds. I can become very interested in almost any problem which is hard enough that I can't solve it easily.

I have a deserved reputation for being able to produce heavily optimised, but nevertheless bug-free and readable code, but I also know how to hack together sloppy, bug-ridden prototypes, and I know which style is appropriate when, and how to slide along the continuum between them.

I've worked in telecoms, commercial research, banking, university research, chip design, server virtualization, university teaching, sports physics, a couple of startups, and occasionally completely alone.

I've worked on many sizes of machine. I've written programs for tiny 8-bit microcontrollers and gigantic servers, and once upon a time every IBM machine in the Maths Department in Imperial College was running my partial differential equation solvers in parallel in the background.

I'm smart and I get things done. I'm confident enough in my own abilities that if I can't do something I admit it and find someone who can.

I know what it means to understand a thing, and I know when I know something. If I understand a thing then I can usually find a way to communicate it to other people. If other people understand a thing even vaguely I can usually extract the ideas from them and work out which bits make sense.

Tuesday, February 5, 2019

The Unexpected Appearance of Schlemiel, the Painter

;; The Unexpected Appearance of Schlemiel, the Painter

;; I was doing some statistics one day, and I defined:

;; the average of a finite sequence
(defn average [sq] (/ (reduce + sq) (count sq)))

;; and the square of a number
(defn square [x] (* x x))

;; and a way of forgetting about all the fiddly little digits at the end
(defn twosf   [x]  (float (/ (Math/round (* x 100.0)) 100))) 

;; but for the variance I was a little torn between:
(defn variance-one [sq]
  (let [av (average sq)]
    (average (map #(square (- % av)) sq)))) 
;; and 
(defn variance-two [sq]
  (let [sqdiff #(square (- % (average sq)))]
    (average (map  sqdiff sq))))

;; and (I have a regrettable weakness for the terse...) 
(defn variance-one-liner [sq] (average (map #(square (- % (average sq))) sq)))

;; but I was surprised when I noticed this: 

(let [s (repeatedly 1000 #(rand))]
  (twosf (reduce + s)) ;; just to force the sequence to be generated before timing things
  [(time (twosf (reduce + s)))
   (time (twosf (average  s)))
   (time (twosf (variance-one s)))
   (time (twosf (variance-two s)))
   (time (twosf (variance-one-liner s)))])

;; "Elapsed time: 0.535715 msecs"
;; "Elapsed time: 0.834523 msecs"
;; "Elapsed time: 1.417108 msecs"
;; "Elapsed time: 251.650722 msecs"
;; "Elapsed time: 248.196331 msecs"
;; [496.83 0.5 0.09 0.09 0.09]

;; It seems that all these functions are correct, in the sense that they are producing
;; correct-looking answers, and yet one of them is orders of magnitude faster.

;; What is going on here, and why?

Thursday, December 13, 2018

Reinforcement Learning : Exploration vs Exploitation : Multi-Armed Bandits

;; Reinforcement Learning : Exploration vs Exploitation : Multi-Armed Bandits

;; I'm reading the excellent:

;; Reinforcement Learning: An Introduction
;; by Richard S. Sutton and Andrew G. Barto

;; The book's website, on which is available a complete pdf, is here:
;; http://www.incompleteideas.net/book/the-book.html

;; In Chapter 2, they introduce multi-armed bandits as a simplified model problem 

;; On the basis that you don't understand anything you can't explain to a computer, I thought I'd code it up:

;; Here is a 2 armed bandit
(defn bandit [action]
  (case action
    :arms? [:right :left]
    :right (if (< (rand) 0.5) 4 0)
    :left (if (< (rand) 0.2) 5 0)

;; We can ask it how many arms it's got, and what they're called
(bandit :arms?) ; [:right :left]

;; And we can pull those arms. Rewards are variable.
(bandit :right) ; 4 ; 4 ; 4 ; 0 ; 0 ; 0 ; 0
(bandit :left) ; 5 ; 0 ; 0 ; 0 ; 5 ; 0 ; 5 ; 0

;; Once we pull an arm, we'll have an action/reward pair
(bandit :right) ; 4
;; the pair would be:
[:right 4]

;; Here's a function that yanks an arm at random, and gives us such a pair
(defn random-yank [bandit]
  (let [a (rand-nth (bandit :arms?))]
    [a (bandit a)]))

(random-yank bandit) ; [:left 0]
(random-yank bandit) ; [:right 4] 

;; And a utility function to take the average of a sequence. We need to be able to provide a default value if the sequence is empty.
(defn average
  ([seq default] (if (empty? seq) default (/ (reduce + seq) (count seq))))
  ([seq] (average seq 0)))

;; with some examples
(average [1 2 3 4 5]) ; 3
(average (list) 10) ; 10
(average (list 1) 2) ; 1
(average [] 100) ; 100

;; If we just pull arms at random we get an average reward of about 1.5

(float (average (map second (repeatedly 1000 #(random-yank bandit))))) ; 1.49

;; Since we can see the code for this particular bandit, we know that
;; the expected value of pulling the right arm is 2 (a half-chance of
;; a reward of 4) and the expected reward for the left arm is 0.2*5 = 1

;; So if we were seeking to maximize reward, we'd probably be best to pull the right arm all the time.

