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22 | <a href="./Home">MLton 20180207</a>\r | |
23 | </div>\r | |
24 | </div>\r | |
25 | <div id="header">\r | |
26 | <h1>Fixpoints</h1>\r | |
27 | </div>\r | |
28 | <div id="content">\r | |
29 | <div id="preamble">\r | |
30 | <div class="sectionbody">\r | |
31 | <div class="paragraph"><p>This page discusses a framework that makes it possible to compute\r | |
32 | fixpoints over arbitrary products of abstract types. The code is from\r | |
33 | an Extended Basis library\r | |
34 | (<a href="https://github.com/MLton/mltonlib/blob/master/com/ssh/extended-basis/unstable/README"><span class="monospaced">README</span></a>).</p></div>\r | |
35 | <div class="paragraph"><p>First the signature of the framework\r | |
36 | (<a href="https://github.com/MLton/mltonlib/blob/master/com/ssh/extended-basis/unstable/public/generic/tie.sig"><span class="monospaced">tie.sig</span></a>):</p></div>\r | |
37 | <div class="listingblock">\r | |
38 | <div class="content"><div class="highlight"><pre><span class="cm">(**</span>\r | |
39 | <span class="cm"> * A framework for computing fixpoints.</span>\r | |
40 | <span class="cm"> *</span>\r | |
41 | <span class="cm"> * In a strict language you sometimes want to provide a fixpoint</span>\r | |
42 | <span class="cm"> * combinator for an abstract type {t} to make it possible to write</span>\r | |
43 | <span class="cm"> * recursive definitions. Unfortunately, a single combinator {fix} of the</span>\r | |
44 | <span class="cm"> * type {(t -> t) -> t} does not support mutual recursion. To support</span>\r | |
45 | <span class="cm"> * mutual recursion, you would need to provide a family of fixpoint</span>\r | |
46 | <span class="cm"> * combinators having types of the form {(u -> u) -> u} where {u} is a</span>\r | |
47 | <span class="cm"> * type of the form {t * ... * t}. Unfortunately, even such a family of</span>\r | |
48 | <span class="cm"> * fixpoint combinators does not support mutual recursion over different</span>\r | |
49 | <span class="cm"> * abstract types.</span>\r | |
50 | <span class="cm"> *)</span><span class="w"></span>\r | |
51 | <span class="k">signature</span><span class="w"> </span><span class="n">TIE</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">sig</span><span class="w"></span>\r | |
52 | <span class="w"> </span><span class="k">include</span><span class="w"> </span><span class="n">ETAEXP'</span><span class="w"></span>\r | |
53 | <span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">etaexp</span><span class="w"></span>\r | |
54 | <span class="w"> </span><span class="cm">(** The type of fixpoint witnesses. *)</span><span class="w"></span>\r | |
55 | \r | |
56 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">fix</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">Fix</span><span class="p">.</span><span class="n">t</span><span class="w"></span>\r | |
57 | <span class="w"> </span><span class="cm">(**</span>\r | |
58 | <span class="cm"> * Produces a fixpoint combinator from the given witness. For example,</span>\r | |
59 | <span class="cm"> * one can make a mutually recursive definition of functions:</span>\r | |
60 | <span class="cm"> *</span>\r | |
61 | <span class="cm"> *> val isEven & isOdd =</span>\r | |
62 | <span class="cm"> *> let open Tie in fix (function *` function) end</span>\r | |
63 | <span class="cm"> *> (fn isEven & isOdd =></span>\r | |
64 | <span class="cm"> *> (fn 0 => true</span>\r | |
65 | <span class="cm"> *> | 1 => false</span>\r | |
66 | <span class="cm"> *> | n => isOdd (n-1)) &</span>\r | |
67 | <span class="cm"> *> (fn 0 => false</span>\r | |
68 | <span class="cm"> *> | 1 => true</span>\r | |
69 | <span class="cm"> *> | n => isEven (n-1)))</span>\r | |
70 | <span class="cm"> *)</span><span class="w"></span>\r | |
71 | \r | |
72 | <span class="w"> </span><span class="cm">(** == Making New Witnesses == *)</span><span class="w"></span>\r | |
73 | \r | |
74 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">pure</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="p">(</span><span class="n">'a</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">UnOp</span><span class="p">.</span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="n">Thunk</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
75 | <span class="w"> </span><span class="cm">(**</span>\r | |
76 | <span class="cm"> * {pure} is a more general version of {tier}. It is mostly useful for</span>\r | |
77 | <span class="cm"> * computing fixpoints in a non-imperative manner.</span>\r | |
78 | <span class="cm"> *)</span><span class="w"></span>\r | |
79 | \r | |
80 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">tier</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="p">(</span><span class="n">'a</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">Effect</span><span class="p">.</span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="n">Thunk</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
81 | <span class="w"> </span><span class="cm">(**</span>\r | |
82 | <span class="cm"> * {tier} is used to define fixpoint witnesses for new abstract types</span>\r | |
83 | <span class="cm"> * by providing a thunk whose instantiation allocates a mutable proxy</span>\r | |
84 | <span class="cm"> * and a procedure for updating it with the result.</span>\r | |
85 | <span class="cm"> *)</span><span class="w"></span>\r | |
86 | \r | |
87 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">id</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
88 | <span class="w"> </span><span class="cm">(** {id x} is equivalent to {pure (const (x, id))}. *)</span><span class="w"></span>\r | |
89 | \r | |
90 | <span class="w"> </span><span class="cm">(** == Combining Existing Witnesses == *)</span><span class="w"></span>\r | |
