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author Robert McIntyre <rlm@mit.edu>
date Sun, 16 Oct 2011 07:32:21 -0700
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1 #+title: Calling it quits without parentheses
2 #+author: Robert McIntyre & Dylan Holmes
3 #+EMAIL: rlm@mit.edu
4 #+MATHJAX: align:"left" mathml:t path:"../MathJax/MathJax.js"
5 #+STYLE: <link rel="stylesheet" type="text/css" href="../css/aurellem.css" />
6 #+OPTIONS: H:3 num:t toc:t \n:nil @:t ::t |:t ^:t -:t f:t *:t <:t
7 #+SETUPFILE: ../templates/level-0.org
8 #+INCLUDE: ../templates/level-0.org
9
10 [TABLE-OF-CONTENTS]
11
12 # * Calling it quits without parentheses
13 Is it possible to make a =quit= command for Clojure such that simply
14 typing the word =quit= at an REPL exits the REPL? An intuitive =quit=
15 command would be especially useful for people new to Lisp syntax or
16 the REPL; the challenge is to make a genuine Clojure command that
17 executes without being called with parentheses. We found three
18 solutions.
19
20 #+srcname: header
21 #+begin_src clojure :results silent
22 (ns abomination.no-parens
23 (:use clojure.contrib.def))
24 #+end_src
25
26 * By modifying =toString=
27 #+srcname: toString
28 #+begin_src clojure
29 (in-ns 'abomination.no-parens)
30 (gen-class :name abomination.no-parens.Quit
31 :prefix quit-)
32
33 (defn quit-toString
34 [this]
35 (System/exit 0))
36
37 (defvar quit (abomination.no-parens.Quit.)
38 "a sneaky way to support a `quit` command")
39 #+end_src
40
41 When you type any variable at the REPL, the REPL attempts to print it as a nicely-formatted string by calling its =toString=
42 method. Our trick is to define a class with a =toString= method
43 that exits the REPL; this trick ensures that any variable of that class will close
44 the REPL when the REPL attempts to print it.
45
46 First, we use =gen-class= to make a new class named Quit; in that same line, we use
47 =:prefix= to establish the convention that any function named
48 =quit-[something]= will be adopted as the =[something]= method for the
49 newly-defined Quit class. We use this convention to write our own
50 =toString= method for Quit.
51
52 Next, we define a suitable =toString= method for the Quit class so that
53 attempting to print an instance of the Quit class has the effect of
54 closing the REPL. We do this by defining a function =quit-toString=
55 which closes the REPL; by the convention established above, the Quit
56 class automatically adopts =quit-toString= as its =toString= method.
57
58 Finally, we use =defvar= to create an instance of the Quit class; we
59 name this instance =quit=. Now when you type =quit= into the REPL, the
60 REPL executes the =toString= method of the Quit class, exiting the REPL instead of returning a
61 string.
62
63 #+begin_src clojure :exports both
64 (binding [*compile-path* "/home/r/aurellem/classes"]
65 (compile 'abomination.no-parens))
66 #+end_src
67
68 #+results:
69 : abomination.no-parens
70
71 * By wrapping the command in a lazy sequence
72 #+srcname: lazy-seq
73 #+begin_src clojure :results silent
74 (in-ns 'abomination.no-parens)
75 (defvar quit* (lazy-seq :the-great-bringer-of-death! (System/exit 0))
76 "the first time it's evaulated it will kill the JVM")
77 #+end_src
78
79 * By =delay=-ing the command
80 #+srcname: delay
81 #+begin_src clojure :results silent
82 (in-ns 'abomination.no-parens)
83 (defvar quit** (delay (System/exit 0))
84 "when this is evaulated at the REPL, it will exit the JVM.")
85 #+end_src
86
87 The same thing, accomplished in a much more elegant and clojureish
88 way.
89
90 #+begin_src clojure :tangle no_parens.clj :results silent :exports none :noweb yes
91 <<header>>
92 <<toString>>
93 <<lazy-seq>>
94 <<delay>>
95 #+end_src
96
97 # STUD CRUFT PIZZA
98
99 #+begin_quote
100 And death i think is no parenthesis
101 \mdash{}E. E. Cummings
102 #+end_quote