(float (average (map bandit (repeat 10000 :right)))) ; 1.9912 
(float (average (map bandit (repeat 10000 :left )))) ; 0.985

;; The interesting question is, if we don't know how the bandit works, how should we design an algorithm that gets the most reward?
;; (Or at least do better than yanking arms at random!)

;; One thing our algorithm is going to have to do is keep some state to record what happens.
;; Let's start by recording the results of all pulls to date:

;; At first, we know nothing, so we can set up a table to represent that we know nothing
(defn initial-state [bandit]
  (into {} (for [k (bandit :arms?)] [k (list)])))

;; We haven't pulled either arm yet
(initial-state bandit) ; {:right (), :left ()}

;; When we get a new action reward/pair, we'll add the result to our state
(defn update-state [state [action reward]]
  (update-in state [action] #(conj % reward)))

;; here are some examples of using update-state
(update-state {:right (), :left ()} [:right 2]) ; {:right (2), :left ()}
(reduce update-state {:right (), :left ()} [[:right 2] [:left 3]  [:right 4] [:right 5]]) ; {:right (5 4 2), :left (3)}

;; here's how we can use it to record the result of ten random yanks
(reduce update-state
        (initial-state bandit)
        (repeatedly 10 #(random-yank bandit))) ; {:right (4 4 0 0 0), :left (0 0 0 0 5)}

;; Once we actually have some data, we can make estimates of the expected rewards

;; mapvals applies a function to every value in a map, returning a new map with the same keys
(defn mapvals [m f] (into {} (for [[k v] m] [k (f v)]))) 

;; examples
(mapvals {} inc) ; {}
(mapvals {:a 1} inc) ; {:a 2}
(mapvals {:a 1, :b 2} inc) ; {:a 2, :b 3}
(mapvals {:a 1, :b 2, :c 3} #(* % %)) ; {:a 1, :b 4, :c 9}

;; In the book, Q_t(a) is the current estimate (at time t)
;; We'll use as our estimate of the value of an action the average value seen so far, or zero if we have no information

(defn Q [state] (mapvals state #(average % 0)))

;; examples
(Q '{:right (5 4 2), :left (3)}) ; {:right 11/3, :left 3}
(Q '{:right (5 4 2), :left ()}) ; {:right 11/3, :left 0}
(Q (initial-state bandit)) ; {:right 0, :left 0} 
(Q (update-state (initial-state bandit) (random-yank bandit))) ; {:right 0, :left 2}

;; let's check that we get roughly what we expect in the long run
(Q (reduce update-state (initial-state bandit)
        (repeatedly 10000 #(random-yank bandit)))) ; {:right 9832/5015, :left 1027/997}

;; If we have estimates of the value of each arm, then a good way to
;; use them is to pull the arm with the highest estimate.

;; This is called 'exploitation', as opposed to 'exploration', which
;; is when you try things you think may be suboptimal in order to get
;; information

;; The 'greedy' action is the one with the highest expected value. Of
;; course there may be more than one greedy action especially at first.

;; To help with this, another utility function:

;; max-keys finds the keys with the highest value in a map, and returns a list with just these keys and values
(defn max-keys [m]
  (let [slist (reverse (sort-by second m)) 
        [_ max] (first slist)]
    (take-while #(= (second %) max) slist)))

;; examples
(max-keys {}) ; ()
(max-keys {1 0}) ; ([1 0])
(max-keys {1 0, 2 0}) ; ([2 0] [1 0])
(max-keys {1 0, 2 1}) ; ([2 1])
(max-keys {1 0, 2 1, 3 -1 , 4 -3, 5 2, 6 2}) ; ([6 2] [5 2])

;; if there is a tie for the greedy action, we can choose at random between the candidates
;; And so we can go from estimates to greedy action like this:
(defn greedy-action [estimates]
  (first (rand-nth (max-keys estimates))))

;; examples
(greedy-action '{:right 10, :left 3}) ; :right
(greedy-action '{:right 10, :left 3 :centre 20}) ; :centre
(greedy-action '{:right 10, :left 3 :centre 3}) ; :right
(greedy-action '{:right 3, :left 3 :centre 3}) ; :right

(greedy-action (Q '{:right (5 4 2), :left (3)})) ; :right
(greedy-action (Q '{:right (), :left (3)})) ; :left
(greedy-action (Q (initial-state bandit))) ; :left

;; after a lot of random pulls, the greedy action should reliably be the one with the highest expected payoff
(greedy-action (Q (reduce update-state (initial-state bandit)
        (repeatedly 10000 #(random-yank bandit))))) ; :right 

;; OK, so we have our stage set, a way of recording what's happened, and some helpful functions defined.