91 | \r | |
92 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">iso</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">'b</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="p">(</span><span class="n">'a</span><span class="p">,</span><span class="w"> </span><span class="n">'b</span><span class="p">)</span><span class="w"> </span><span class="n">Iso</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
93 | <span class="w"> </span><span class="cm">(**</span>\r | |
94 | <span class="cm"> * Given an isomorphism between {'a} and {'b} and a witness for {'b},</span>\r | |
95 | <span class="cm"> * produces a witness for {'a}. This is useful when you have a new</span>\r | |
96 | <span class="cm"> * type that is isomorphic to some old type for which you already have</span>\r | |
97 | <span class="cm"> * a witness.</span>\r | |
98 | <span class="cm"> *)</span><span class="w"></span>\r | |
99 | \r | |
100 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">product</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="p">(</span><span class="n">'a</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'b</span><span class="w"> </span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="p">(</span><span class="n">'a</span><span class="p">,</span><span class="w"> </span><span class="n">'b</span><span class="p">)</span><span class="w"> </span><span class="n">Product</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
101 | <span class="w"> </span><span class="cm">(**</span>\r | |
102 | <span class="cm"> * Dependent product combinator. Given a witness for {'a} and a</span>\r | |
103 | <span class="cm"> * constructor from a {'a} to witness for {'b}, produces a witness for</span>\r | |
104 | <span class="cm"> * the product {('a, 'b) Product.t}. The constructor for {'b} should</span>\r | |
105 | <span class="cm"> * not access the (proxy) value {'a} before it has been fixed.</span>\r | |
106 | <span class="cm"> *)</span><span class="w"></span>\r | |
107 | \r | |
108 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">*`</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">'b</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="p">(</span><span class="n">'a</span><span class="p">,</span><span class="w"> </span><span class="n">'b</span><span class="p">)</span><span class="w"> </span><span class="n">Product</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
109 | <span class="w"> </span><span class="cm">(** {a *` b} is equivalent to {product (a, const b)}. *)</span><span class="w"></span>\r | |
110 | \r | |
111 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">tuple2</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">'b</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="p">(</span><span class="n">'a</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">'b</span><span class="p">)</span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
112 | <span class="w"> </span><span class="cm">(**</span>\r | |
113 | <span class="cm"> * Given witnesses for {'a} and {'b} produces a witness for the product</span>\r | |
114 | <span class="cm"> * {'a * 'b}.</span>\r | |
115 | <span class="cm"> *)</span><span class="w"></span>\r | |
116 | \r | |
117 | <span class="w"> </span><span class="cm">(** == Particular Witnesses == *)</span><span class="w"></span>\r | |
118 | \r | |
119 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">function</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="p">(</span><span class="n">'a</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'b</span><span class="p">)</span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
120 | <span class="w"> </span><span class="cm">(** Witness for functions. *)</span><span class="w"></span>\r | |
121 | <span class="k">end</span><span class="w"></span>\r | |
122 | </pre></div></div></div>\r | |
123 | <div class="paragraph"><p><span class="monospaced">fix</span> is a <a href="TypeIndexedValues">type-indexed</a> function. The type-index\r | |
124 | parameter to <span class="monospaced">fix</span> is called a "witness". To compute fixpoints over\r | |
125 | products, one uses the <span class="monospaced">*`</span> operator to combine witnesses. To provide\r | |
126 | a fixpoint combinator for an abstract type, one implements a witness\r | |
127 | providing a thunk whose instantiation allocates a fresh, mutable proxy\r | |
128 | and a procedure for updating the proxy with the solution. Naturally\r | |
129 | this means that not all possible ways of computing a fixpoint of a\r | |
130 | particular type are possible under the framework. The <span class="monospaced">pure</span>\r | |
131 | combinator is a generalization of <span class="monospaced">tier</span>. The <span class="monospaced">iso</span> combinator is\r | |
132 | provided for reusing existing witnesses.</p></div>\r | |
133 | <div class="paragraph"><p>Note that instead of using an infix operator, we could alternatively\r | |
134 | employ an interface using <a href="Fold">Fold</a>. Also, witnesses are eta-expanded\r | |
135 | to work around the <a href="ValueRestriction">value restriction</a>, while\r | |
136 | maintaining abstraction.</p></div>\r | |
137 | <div class="paragraph"><p>Here is the implementation\r | |
138 | (<a href="https://github.com/MLton/mltonlib/blob/master/com/ssh/extended-basis/unstable/detail/generic/tie.sml"><span class="monospaced">tie.sml</span></a>):</p></div>\r | |
139 | <div class="listingblock">\r | |
140 | <div class="content"><div class="highlight"><pre><span class="k">structure</span><span class="w"> </span><span class="n">Tie</span><span class="w"> </span><span class="p">:></span><span class="w"> </span><span class="n">TIE</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
