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Apr 22, 2 022 Thinking in Clojure

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### Transcript of Clojure 1a

• 1. Thinking in ClojureMar 29, 2013

2. Jumping in Well quickly go through Clojures data types, somebasic functions, and basic syntax Then well get to the good stuff!2 3. Clojures data types Clojure has: Lists, enclosed in parentheses and separated by spaces or commas:(a 17 "Plenty of parentheses") Functions: (fn [x] (first(rest x)) Numbers: All Java numeric types, plus ratios and exact decimals:5, 5.3, 5.3e30, 077, 0xFF00FF, 3/5, 5.3M Strings, as in Java: "She said "Hello"" Characters: a, 5, n, newline, tab, etc. The booleans true and false nil, equivalent to Javas null Symbols, which stand for themselves: :meadow, :CIS-554 Vectors: [5 :a "hi!"] Maps: {:one 1, :two 2} Sets: #{:prolog :clojure}3 4. Some basic Clojure functions Syntax of a function call: (function args) Basic operationssequences (seq) are lists, sets, maps, vectors: (quote arg) or arg, to keep arg from being evaluated (first seq) is the first element in the sequence (or nil) (rest seq) is what remains when the first element is removed (or nil) (cons arg seq) returns a new sequence with arg as its first element (= args) tests whether its args are equal (empty? seq), (list? seq), (seq? arg), (nil? arg) are more tests Basic arithmetic (+ args), (- args), (* args), (/ args) , (< args), etc. Basic logic (and args), (or args), (not arg), (if condition result1 result2) Defining values (def name value) defines the name to be the given value (defn name argv value) is shorthand for (def name argv value), where argv is a vector 4 5. Functions and special forms The arguments to a function are evaluated before the function is called Example: (* 2 (+ 3 4)) The function * is called with the arguments 2 and 7 A special form looks just like a function, but it gets its argumentsunevaluated The special form itself decides when and whether to evaluate its arguments quote does not evaluate its argument if evaluates its first argument, then decides which of the second and third arguments to evaluate Clojure allows you to define your own special forms This means you can define your own control structures 5 6. A typical Clojure function (defn first-double-letter"Returns the first doubled letter in a string, or nil."[s](if (< (count s) 2)nil(if (= (first s) (second s))(first s)(first-double-letter (rest s)) ) ) ) user=> (first-double-letter "Pennsylvania")nuser=> (first-double-letter (1 2 3 4 3 5 5 4 6))56 7. Its easier with cond cond is an if then else if then else construct:(cond test1 result1 test2 result2 testN resultN) It requires an even number of parameters (one result for each test) The symbol :else may be used as the last test (defn first-double-letter"Returns the first doubled letter in a string, or nil."[s](cond (< (count s) 2) nil(= (first s) (second s)) (first s):else (first-double-letter (rest s)) ) ) user=> (first-double-letter "Pennsylvania")nuser=> (first-double-letter (1 2 3 4 3 5 5 4 6))5 7 8. Its all about recursion Some rules of doing recursion: 1. Handle the base cases directly (without recursion) 2. Recur only with a simpler case 3. Dont use global variables 4. Dont look down into the recursionthat will just confuse you In Clojure you are almost always working with a list or some similar sequence Lisp programmers say, Do something with the head, and recur with the tail Clojures terms for head and tail are first and rest This pretty much covers rules 1 and 2 above Clojure doesnt have global variables This covers rule #3 above Rule 4 always holds. Think about what you are doing now, not whatsome recursive call is doing8 9. Functional programming Clojure is functionalwhat does that mean? Functions are like functions in mathcalled with the same arguments,they always return the same result This means: No global variables, no dependence on external values, and no side effects! Functions are values, or first-class objects Functions can be passed as parameters to functions, returned as the value of functions, created as needed, stored in data structures, and there are operations on functions that produce new functions The blub paradox appliesthe value added is substantial, but not obvious to an imperative or object-oriented programmer Data is immutable (like strings in Java) Clojures data structures are designed to make this efficient Immutable data greatly simplifies concurrent programming Because data is immutable, loops are unnecessary (use recursion instead!)9 10. Costs and benefits Costs of functional programming Its weird and unfamiliar How can you do anything without objects, mutable variables, or loops? (Loops are used primarily to change the values of things) As a manager, functional programmers are hard to find (and expensive!) Clojure, and Lisp dialects generally, have too many parentheses! Benefits of functional programs Easier to write correct programs Yeah, right! No, really! All data is local and immutable. Easier to write unit tests, because function values depend only on inputs Much easier to write concurrent programs Operations on collections make code simpler and more concise The simpler foundation means less syntax and fewer special cases Some operations, such as equality testing, are really fast But its still weird! 