;; Our first try at a learning algorithm will be 'by hand', as it were.

;; We'll always make the 'greedy' choice.

;; At first, we have no records to go on
(initial-state bandit) ; {:right (), :left ()}

;; expected values for both levers are therefore zero
(Q (initial-state bandit)) ; {:right 0, :left 0}

;; so the greedy action will get chosen at random
(greedy-action (Q (initial-state bandit))) ; :left

;; in this case, we've chosen :left, and the bandit's response is
(bandit :left) ; 0 

;; we record it
(update-state (initial-state bandit) [:left 0]) ; {:right (), :left (0)}

;; and we have a new state
'{:right (), :left (0)}

;; new estimates
(Q '{:right (), :left (0)}) ; {:right 0, :left 0}

;; and again, we choose at random
(greedy-action (Q '{:right (), :left (0)})) ; :left

;; the bandit is not feeling very generous
(bandit :left) ; 0

(update-state '{:right (), :left (0)} [:left 0]) ; {:right (), :left (0 0)}

;; new state:
'{:right (), :left (0 0)}

;; new estimates
(Q '{:right (), :left (0 0)}) ; {:right 0, :left 0}

;; this time we choose :right
(greedy-action (Q '{:right (), :left (0 0)})) ; :right

;; and the bandit pays out! 
(bandit :right) ; 4

(update-state '{:right (), :left (0 0)} [:right 4]) ; {:right (4), :left (0 0)}

;; the greedy action will be :right now, because we have evidence that right is better.
(greedy-action (Q '{:right (4), :left (0 0)})) ; :right

;; You get the idea......

;; Let's automate that....

;; Given a state and a bandit, we decide an action and the bandit
;; responds, producing an action/reward pair, and a new state

(defn greedy-algorithm [bandit state]
  (let [action (greedy-action (Q state))
        reward (bandit action)]
    [[action reward] (update-state state [action reward])]))

(greedy-algorithm bandit (initial-state bandit)) ; [[:left 0] {:right (), :left (0)}]

;; To get something we can iterate:

(defn step [[[a r] state]]
  (greedy-algorithm bandit state))

(iterate step [ [:dummy :dummy] (initial-state bandit)])

;; ([[:dummy :dummy] {:right (), :left ()}]
;;  [[:left 5] {:right (), :left (5)}]
;;  [[:left 0] {:right (), :left (0 5)}]
;;  [[:left 0] {:right (), :left (0 0 5)}]
;;  [[:left 0] {:right (), :left (0 0 0 5)}]
;;  [[:left 0] {:right (), :left (0 0 0 0 5)}]
;;  [[:left 0] {:right (), :left (0 0 0 0 0 5)}]
;;  [[:left 0] {:right (), :left (0 0 0 0 0 0 5)}]
;;  [[:left 0] {:right (), :left (0 0 0 0 0 0 0 5)}]
;;  [[:left 0] {:right (), :left (0 0 0 0 0 0 0 0 5)}]
;;  [[:left 0] {:right (), :left (0 0 0 0 0 0 0 0 0 5)}]
;;  [[:left 0] {:right (), :left (0 0 0 0 0 0 0 0 0 0 5)}]
;;  [[:left 5] {:right (), :left (5 0 0 0 0 0 0 0 0 0 0 5)}]
;;  [[:left 0] {:right (), :left (0 5 0 0 0 0 0 0 0 0 0 0 5)}]
;;  [[:left 0] {:right (), :left (0 0 5 0 0 0 0 0 0 0 0 0 0 5)}]
;;  [[:left 0] {:right (), :left (0 0 0 5 0 0 0 0 0 0 0 0 0 0 5)}]
;;  [[:left 0] {:right (), :left (0 0 0 0 5 0 0 0 0 0 0 0 0 0 0 5)}]

;; In this case, the greedy algorithm happens to get a payout on its
;; first try, and decides that it will pull that arm for ever. It
;; never even tries the other arm.

