Clojure Q&A

A work in progress; a new approach to teaching Clojure to relative novices.

Transcript

1. Is this talk about Clojure? Howard M. Lewis Ship — hlship@gmail.com — @hlship
2. a: Yes, because Howard loves to talk about Clojure.
3. q: Does Howard know what he's talking about?
4. a: Hopefully. He's been coding Clojure full time for several years. And he's done some interesting stuff before that. He's also written some well-favored open source libraries for Clojure.
5. syntax
6. q: Is "Zanzibar" a form?
7. a: Yes, because "Zanzibar" is a string. In fact, it is an instance of java.lang.String. Clojure uses the underlying host's facilities where possible.
8. q: Is 5431 a form?
9. a: Yes, because 5431 is a literal number. In fact, it is an instance of java.lang.Long.
10. q: Is true a form?
11. a: Yes, because true and false are booleans. In fact, it is an instance of java.lang.Boolean.
12. q: Is null a form?
13. a: No. I think you mean nil. Java's null is Clojure's nil. It's about heritage.
14. q: Is \J a form?
15. a: Yes, because \J is a character. It is an instance of java.lang.Char. \newline, \tab, and a few others work as well. There's also Unicode character support.
16. q: Is john-brunner a form?
17. a: Yes, because john-brunner is a symbol. Clojure symbols start with a letter (or certain punctuation) and can contain most other characters.
18. q: Is :science-fiction a symbol?
19. a: No. :science-fiction is a keyword. Keywords look like symbols, but start with a colon. Symbols or keywords can contain colons, but that is rarely used.
20. q: Is clojure.core/reduce a form?
21. a: Yes, because symbols can be namespace qualified. Symbols can contain a namespace, a slash, and a simple symbol. Watch out: simple symbols can start with a slash. clojure.core// is also a symbol.
22. q: Is ::donald-hogan a keyword?
23. a: Yes, because ::donald-hogan starts with a colon. Two colons is a shortcut for qualifying the keyword into the current namespace.
24. q: Are those all of the Clojure forms?
25. a: No. There's a couple of more literal types, such as regular expressions and ratios, and some more number types. There's also collection forms.
26. collections
27. q: Is () a form?
28. a: Yes, because () is the empty list. In most cases, nil and () are treated identically but they aren't the same thing.
29. q: Is (1 2 3) a form?
30. a: Yes, because (1 2 3) is a sequence of forms surrounded by open and close parenthesis.
31. q: Is ("Howard" ("Lewis" "Ship")) a list?
32. a: Yes, because lists are forms that can be contained inside other lists.
33. q: ("Stand on Zanzibar" "John Brunner" 1968 "ISBN 0-09-919110-5" :science-fiction) Is this a list?
34. a: Yes. Lists can contain a mix of different types of forms. Forms are separated by whitespace, including blank lines. The comma character is also whitespace in Clojure.
35. q: Is [1 2 3] a list?
36. a: No. [1 2 3] is a vector. Vectors are similar to lists, but support indexed access to the elements. We'll come back to that later.
37. q: Is {:genre} a form?
38. a: No. Curly braces denote a map form, but map forms must have an matching number of keys and values.
39. q: Is {:title "Stand on Zanzibar", :genre :science-fiction} a map?
40. a: Yes, because there is a value for every key. Keys and values can be any form, even collections. Remember: commas are whitespace. Some people like to put commas after each value.
41. q: Is #{:science-fiction :romance :western :fantasy} a form?
42. a: Yes. This is a set. Like the other collections, the values inside the set can be any other form, even collections or nil. Unlike lists or vectors, the values inside a set are un-ordered.
43. q: Is ; TODO: Fix this later a form?
44. a: No. It is a comment. Comments start with a ; and continue to the end of the line. By convention, a single ; is a comment for the same line. A ;; denotes a block within a definition. A ;;; denotes a section within a source file.
45. evaluation
46. q: What does 37 evaluate to?
