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author Robert McIntyre <rlm@mit.edu>
date Fri, 28 Oct 2011 00:06:37 -0700
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rlm@2 7 <title>Categorical Programs</title>
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rlm@2 123 <div id="content">
rlm@2 124
rlm@2 125
rlm@2 126
rlm@2 127
rlm@2 128
rlm@2 129 <h1>Categorical Programs</h1>
rlm@2 130
rlm@2 131
rlm@2 132 <div id="table-of-contents">
rlm@2 133 <h2>Table of Contents</h2>
rlm@2 134 <div id="text-table-of-contents">
rlm@2 135 <ul>
rlm@2 136 <li><a href="#sec-1">1 The Category of Types </a>
rlm@2 137 <ul>
rlm@2 138 <li><a href="#sec-1-1">1.1 Building new data types out of existing ones </a></li>
rlm@2 139 </ul>
rlm@2 140 </li>
rlm@2 141 </ul>
rlm@2 142 </div>
rlm@2 143 </div>
rlm@2 144
rlm@2 145 <div id="outline-container-1" class="outline-2">
rlm@2 146 <h2 id="sec-1"><span class="section-number-2">1</span> The Category of Types </h2>
rlm@2 147 <div class="outline-text-2" id="text-1">
rlm@2 148
rlm@2 149 <p>Every computer language with data types (such as Integers,
rlm@2 150 Floats, or
rlm@2 151 Strings) corresponds with a certain category; the objects of the category are the data types
rlm@2 152 of the language, and there is one arrow between data types \(A\) and \(B\)
rlm@2 153 for every function in the language whose argument has type \(A\) and whose return value
rlm@2 154 has type \(B\).
rlm@2 155 </p>
rlm@2 156
rlm@2 157
rlm@2 158
rlm@2 159 <pre class="src src-clojure">(<span style="color: #f0dfaf; font-weight: bold;">ns</span> categorical.morphisms)
rlm@2 160
rlm@2 161 </pre>
rlm@2 162
rlm@2 163
rlm@2 164
rlm@2 165
rlm@2 166
rlm@2 167 </div>
rlm@2 168
rlm@2 169 <div id="outline-container-1-1" class="outline-3">
rlm@2 170 <h3 id="sec-1-1"><span class="section-number-3">1.1</span> Building new data types out of existing ones </h3>
rlm@2 171 <div class="outline-text-3" id="text-1-1">
rlm@2 172
rlm@2 173
rlm@2 174 <p>
rlm@2 175 We will now discuss two processes which combine existing
rlm@2 176 data types to yield composite data types. These higher-order processes will be
rlm@2 177 <i>functorial</i>, which means that they will recombine not only the objects of
rlm@2 178 our category (the data types), but the arrows as well (functions with
rlm@2 179 typed input and output).
rlm@2 180 </p>
rlm@2 181 <p>
rlm@2 182 The <code>product</code> functor combines two data types, \(A\) and \(B\), producing
rlm@2 183 a new type, denoted \(A\times B\). Conceptually, values with the data type \(A\times
rlm@2 184 B\) have two components, the first of which is a value of type \(A\), the
rlm@2 185 second of which is a value of type \(B\). Concretely, the data type \(A\times B\)
rlm@2 186 can be implemented as pairs of values, the first of which is
rlm@2 187 of type \(A\) and the second of which is of type \(B\). With this
rlm@2 188 implementation in mind, you can see that the type \(A\) and type \(B\)
rlm@2 189 constituents of type \(A\times B\) value can be recovered; the so-called
rlm@2 190 projection functors <code>first</code> and <code>second</code> recover them. A third
rlm@2 191 functor, the <code>diagonal</code> functor, completes our toolkit; given
rlm@2 192 value \(a\) of type \(A\), it produces the value \((a,a)\) of type \(A\times A\).
rlm@2 193 </p>
rlm@2 194
rlm@2 195
rlm@2 196
rlm@2 197 <pre class="src src-clojure">(<span style="color: #f0dfaf; font-weight: bold;">defn</span> <span style="color: #f0dfaf;">product</span>[a b] (<span style="color: #8cd0d3;">list</span> a b))
rlm@2 198 (<span style="color: #f0dfaf; font-weight: bold;">def</span> <span style="color: #f0dfaf;">projections</span> (<span style="color: #8cd0d3;">list</span> first second))
rlm@2 199 (<span style="color: #f0dfaf; font-weight: bold;">defn</span> <span style="color: #f0dfaf;">diagonal</span>[a] (product a a))
rlm@2 200
rlm@2 201 </pre>
rlm@2 202
rlm@2 203
rlm@2 204
rlm@2 205
rlm@2 206 <p>
rlm@2 207 The <code>coproduct</code> functor combines two data types, \(A\) and \(B\),
rlm@2 208 producing a new type, denoted \(A+B\). Conceptually, you construct the
rlm@2 209 \(A+B\) datatype by creating labelled versions of the data types \(A\) and
rlm@2 210 \(B\) &mdash; we'll call them black-\(A\) and white-\(B\) &mdash; and
rlm@2 211 then creating a data type \(A+B\) which contains all the black-\(A\)
rlm@2 212 values and all the white-\(B\) values. Concretely, you can implement the
rlm@2 213 \(A+B\) data type using label-value pairs: the first value in the pair
rlm@2 214 is either the label `black' or the label `white'; the second
rlm@2 215 value must have type \(A\) if the label is black, or type \(B\) if the
rlm@2 216 label is white.
rlm@2 217 </p>
rlm@2 218 <p>
rlm@2 219 Now, the <code>product</code> functor came with projection functors <code>first</code>
rlm@2 220 and <code>second</code> that take \(A\times B\) values and return (respectively) \(A\) values and
rlm@2 221 \(B\) values. The <code>coproduct</code> functor, conversely, comes with two
rlm@2 222 injection functors <code>make-black</code> and <code>make-white</code> that perform the
rlm@2 223 opposite function: they take (respectively) \(A\) values and \(B\) values,
rlm@2 224 promoting them to the \(A+B\) datatype by making the \(A\)'s into
rlm@2 225 black-\(A\)'s and the \(B\)'s into white-\(B\)'s.
rlm@2 226 </p>
rlm@2 227
rlm@2 228
rlm@2 229 </div>
rlm@2 230 </div>
rlm@2 231 </div>
rlm@2 232 <div id="postamble">
rlm@2 233 <p class="date">Date: 2011-07-01 12:28:39 CDT</p>
rlm@2 234 <p class="author">Author: Dylan Holmes</p>
rlm@2 235 <p class="creator">Org version 7.5 with Emacs version 23</p>
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