;; Try again:

(iterate step [ [:dummy :dummy] (initial-state bandit)])
;;([[:dummy :dummy] {:right (), :left ()}]
;;  [[:right 0] {:right (0), :left ()}]
;;  [[:right 0] {:right (0 0), :left ()}]
;;  [[:left 0] {:right (0 0), :left (0)}]
;;  [[:right 4] {:right (4 0 0), :left (0)}]
;;  [[:right 4] {:right (4 4 0 0), :left (0)}]
;;  [[:right 4] {:right (4 4 4 0 0), :left (0)}]
;;  [[:right 4] {:right (4 4 4 4 0 0), :left (0)}]
;;  [[:right 4] {:right (4 4 4 4 4 0 0), :left (0)}]
;;  [[:right 4] {:right (4 4 4 4 4 4 0 0), :left (0)}]
;;  [[:right 0] {:right (0 4 4 4 4 4 4 0 0), :left (0)}]
;;  [[:right 0] {:right (0 0 4 4 4 4 4 4 0 0), :left (0)}]
;;  [[:right 4] {:right (4 0 0 4 4 4 4 4 4 0 0), :left (0)}]
;;  [[:right 0] {:right (0 4 0 0 4 4 4 4 4 4 0 0), :left (0)}]
;;  [[:right 4] {:right (4 0 4 0 0 4 4 4 4 4 4 0 0), :left (0)}]
;;  [[:right 0] {:right (0 4 0 4 0 0 4 4 4 4 4 4 0 0), :left (0)}]

;; In this case, it tried the right arm a couple of times, then had a
;; go with the left arm, then went back to the right arm, won a
;; payout, and then got hung up on pulling the right arm repeatedly.

;; We've got a couple of problems here!

;; First is that the algorithm has clearly got into a state where it
;; always pulls the left arm (in the first case), and the right
;; arm (in the second case).

;; It can't be doing the right thing in both cases.

;; Secondly the state is growing linearly, as the algorithm remembers
;; all previous results. That's giving us algorithmic complexity
;; problems and the calculation will get slower and slower, and
;; eventually run out of memory.

Sunday, December 6, 2015

Define Macro

;; When I say

(def a (* 4 5)) ;-> #'user/a

;; I'd rather the repl told me that I'd just assigned the value 20 to
;; something rather than that I'd just assigned something to user/a

a ;-> 20

;; A macro for this is easy

(defmacro define [var expr]
  `(let [tmp# ~expr]
     (def ~var tmp#) tmp#))

(define a (* 4 5)) ;-> 20

;; I'd also like to be able to say, in a scheme-like manner

'(define (square x) (* x x))

;; meaning
(defn square [x] (* x x)) ; #'user/square

;; So I can modify my define macro so:

(defmacro define [var expr]
  (cond (symbol? var)  `(let [tmp# ~expr] (def ~var tmp#) tmp#)
        (list? var)  `(defn ~(first var) ~(into [] (rest var)) ~expr)))

(macroexpand '(define a (* 20 20))) ;-> (let* [tmp__1986__auto__ (* 20 20)] (def a tmp__1986__auto__) tmp__1986__auto__)
(macroexpand '(define (square x) (* x x))) ;-> (def square (clojure.core/fn ([x] (* x x))))

(define a (* 20 20)) ; 400
(define (cube x) (* x x x)) ;-> #'user/cube
(cube 20) ;-> 8000

;; I must say, I haven't actually tried this in practice yet, but it looks like it might work

(define (random-error)
  (+ (rand) -0.5)) ;-> #'user/random-error

(random-error) ;-> -0.28442993770155234
(random-error) ;-> 0.2911519817783499
(random-error) ;-> -0.4037254523155406

(define bell (/ (reduce + (repeatedly 10 random-error)) 10)) ;-> 0.015416035491431623

;; I'm sure someone will let me know if it's broken.

;; This is a bit sub-optimal:
(define square (fn[x] (* x x))) ; #function[user/eval8586$tmp--8558--auto----8587]

;; Any suggestions for improvements?

Sunday, November 22, 2015

An Introduction to the Lambda Calculus

;; Done Twice as a "Dojo" at Villiers Park on Thursday 19th March 2015
;; To groups of about 15 ultra-clever teenagers who were thinking about doing Computer Science at university

;; The first group got as far as higher order functions in an hour.

;; The second group went a bit faster, and we had a bit more time, about an hour and a half,
;; and so we got right to iterative-improve and finding square roots of anything using it.

;; Environment DrRacket (version 6.1)
;; Language R5RS (Revised Revised Revised Revised Revised Report on the Algorithmic Language Scheme)
;; One person sits at the computer, one person helps them, the rest tell them what to do
;; Every time they achieve something significant, rotate audience->copilot->pilot->audience
;; Notes on back of hand: 
(define crib 
  '( 2 3 + (+ 2 3) lambda define square < #t #f if abs Heron average improve make-improver error good-enough good-enough-guess iterative-improve ))

;; Introduction to the Lambda Calculus

;; More precisely, an introduction to the algorithmic language Scheme, which is what you get if you start with 
;; the lambda calculus and you trick it out with some extra stuff that often comes in handy, true and false and if 
;; and define and also some types of numbers, like integers and fractions, and adding, and multiplying.

;; You can build all that stuff starting from scratch with just lambda, and it's a nice thing to do if you want 
;; to understand how it all works, but I reckon you're already ok at that sort of thing. 