47. a: 37. Literal values evaluate to themselves. user=> 37 ➠ 37
48. user=> 37 ➠ 37 Namespace prompt Evaluation output Input Expression
49. q: What does :science-fiction evaluate to?
50. a: It evaluates to :science-fiction. Keywords always evaluate to themselves. user=> :science-fiction ➠ :science-fiction
51. q: What does [1 2 3] evaluate to?
52. a: It evaluates to [1 2 3]. Vectors, maps, and sets evaluate to themselves.
53. q: What does [space 1999] evaluate to?
54. a: Unknown, or an error. space is a symbol that has not been defined.
56. q: What does = evaluate to?
57. a: A function, from the clojure.core namespace. = is a symbol, and symbols evaluate to something else. Everything in the clojure.core namespace is automatically available by default.
58. user=> = ➠ #object[clojure.core$_EQ_ …] How Clojure outputs Java objects
59. q: What does () evaluate to?
60. a: () evaluates to itself, (). As we'll see, this is an exception to the general rule.
61. q: What does this evaluate to? (= :science-fiction :fantasy)
62. a: It evaluates to false. A list form evaluates as a function call. The = function compares two or more values for equality.
63. q: What does (=) evaluate to?
64. a: An error. Each function has an arity, or number of parameters it expects. It is an error to invoke a function with the wrong number of arguments. user=> (=) clojure.lang.ArityException: Wrong number of args (0) passed to: core/=
65. q: What does (= 42 42.) evaluate to?
66. a: false. The = function does not coerce its arguments, and the java.lang.Long 42 does not equal the java.lang.Double 42. You can use == instead; it only works for numbers and does some coercion.
67. q: What does (1 2 3) evaluate to?
68. a: An error. The first form inside a list must be a function. user=> (1 2 3) java.lang.ClassCastException: java.lang.Long cannot be cast to clojure.lang.IFn
69. q: What does '(flip flop) evaluate to?
70. a: It evaluates to (flip flop). The leading quote prevents evaluation of the list. '(flip flop) ➠ (flip flop) The symbols are not evaluated and do not have to be previously defined.
71. q: What does String evaluate to?
72. a: The Java Class java.lang.String. The classes in the java.lang package are automatically available. Other classes can be imported, or fully qualified (e.g., java.util.ArrayList).
73. q: What does (println "Outstanding!") evaluate to?
74. a: It evaluates to nil. More importantly, it has a side effect. user=> (println "Outstanding!") Outstanding! ➠ nil
75. q: What does this evaluate to? (def book {:title "Stand on Zanzibar" :author "John Brunner"})
76. a: It evaluates to a Var, a holder of references to other things. The Var prints itself as #'user/book. A side effect is that the current namespace, user, is modified: a new Var is created and added to the namespace.
77. user=> (def book {:title "Stand on Zanzibar" :author "John Brunner"}) ➠ #'user/book
78. clojure.core = def defn String … user book class java.lang.String {:title "Stand on Zanzibar" :author "John Brunner"} a function a function a function
79. q: What does book evaluate to?
80. a: It evaluates to: {:title "Stand on Zanzibar" :author "John Brunner"} This is the value stored in the Var identified by the book symbol.
81. functions
82. q: What does this evaluate to? (defn add-one [x] (+ 1 x))
83. a: It evaluates to the Var #'user/add-one. It also adds the symbol mapping to the user namespace.
84. (defn add-one [x] (+ 1 x)) New Symbol Function Body Parameter symbols
85. clojure.core = def defn String … user book add-one class java.lang.String {:title "Stand on Zanzibar" :author "John Brunner"} a function a function a function a function
86. q: What is the purpose of this string? (defn add-one "Returns the value of x incremented by one." [x] (+ 1 x))
87. a: It adds :doc metadata to the Var. The clojure.repl/doc function will print it out.
88. user=> (doc add-one) ------------------------- user/add-one ([x]) Returns the value of x incremented by one. ➠ nil
90. q: What does this evaluate to? (let [mystery :science-theater] mystery)
91. a: It evaluates to :science-theater. The let macro is a way to define local symbols. mystery is a local symbol, and becomes the evaluation of the let.
