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1 #+title: Calling it quits without parentheses
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2 #+author: Robert McIntyre & Dylan Holmes
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3 #+EMAIL: rlm@mit.edu
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4 #+MATHJAX: align:"left" mathml:t path:"../MathJax/MathJax.js"
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5 #+STYLE: <link rel="stylesheet" type="text/css" href="../css/aurellem.css" />
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6 #+OPTIONS: H:3 num:t toc:t \n:nil @:t ::t |:t ^:t -:t f:t *:t <:t
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7 #+SETUPFILE: ../templates/level-0.org
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8 #+INCLUDE: ../templates/level-0.org
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9
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10 [TABLE-OF-CONTENTS]
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11
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12 # * Calling it quits without parentheses
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13 Is it possible to make a =quit= command for Clojure such that simply
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14 typing the word =quit= at an REPL exits the REPL? An intuitive =quit=
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15 command would be especially useful for people new to Lisp syntax or
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16 the REPL; the challenge is to make a genuine Clojure command that
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17 executes without being called with parentheses. We found three
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18 solutions.
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19
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20 #+srcname: header
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21 #+begin_src clojure :results silent
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22 (ns abomination.no-parens
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23 (:use clojure.contrib.def))
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24 #+end_src
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25
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26 * By modifying =toString=
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27 #+srcname: toString
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28 #+begin_src clojure
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29 (in-ns 'abomination.no-parens)
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30 (gen-class :name abomination.no-parens.Quit
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31 :prefix quit-)
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32
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33 (defn quit-toString
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34 [this]
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35 (System/exit 0))
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36
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37 (defvar quit (abomination.no-parens.Quit.)
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38 "a sneaky way to support a `quit` command")
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39 #+end_src
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40
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41 When you type any variable at the REPL, the REPL attempts to print it as a nicely-formatted string by calling its =toString=
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42 method. Our trick is to define a class with a =toString= method
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43 that exits the REPL; this trick ensures that any variable of that class will close
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44 the REPL when the REPL attempts to print it.
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45
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46 First, we use =gen-class= to make a new class named Quit; in that same line, we use
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47 =:prefix= to establish the convention that any function named
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48 =quit-[something]= will be adopted as the =[something]= method for the
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49 newly-defined Quit class. We use this convention to write our own
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50 =toString= method for Quit.
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51
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52 Next, we define a suitable =toString= method for the Quit class so that
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53 attempting to print an instance of the Quit class has the effect of
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54 closing the REPL. We do this by defining a function =quit-toString=
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55 which closes the REPL; by the convention established above, the Quit
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56 class automatically adopts =quit-toString= as its =toString= method.
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57
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58 Finally, we use =defvar= to create an instance of the Quit class; we
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59 name this instance =quit=. Now when you type =quit= into the REPL, the
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60 REPL executes the =toString= method of the Quit class, exiting the REPL instead of returning a
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61 string.
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62
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63 #+begin_src clojure :exports both
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64 (binding [*compile-path* "/home/r/aurellem/classes"]
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65 (compile 'abomination.no-parens))
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66 #+end_src
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67
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68 #+results:
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69 : abomination.no-parens
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70
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71 * By wrapping the command in a lazy sequence
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72 #+srcname: lazy-seq
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73 #+begin_src clojure :results silent
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74 (in-ns 'abomination.no-parens)
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75 (defvar quit* (lazy-seq :the-great-bringer-of-death! (System/exit 0))
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76 "the first time it's evaulated it will kill the JVM")
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77 #+end_src
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78
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79 * By =delay=-ing the command
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80 #+srcname: delay
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81 #+begin_src clojure :results silent
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82 (in-ns 'abomination.no-parens)
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83 (defvar quit** (delay (System/exit 0))
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84 "when this is evaulated at the REPL, it will exit the JVM.")
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85 #+end_src
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86
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87 The same thing, accomplished in a much more elegant and clojureish
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88 way.
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89
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90 #+begin_src clojure :tangle no_parens.clj :results silent :exports none :noweb yes
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91 <<header>>
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92 <<toString>>
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93 <<lazy-seq>>
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94 <<delay>>
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95 #+end_src
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96
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97 # STUD CRUFT PIZZA
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98
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99 #+begin_quote
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100 And death i think is no parenthesis
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101 \mdash{}E. E. Cummings
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102 #+end_quote
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