;; So we'll start from something that can do basic arithmetic, and we'll learn how to find square roots of things.

;; This is an evaluator. You can ask it the values of things.


;; We can apply the procedure to the two numbers

(+ 2 3)

;; Can you tell me the square of 333?

(* 333 333)

;; The brackets mean (work out the value of the things in the brackets, and then do the first thing to the other things)

;; So what do you get if you add the squares of 3 and 4?

(+ (* 3 3) (* 4 4))

;; We have procedures for * and + , but if we ask the evaluator what & means, or what square means
;; it will just say 'I have no clue'.

;; It might be nice if we had a procedure for squaring things

;; How you make a procedure is with this thing called lambda, which is sort of a rewriting sort of thing.

;; Try (lambda (x) (* x x)), which means 'make me a thing which, when I give the thing x, gives me the value of (* x x) instead' 

(lambda (x) (* x x))

;; #<procedure>, it says, which is very like what you get when you type in +, and it says #<procedure:+>.

;; So we hope we've made a procedure like + or *

;; How shall we use it to get the square of 333?

((lambda (x) (* x x)) 333)

;; Now obviously, typing out (lambda (x) (* x x)) every time you mean square is not brilliant, 
;; so we want to give our little squaring-thing a name.

(define square (lambda (x) (* x x)))

;; Now how do we find the square of 333?

(square 333) ; 110889

;; So lambda is allowing us to make new things, to turn complicated procedures into simple things 

;; and define is allowing us to give things names

;; So now let's make a procedure that takes two things, and squares them both, 
;; and adds the squares together, and let's call it pythag

(define pythag 
  (lambda (x y) 
    (+ (square x) (square y))))

(pythag 3 4)

;; OK, great, now can you figure out how the procedure < works?

( < 3 4)
( < 4 3)
( < 3 4 6)
( < 3 4 2)

;; Notice that these #t and #f things are things that the evaluator knows the value of:
;; They're called true and false.


;; So now the last piece of the puzzle:

;; if takes three things:

(if #t 1 2) ;1
(if #f 1 2) ;2

;; So we've got numbers and *,+,-,/, and we've got #t #f and if, and we've got lambda, and define

;; And so all the stuff we've got above, we can think of it as a reference manual for a little language.

;; We can build the whole world out of this little language.

;; That's what God used to build the universe, and any other universes that might have come to His mind.
;; And we can use it too.

;; Here's the manual

(* 2 3)
(define forty-four 44)
(lambda (x) (* x x))
((lambda (x) (* x x)) 3)
(if (< 2 3) 2 3)

;; And if we understand these few lines, then we understand the whole thing, and we can fit the little pieces together like this:

(define square (lambda (x) (* x x)))
(square 2)

;; So now I want you to use the bits to make me a function, call it absolute-value, which if you give it a number gives you back
;; the number, if it's positive, and minus the number, if it's negative.

(define absolute-value (lambda (x) (if (> x 0) x (- x))))

(absolute-value 1)
(absolute-value -3)
(absolute-value 0)

;; So I've taught you most of the rules for Scheme, which is a sort of super-advanced lambda calculus, and so if you understand 
;; the bits above, then you've got the hang of the lambda calculus plus some more stuff.

;; And it's a bit like chess. The rules of chess are super-simple, you can explain them to babies, 
;; like Dr Polgar did to Judit and her sisters.
;; But that doesn't make the babies into good chess players yet. They have to practise.

;; How are we doing for time? We've done the whole of the lambda calculus, plus some extra bits. We should feel pretty smug.

;; (In both cases, this had taken about 35 minutes)


;; Let's do a little practice exercise. Like a very short game of chess, now I've explained most of the rules.

;; So once upon a time there was this guy, believe it, called 'Hero of Alexandria'.

;; Or sometimes he seems to have been called 'Heron of Alexandria', like Hero was the short version, 
;; like he was sometimes called Jack and sometimes called John.

;; Whatever, Hero invented the syringe, and the vending machine, and the steam engine, and the windmill, and the rocket, 
;; and the shortest path theory of reflection of light, and did some theatre stuff, 
;; and he was like Professor of War at the big library in Alexandria.

;; You get the impression that if Alexandria had lasted just a little bit longer, 
;; the whole industrial revolution would have kicked off right there, and the Romans would have walked on the moon in about AD400.

;; And we'd all be immortal, and live amongst the stars. So you should take the fall of the Roman Empire *very* personally.

;; And one of his things was a way of finding the square roots of numbers, 
;; which is so good that it was how people found square roots right up until the invention of the computer.

;; So I'm going to explain that method to you, and you're going to explain it to this computer, and then you can get the computer
;; to calculate square roots for you, really fast. And after that you're only a couple of steps away from cracking the 
;; Enigma codes and winning the second world war and inventing the internet and creating an artificial intelligence 
;; that will kill us all just 'cos it's got better things to do with our atoms. I'm not joking.