92. q: What does this evaluate to? (let [mystery (fn [x] (* 2 x))] (mystery 3000))
93. a: It evaluates to 6000. mystery is a local symbol for an anonymous function. Passing 3000 to mystery returns 6000.
94. q: What does this evaluate to? (let [mystery #(str "* " % " *")] (mystery "YOW"))
95. a: It evaluates to "* YOW *". str is a function that assembles a string. #() is an even more terse way to write an anonymous function. The % symbol is the parameter.
96. q: Is #(str "* " % " *") a form?
97. a: No, it is a reader macro. The reader is the part of Clojure that parses a stream of characters into forms. The #(…) sequence is converted into a (fn [x] …) form before being evaluated.
98. q: What are we doing here? (defn inc-by ([x] (inc-by x 1)) ([x delta] (+ x delta)))
99. a: We're defining a function with two arities. user=> (inc-by 10) ➠ 11 user=> (inc-by 10 5) ➠ 15
100. q: Are Clojure functions pure?
101. a: It depends. Pure - mathematically pure - functions will always return the same value for the same parameters. Generally, Clojure functions are pure.
102. • A function is pure if it only calls pure functions. • A function is pure if it performs no side-effects. • A pure function must not reference mutable state. • Input or output of any kind is impure: • Reading from the keyboard • Reading from a file, socket, etc. • System.currentTimeMillis(), etc. • Randomness is impure.
103. q: Why do we care about purity?
104. a: Pure functions are easier to reason about. They tend to be simpler. You don’t have to think about anything outside the function’s body and parameters. You can test them easily, and experiment with them using the REPL. Pure functions compose better than impure functions.
105. a: A pure function is independent from time. With multiple threads, it does not matter what threads are doing which calculations. Computing the result of a pure function can be deferred until the value is needed. The result can be cached, using the parameters as a key. This is called memoization.
106. q: Are Java methods pure?
107. a: It depends. Java objects are often mutable. They contain fields whose value can be updated at any time. Mutability works against purity. Basically, any mutable field anywhere is an additional invisible parameter of the Java method. This makes reasoning about Java much harder.
108. q: Are Clojure collections pure?
109. a: Yes, for vectors, maps, and sets. Clojure collections are persistent (aka immutable). They can act as functions. As functions, they are pure.
110. q: What does this evaluate to? ([:arthur :ford :zaphod] 1)
111. a: It evaluates to :ford. A vector can acts as a function; the parameter is the index.
112. q: What does this evaluate to? ({:author "John Brunner"} :author)
113. a: It evaluates to "John Brunner". Matching keys return their value. Non-matching keys return nil, or the second parameter.
114. q: What does this evaluate to? (#{:blinky :pinky :inky :clyde} :pinky)
115. a: It evaluates to :pinky. Sets return the matching value if found, or nil. This is idiomatic for testing for inclusion in the set.
116. higher order functions
117. q: What is truth?
118. a: I'm not sure, I think it is related to beauty. Truthy is easier. nil and false are falsey. Anything else is truthy.
119. q: Is odd? a predicate? (doc odd?) ------------------------- clojure.core/odd? ([n]) Returns true if n is odd, throws an exception if n is not an integer
120. a: Yes, because odd? is a 1-arity function that returns a truthy response. Predicates end in ? by convention.
121. q: What does this evaluate to? (iterate inc 1) inc is a function that adds one to a number
122. a: It evaluates to (1 2 3 4 5 …) iterate keeps passing a value through a function, building an infinite list from it, as if: (1 (inc 1) (inc (inc 1)) (inc (inc (inc 1))) …) Clojure handles infinite data really well via lazy evaluation.
124. q: What does this symbolize? f ?
125. a: A filter. The filter function is a predicate. Each collection value is passed, in turn, to the predicate. Using filter, only the truthy predicate results are included in the result. remove is the opposite; truthy predicate results are removed, falsey are retained.
