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Add notes and exercises from Day 1 of Clojure
author IBBoard <dev@ibboard.co.uk>
date Sat, 01 Jun 2019 20:10:59 +0100
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1 Install Clojure with "curl -O https://download.clojure.org/install/linux-install-1.10.0.442.sh; chmod +x linux-install-1.10.0.442.sh; sudo ./linux-install-1.10.0.442.sh" (it's a downloader that downloads the real package)
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2
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3 Clojure has a repl (clj), which needs rlwrap ("sudo zypper in rlwrap"). This then downloads some MORE files from Maven. Don't you just LOVE the Java ecosystem?!
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4
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5 Clojure is a Lisp dialect. It feels a little like Prolog in that it's logic and brackets galore, but Prolog is about predicates and pattern matching whereas Lisp has more of an imperative language with odd syntax
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6 but sensible things like "if" statements. It uses strong, dynamic typing.
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7
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8 Running commands at the repl and prints output *and* the return value (which can be "nil").
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9
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10 Commands are prefix, not infix:
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11 "Normal" languages: 1 + 1
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12 Clojure: (+ 1 1)
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14 Division of integers creates a (normalised) "ratio" type to maintain accuracy:
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15 (/ 2 4) → 1/2
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16 (/ 2.0 4) → 0.5
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17
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18 Prefix does have its advantages, though - it's more like a function and can have variable numbers of parameters:
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19 (+ 2 2 2 2) → 8
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20
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21 And it can be used to check order:
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22 (< 1 2 3) → true
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23 (< 1 3 2 4) → false
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24
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25 Strings are double-quoted, but characters can be escaped with a backslash outside quotes to become a string. When done with "str" it makes a string:
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26 (str \h \e \l \l \o) → "hello"
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27 This seems like it would rarely be useful.
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28
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29 Strings and characters are different (just like in Java)
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30 (= "a" \a) → false
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31
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32 Clojure can pass code as arguments, e.g.:
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33 (if true (println "It's true!"))
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34 but that's quite normal for functional languages.
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35
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36 The "if" function is basically "if <condition> <true>[ <false>]".
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37
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38 Note: 0 and "" are true but nil is false, so only false and the null value are falsey.
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39
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40 Clojure has four main data structures - lists (for code), vectors (for data), sets and maps.
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41
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42 You can't build a list Python style with "(1 2 3)". You need either "(list 1 2 3)" or the odd syntax "'(1 2 3)".
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43 Lists can then be operated on with "first", "last", "rest" and "cons" (construct from head and tail) functions.
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44 There are also functions like "nth" (nth <list> <0-based index>)
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45
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46 Vectors use sequare brackets. They're like a list, but you can implicitly index them (i.e. you don't need "nth") by doing ([vector] pos) because they're treated as functions.
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47
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48 first, last and rest can be used for pattern matching.
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49
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50 Sets use hash followed by squiggly brackets (which is easily confused with a map). Sorted sets use "(sorted-set <values> …)".
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51 Manipulating sets is less clean - (clojure.set/union <set1> <set2>) for unioning sets and (clojure.set/difference <set1> <set2>) for difference.
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52 Sets are also functions and so you can check if an item is in a set with (#{set} <value>). Returns "nil" if not in the set, else returns the value.
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53
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54 Maps are defined with squiggly brackets and have implicit key-value positions, so you can write"{:chewie :wookie :lea :human}" for keys :chewie and :lea with values :wookie and :human.
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55 You can also put in commas, though.
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56
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57
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58 ":text" appears to be a unique identifier ("keyword"), but the book doesn't mention this until three pages after it started using it. They're like Ruby symbols and Prolog/Erland atoms.
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59 Symbols point to something else and keywords point to themselves (although it isn't made clear what that means).
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60 Keywords are also functions, so (map key) and (key map) are equivalent.
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61
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62 Maps can be merged with "merge" or with "merge-with", which takes a function to calculate the merged value.
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63
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64 Variables are assigned to with "(def <variable-name> <value>)". Functions are defined with "(defn <func-name> [<params list>] (<body>))".
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65 There's also an optional docstring after <func-name>. Docs can be read with "(doc <func-name>)".
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66
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67 Function arguments can be quite complex and use pattern matching, e.g. getting the second value in a vector with:
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68 (defn line-end [[_ second]] second)
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69 which takes a single arg (the vector "[_ second]") and the body just returns "second". Clojure calls this "destructuring". It ignores any additional values (e.g. "[_ c]" matches "[1 2 3 4 5]" and puts "2" in c and ignores the trailing 3,4,5).
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70
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71 Where "def" is used for long-term variables (which are created in user-space - the return result is #'user/<def-name>), "let" is used for local variables, which is useful within function definitions, e.g.:
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72 (defn centre_space [board] (let [[_ [_ c]] board] c))
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73 which is a function with one argument (board) where the function body uses "let" with a vector containing a pattern to match and a variable to match against, and then a function that uses that vector.
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74
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75 There are also anonymous functions that use "(fn [args] (body))" - like a defn, but without a name. Alternatively, "#(function-body-with-%-as-a-variable)" defines an anonymous "reader macro" function with % bound to each value.
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76 (map (fn [w] (* 2 (count w))) my_list)
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77 is equivalent to:
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78 (map #(* 2 (count %)) my_list)
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79
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80 As well as the map function, there's "apply" and "filter".
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81
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82 Also, some helper functions exist, like "odd?".