;; So careful.... What I've just given you is the first step on the path that leads to becoming a mighty and powerful wizard.
;; And with great power comes great something or other, you'll find it on the internet, so remember that.


;; So imagine you want to find the square root of 9. And you're a bit stuck, so you say to your friend, "What's the square root of nine?", and he says it's three.

;; How do you check?

(* 3 3)

;; Bingo. There's another way to check

(/ 9 3)

;; That's what it means to be the square root of something. If you divide the something by the square root, you get the square root back.

;; But what if your friend had said "err,.. 2 or something?"

(/ 9 2)

;; Notice that the number you put in is too low, but the number you got back is too high.

;; So Heron says, let's take the average.

;; So we need an average function

(define average (lambda (a b) (/ (+ a b) 2)))

(average 2 (/ 9 2)) ; 3 1/4

;; three and a quarter, that's like a much better guess, it's like you'd found a cleverer friend.

;; so try again.

(average 3.25 (/ 9 3.25)) ; 3.009615...

;; and again 

(average 3.0096 (/ 9 3.0096)) ; 3.0000153..

(average 3.0000153 (/ 9 3.0000153)) ; 3.000000000039015

;; So you see this little method makes guesses at the square root of nine into much better guesses.

;; We see that this is kind of a repetitive type thing, and if you see one of those, your first thought should be, 
;; I wonder if I can get the computer to do that for me?

;; Can you make a function which takes a guess at the square root of nine, and gives back a better guess?

(define improve-guess (lambda (guess) (average guess (/ 9 guess))))

;; I'd better show you how to format these little functions so that they're easier to read

(define improve-guess 
  (lambda (guess) 
    (average guess (/ 9 guess))))

;; The evaluator doesn't notice the formatting, and it makes it a bit more obvious what's getting replaced by what.

(improve-guess 4) ; 3 1/8
(improve-guess (improve-guess 4)) ; 3 1/400
(improve-guess (improve-guess (improve-guess 4))) ; 3 1/960800

;; We all know what the square root of nine is, let's look at a more interesting number, two. 
;; It's a bit of an open question whether 'the square root of two' is a number, or whether it's just a noise 
;; that people make with their mouths shortly after you show them a square and tell them about Pythagoras' theorem.

;; Pythagoras used to have people killed for pointing out that you couldn't write down the square root of two.

;; I've got a bit of a confession to make. 

;; Someone's already explained to this computer how to find square roots

(sqrt 9)          ; so far so good!
(sqrt 2)          ; 1.4142135623730951   hmmm. let's check.

(square (sqrt 2)) ; 2.0000000000000004

;; So it turns out that this guy's just said, if you can't come up with the square root of two, just lie, and come up with something
;; that works, close as dammit. 

;; Which is like, bad practice, and tends to lead to Skynet-type behaviour in the long run.

;; So let's see what Hero would have said about it.

;; We need a new function that makes guesses better at being square roots of two.
;; It's a bit dirty, but let's just call that improve-guess as well.

;; That's called redefinition, or 'mutation', and it's ok when you're playing around, 
;; but it's a thing you should avoid when writing real programs, because, you know, Skynet issues.

;; Hell, no-one ever got more powerful by refraining from things.

(define improve-guess 
  (lambda (guess) 
    (average guess (/ 2 guess))))

;; Anyone make a guess? 

(improve-guess 1) ; 1 1/2

;; Any good?

(square (improve-guess 1)) ; 2 1/4

;; How wrong?

(- (square (improve-guess 1)) 2) ; 1/4

;; OK, I want you to notice that we've just done the same thing twice

(define improve-guess-9 (lambda (guess) (average guess (/ 9 guess))))
(define improve-guess-2 (lambda (guess) (average guess (/ 2 guess))))

;; Now whenever you see that you've done the same thing twice, and there's this sort of grim inevitability 
;; about having to do it a third time someday, you should think:

;; Hey, this looks like exactly the sort of repetitive and easily automated task that computers are good at.

;; And so now I want you to make me (and this is probably the hard bit of the talk...) a function which 
;; I give it a number and it gives me back a function which makes guesses at square roots of the number better.

(define make-improve-guess
  (lambda (n)
    (lambda (guess)
      (average guess (/ n guess)))))

;; And now we can use that to make square root improvers for whatever numbers we like

(define i9 (make-improve-guess 9))

(i9 (i9 (i9 (i9 1)))) ; 3 2/21845

(define i2 (make-improve-guess 2))

(i2 (i2 (i2 (i2 1)))) ; 1 195025/470832

;; The first group got this far in about an hour, which was all we had time for, and then we stopped and I waffled for a bit.

;; Now how good are our guesses, exactly?