126. q: What does this evaluate to? (take 10 (filter odd? (iterate inc 1))
127. a: It evaluates to 
 (1 3 5 7 9 11 13 15 17 19). take will return up to N values from a collection. take and filter are lazy.
128. q: Isn't that getting nested a bit deep?
129. a: Yes, so we would use the ->> ("thread last") macro. (->> (iterate inc 1) (filter odd?) (take 10)) This makes the flow of transformations easier to understand.
130. q: What does this symbolize? f
131. a: A mapping operation. The provided function can convert one cube to one pyramid. f The map function converts a collection of cubes to a collection of pyramids.
132. q: What does this evaluate to? (map inc [5 10 50])
133. a: It evaluates to (6 11 51). map does not return a vector; this is a lazy list.
134. q: What does this evaluate to? (take 10 (map * (iterate inc 1) (iterate inc 1))) * is, of course, multiplication.
135. a: It evaluates to (1 4 9 16 25 36 49 64 81 100). f The arity of the mapping function must match the number of collections passed to map.
136. q: Are lazy collections pure?
137. a: Yes. Mostly. A lazy collection’s internal state does mutate, as lazy values are computed. A lazy collection’s externally visible state is pure, as long as the mapping function (for map) or predicate (for filter and reduce) is pure. Of course, the values in the collection need to be pure as well.
138. q: What is a race condition?
139. a: A race condition occurs when two different threads read and mutate the same memory, at the same time.
142. q: Are race conditions the only risk?
143. a: Heck no! There are deadlocks where two or more threads just stop. Or … sometimes different threads see different values in the same mutable field. Or things can just get slow (because of locks).
145. q: Are lazy collections thread safe?
146. a: Yes. That internal mutable state is properly protected by locks. All threads will see a consistent view of the lazy collection.
147. back to higher order functions
148. Some Coders (def coders [{:name "Rich" :language :clojure} {:name "Guido" :language :python} {:name "Bjarne" :language :c++} {:name "Charles" :language :forth} {:name "Brendan" :language :javascript}])
149. q: How can I get the names of all the coders?
150. a: (map :name coders) will evaluate to:
 ("Rich" "Guido" "Bjarne" "Charles" "Brendan") A keyword can act as a function for accessing a value from a map. Remember that map is a data structure, and map is a function for transforming collections.
151. q: How can I get the set of first letters of the coders’ names?
152. a: The output of one mapping operation can be the input to the next. (->> coders (map :name) (map #(subs % 0 1)) set) ➠ #{"G" "R" "C" "B"} subs extracts a substring from a string. set converts a collection to a set. Notice how "B" is only present once.
153. q: What does this symbolize? f
154. a: A reducing operation. f The reducing function accepts an accumulator and a collection value, and returns a new accumulator. It reduces a collection to a single accumulated value.
155. q: What does (reduce + [1 20 300 4000]) evaluate to?
156. a: It evaluates to 4321 as follows: (+ 1 20) ➠ 21 (+ 21 300) ➠ 321 (+ 321 4000) ➠ 4321 The first value in the collection, 1, is treated as the initial accumulator.
157. q: What is reduce useful for?
158. a: Lots of things, like deriving meaning from data. Example: finding the frequencies of specific values.
159. (reduce (fn [m v] (update m v (fnil inc 0))) {} [1 3 5 10 5 3]) ➠{1 1, 3 2, 5 2, 10 1}
160. 1. Start with an empty map 2. For each collection value: 1. Update the map, using the value as a key 2. Increment the map value (the count) 3. When the map value is nil, use 0 as default (reduce (fn [m v] (update m v (fnil inc 0))) {} [1 3 5 10 5 3])
161. (require '[clojure.string :as str]) (def text "Any programming language can be divided into two parts: some set of fundamental operators that play the role of axioms, and the rest of the language, which could in principle be written in terms of these fundamental operators.\n") (def sep #"[\p{Space}|\p{Punct}]+") (->> (str/split text sep) (map count) frequencies) ➠ {7 2, 4 6, 6 1, 3 8, 2 8, 11 3, 9 3, 5 5, 8 2}