10 11. Easier to write correct programs Programs are easier to write when all data is local When relevant values can be changed elsewhere in the programpossibly in many placesits harder to see all the connections Functions in a functional language get all relevant input from the parameter list Unit testing is easier, because there are no dependencies on functions thatmay or may not have been called previously There is no need for a setUp method Functional programming supports powerful operations on sequences The imperative and object-oriented programming styles have been characterized as word by word programming Some sequence operations, such as membership testing, are provided for you In a functional language, any function can be a sequence operation 11 12. The problem of state Nonfunctional programming language are stateful or have state The state of a program is given by (1) the values of all the variables throughout the program, and (2) the current locus of execution That can be a huge amount of information to keep track of! Object-oriented programmers try to control complexity by having objects be responsible for their own state, and loosely coupled (not very dependent on) other objects Methods often have side effects, that is, they modify state Functional languages try to avoid having state at all This isnt always easy Purely functional languages cannot have side effects Since I/O is a side effect, this is an even more difficult restriction 12 13. Maintaining state, functionally Sometimes you just need state Consider an adventure game You need to keep track of where you are, where other objects are, what you areholding, which paths are blocked or open, etc.this is your state You do not need to keep track of permanent, immutable data; for example, most pathsbetween rooms are fixed and unchangingthis isnt part of the state In a functional program, a state is just an immutable (and usually just one)data item The data item can be quite complex, such as a dictionary States are immutable, but you can always create a new state that is a variation of a given state With carefully designed data structures, not as much storage is required as you mightexpect So the functional solution to maintaining state is: Pass one state into a function, get a new (and different) state back!13 14. Clojures I/O compromise A purely functional program has no side effects I/O is a side effect Therefore: A purely functional program cannot do I/O! In Clojure, all functions return a value (print args) and (println args) return nil Clojure allows side effects in two well-defined places: (do args) evaluates all its arguments in order, but returns only the value of the last one When a function (fn argv args) is called, the arguments are evaluated in order, but only the last value is returned Example: (defn powers "Computes cube and square" [] (def n (read)) (println (* n n n)) (println (* n n)) n)14 15. Lists are immutableHere is a typical list:my-list AB CwHere is(cons w my-list)Here is (rest my-list)Notice that my-list remains unchangedVectors, hash maps, sorted maps, hash sets, and sorted setsare similarly immutable15 16. Functions are just values user=> (cons w (a b c))(w a b c) user=> (defn swap-args [f x y] (f y x))#user/swap-args user=> (swap-args cons (a b c) w)(w a b c) user=> (defn apply-n-times [f x n] "Apply f to x, n times: f(f(f..(n)...))" (if (zero? n) x (apply-n-times f (f x) (dec n)) ) )#user/apply-n-times user=> (apply-n-times (fn [x] (* 2 x)) 1 10)102416 17. Collatz, the hard way Definition: collatz(1) = 1collatz(n) = collatz(n / 2) if n is evencollatz(n) = collatz(3 * n + 1) user=> (defn collatz [n](let [ do-even (fn [n] (collatz (/ n 2))) do-odd (fn [n] (collatz (inc (* 3 n)))) ](print n " ")(if (= n 1) 1(if (even? n) (do-even n) (do-odd n)) ) ) ) #user/collatzuser=> (collatz 7)7 22 11 34 17 5226 134020 10 5 16 8 4 2 1 117 18. map, filter, and reduce Here are three powerful functions you will find in almost anyfunctional programming language map apply a function to every element of a sequence, returning asequence of results user=> (map even? (3 1 4 1 6)) (false false true false true) filter apply a predicate to every element of a sequence, returning asequence of those that satisfy the predicate user=> (filter even? (3 1 4 1 6)) (4 6) reduce use a function to reduce a sequence to a single value user=> (reduce * (3 1 4 1 6)) 72 18 19. The real problem with state For decades weve been dealing with mutable state Mutable state + concurrency = nondeterminism We use threads and locks and semaphores and so on These are complicated, unsafe, and inefficient As Herb Sutter points out in The Free Lunch is Over, we have hit a 3 GHz barrier Since 2003, computers have not gotten faster We still want them faster Concurrency is the only solution Functional languages, with immutable state, provide a partial solution As Martin Odersky points out, you can hide from concurrency for a while yetbutnot forever Important consequences: All newer languages are gaining functional and concurrent features Older languages, such as Java, are also trying to integrate these features You can run, but you cant hide! 19 20. Oh, and by the way Clojure has Infinite sequences Lazy sequences Exact decimal arithmetic Function composition Function currying Macros And lots more No version of Lisp has ever become mainstream They just get mugged in dark alleys and their ideas stolen!20 21. The End21