(- 2 (square (i2 (i2 (i2 (i2 1))))))

;; We could totally make a function out of that:

(define wrongness (lambda (guess) (- 2 (square guess))))

(wrongness (improve-guess 1)) ; -1/4
(wrongness (improve-guess (improve-guess 1))) ; -1/144
(wrongness (improve-guess (improve-guess (improve-guess 1)))) ; -1/166464
(wrongness (improve-guess (improve-guess (improve-guess (improve-guess 1))))) ; -1/221682772224

;; So we're getting closer! When should we stop? Let's say when we're within 0.00000001

(define good-enough? (lambda (guess) (< (absolute-value (wrongness guess)) 0.00000001)))

(good-enough? (improve-guess (improve-guess 1)))  ; #f

(good-enough? (improve-guess (improve-guess (improve-guess (improve-guess 1))))) ; #t

;; Now, we're doing a bit too much typing for my taste.

;; What we want to do is to say:

;; I'll give you a guess. If it's good enough, just give it back. If it's not good enough, make it better AND TRY AGAIN.

;; This is the hard bit. We need to make a function that calls itself.

;; Go on, have a go

(define good-enough-guess 
  (lambda (guess)
    (if (good-enough? guess) guess
        (good-enough-guess (improve-guess guess)))))

(good-enough-guess 1) ; 1 195025/470832


;; The second group got this far in about an 1hr 10 mins, but they all still seemed keen and we didn't have to stop, so:

;; Now this is as much of the talk as I'd written,
;; but actually we've got the time to go a little bit further, if your brains haven't totally exploded, and you might like the next bit, 
;; because it makes a nice punchline to the whole thing:

;; There's a pattern here, and it's called iterative-improve

;; And iterative improvement is everywhere in the world, for instance you probably got shown the Newton-Raphson solver at school, 
;; which is a thing which can find roots of all sorts of equations very fast, and it works like this, you have an initial guess, and 
;; Newton Raphson is a way of making a guess into a better guess, and you need to know when the answer is good enough so you can stop.

;; Or this morning I had a shower, and I got in the shower and I turned the water on to just a random position and it was too hot, so I turned the handle
;; a bit the other way and it was a bit too cold, so I turned it back just a bit and then it was ok so I stopped.

;; And that's the same pattern, and you see this sort of thing all over, it is how you solve big matrices and so on and so forth.

;; And we have just discovered this pattern, which is kind of a fundamental building block when you're writing programs, like a for loop is another basic pattern.

;; So let's see if we can make a function that takes a guess and a way of improving guesses and a way to tell if we're done yet, and gives us back an answer.

(define iterative-improve
  (lambda (guess improve good-enough?)
    (if (good-enough? guess) guess
        (iterative-improve (improve guess) improve good-enough?))))

(iterative-improve 1 (make-improve-guess 2) good-enough?) ; 1 195025/470832

;; This was where we stopped the second session. Here's some waffle:

;; And I think now you can see that we've abstracted a pattern here that will come in handy for the sorts of things that we're trying to do.

;; That's what this talk has really been about, how to build a language which allows you to solve the problems that you're interested in.

;; So I'd like to tidy up the program that we've just written, and put it into the sort of form that I'd have written it in, if I'd been solving this problem
;; and I'd played around for a bit and found what I thought was a nice expression of the ideas that we've been talking about.

(define square (lambda (x) (* x x)))

(define absolute-value (lambda (x) (if (> x 0) x (- x))))

(define make-improve-guess
  (lambda (n)
    (lambda (guess)
      (average guess (/ n guess))))) ; this bit is Heron's method

(define make-good-enough? 
  (lambda (n tolerance) 
    (lambda (guess)
      (> tolerance
         (absolute-value (- n (square guess)))))))
(define iterative-improve
  (lambda (guess improve good-enough?)
    (if (good-enough? guess) guess
        (iterative-improve (improve guess) improve good-enough?))))

(define make-square-root
  (lambda (guess tolerance)
    (lambda (n)
      (iterative-improve guess (make-improve-guess n) (make-good-enough? n tolerance)))))

;; We can use these bits to make the sort of square root we usually find provided:
(define engineer-sqrt (make-square-root 1.0 0.00000000000001 ))

(engineer-sqrt 2)

;; And here's what we might use, if we needed really good square roots for some reason:
(define over-cautious-engineer-square-root (make-square-root 1 1/1000000000000000000000000000000000000000000000000000000000000000000))

(over-cautious-engineer-square-root 2)

;; And I hope you can see this this program is actually built of lots of tiny simple pieces, 
;; all of which you can understand, and most of which you'll be able to reuse in other contexts.

;; In particular, iterative-improve is a terribly general thing which you can use in lots of ways.

;; And it might have taken us a while to write, although we wrote it as part of a learning-the-language finger-exercise, 
;; but we never have to write it again. It works and it will keep working and we've got in the bank now.

;; Here's the take-home message:

;; If you've got a problem, build yourself a language to solve the problem in. 

;; To do that, you need to start with a language that allows you to abstract what you do into simple pieces
;; which are easy to understand, so that you can see that they're right and they aren't too snarled up with 
;; the little details of the problem you're working on at the moment.

;; And you need a language that allows you to combine the little pieces easily
;; to make new pieces that solve the problem that you're trying to deal with.

;; And there's a sense in which all computer languages are just this lambda calculus.

;; We've done all this in Scheme, which is lambda calculus plus some extra stuff.

;; There's nothing we've done here that can't be done in python, or in ruby, or in perl or in haskell or in lisp.

;; What distinguishes these languages is what extra stuff has already been added to them. 

;; But if a language is good enough, and none of the usual features have actually been taken away, 
;; which does happen sometimes, then if there's anything missing that you need you can always add it yourself.

;; And then you can use the language that you have to build the language that you need.

;; In a sense, making languages is itself an iterative improvement process. 

;; And the big questions are always:

;; How do we make things better? What's good enough? When are we done?

;; Postscript

;; I'll show you a trick now. We've been using it all along and nobody noticed, 
;; but it's the sort of thing that looks like magic, and I don't like magic unless I can cast the spells myself.

(good-enough-guess 1)   ; 1 195025/470832
(good-enough-guess 1.0) ; 1.4142135623746899

;; This is called 'contagion'. There are really two types of numbers.

;; Numbers that look like 432/123 are called 'exact', or 'vulgar fractions'
;; Numbers that look like 1.4142 are called 'inexact', or 'approximate', or 'floating point', or 'decimal fractions'

;; The first type are the sort of numbers that children learn about in school, and that mathematicians use.

;; And the second type are the sort of numbers that engineers use, and they're actually quite a lot more complicated and fuzzy
;; than the exact type. They just sort of work like 'if it's very close, then it's good enough'.

;; The way most computers think about them, they keep about sixteen digits around, and if you want more than that, tough luck.

;; But for some purposes they're better, for instance they're easier to read, and it's a bit of a matter of taste.

;; If you multiply or add an inexact number to an exact number, the answer is always inexact.
;; You can't unapproximate something.

(/ 1 3)   ; 1/3
(/ 1.0 3) ; 0.3333333333333333

;; We all know that 1/3 isn't really 0.33333333333333

;; Mathematicians worry about that sort of thing. Engineers don't. Sometimes aeroplanes crash. Mostly they don't.

Sunday, February 15, 2015

Destructuring Clojure Maps

;; Destructuring Clojure's Maps

;; I can never ever remember how this works, so here is a note to self:

((fn [{a :a}] a) {:a 1}) ; 1

;; And by let-lambda isomorphism

(let [{a :a} {:a 1}] a) ; 1

;; Why on earth is the syntax the wrong way round? Why can't {:a a} match {:a 1}?

;; Similarly

((fn [{a :a b :b}] [a b]) {:a 1 :b 2}) ; [1 2]

(let [{a :a b :b} {:a 1 :b 2}] [a b]) ;  ; [1 2]

;; And with the common pattern where the variables are like the keys:

((fn [{:keys [a b]}] [a b]) {:a 1 :b 2}) ; [1 2]

(let [{:keys [a b]} {:a 1 :b 2}] [ a b ]) ; [1 2]

;; We can destructure recursively (although we may not be wise to if we keep forgetting how it works!)

((fn [{a :a {c :c d :d} :b}] [a c d]) {:a 1 :b {:c 2 :d 3}}) ; [1 2 3]

(let [{a :a {c :c d :d} :b} {:a 1 :b {:c 2 :d 3}}] [a c d]) ; [1 2 3]

;; And we can remember the keys entire on which we have recursed, so:

(let [{a :a {c :c d :d :as b} :b}
      {:a 1 :b {:c 2 :d 3}}]
  [a b c d]) ;-> [1 {:c 2, :d 3} 2 3]

;; Finally a 'real' example, a ring request map containing parameters and a session, both of
;; which have substructure

(def ring-request
  {:params {:action "a" :key "k" :spurious "sp"}
   :session {:data "d" :state "s"}
   :irrelevant "irr"})

;; So the parameters we're interested in look like
{:params {:action :key} :session {:data :state}}

;; And we can extract all the pieces, naming each part, like so:

(defn process-request [{{action :action key   :key   :as params } :params
                        {data   :data   state :state :as session} :session :as request}]
  (println action)
  (println key)
  (println data)
  (println state)
  (println params)
  (println session)
  (println request))

(process-request ring-request)
;; a
;; k
;; d
;; s
;; {:key k, :action a, :spurious sp}
;; {:state s, :data d}
;; {:irrelevant irr, :params {:key k, :action a, :spurious sp}, :session {:state s, :data d}}