141 | <span class="w"> </span><span class="k">open</span><span class="w"> </span><span class="n">Product</span><span class="w"></span>\r | |
142 | <span class="w"> </span><span class="k">infix</span><span class="w"> </span><span class="n">&</span><span class="w"></span>\r | |
143 | <span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">etaexp_dom</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Unit</span><span class="p">.</span><span class="n">t</span><span class="w"></span>\r | |
144 | <span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">etaexp_cod</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="p">(</span><span class="n">'a</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">UnOp</span><span class="p">.</span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="n">Thunk</span><span class="p">.</span><span class="n">t</span><span class="w"></span>\r | |
145 | <span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">etaexp</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">etaexp_dom</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">etaexp_cod</span><span class="w"></span>\r | |
146 | <span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">etaexp</span><span class="w"></span>\r | |
147 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">fix</span><span class="w"> </span><span class="n">aT</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">let</span><span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="p">(</span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="n">ta</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">aT</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="k">in</span><span class="w"> </span><span class="n">ta</span><span class="w"> </span><span class="p">(</span><span class="n">f</span><span class="w"> </span><span class="n">a</span><span class="p">)</span><span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
148 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">pure</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Thunk</span><span class="p">.</span><span class="n">mk</span><span class="w"></span>\r | |
149 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">iso</span><span class="w"> </span><span class="n">bT</span><span class="w"> </span><span class="p">(</span><span class="n">iso</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="p">(_,</span><span class="w"> </span><span class="n">b2a</span><span class="p">))</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">let</span><span class="w"></span>\r | |
150 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="p">(</span><span class="n">b</span><span class="p">,</span><span class="w"> </span><span class="n">fB</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">bT</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">()</span><span class="w"></span>\r | |
151 | <span class="w"> </span><span class="k">in</span><span class="w"></span>\r | |
152 | <span class="w"> </span><span class="p">(</span><span class="n">b2a</span><span class="w"> </span><span class="n">b</span><span class="p">,</span><span class="w"> </span><span class="n">Fn</span><span class="p">.</span><span class="n">map</span><span class="w"> </span><span class="n">iso</span><span class="w"> </span><span class="n">fB</span><span class="p">)</span><span class="w"></span>\r | |
153 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
154 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">product</span><span class="w"> </span><span class="p">(</span><span class="n">aT</span><span class="p">,</span><span class="w"> </span><span class="n">a2bT</span><span class="p">)</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">let</span><span class="w"></span>\r | |
155 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="p">(</span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="n">fA</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">aT</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">()</span><span class="w"></span>\r | |
156 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="p">(</span><span class="n">b</span><span class="p">,</span><span class="w"> </span><span class="n">fB</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">a2bT</span><span class="w"> </span><span class="n">a</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">()</span><span class="w"></span>\r | |
157 | <span class="w"> </span><span class="k">in</span><span class="w"></span>\r | |
158 | <span class="w"> </span><span class="p">(</span><span class="n">a</span><span class="w"> </span><span class="n">&</span><span class="w"> </span><span class="n">b</span><span class="p">,</span><span class="w"> </span><span class="n">Product</span><span class="p">.</span><span class="n">map</span><span class="w"> </span><span class="p">(</span><span class="n">fA</span><span class="p">,</span><span class="w"> </span><span class="n">fB</span><span class="p">))</span><span class="w"></span>\r | |
159 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
160 | <span class="w"> </span><span class="cm">(* The rest are not primitive operations. *)</span><span class="w"></span>\r | |
161 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="k">op</span><span class="w"> </span><span class="n">*`</span><span class="w"> </span><span class="p">(</span><span class="n">aT</span><span class="p">,</span><span class="w"> </span><span class="n">bT</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">product</span><span class="w"> </span><span class="p">(</span><span class="n">aT</span><span class="p">,</span><span class="w"> </span><span class="n">Fn</span><span class="p">.</span><span class="n">const</span><span class="w"> </span><span class="n">bT</span><span class="p">)</span><span class="w"></span>\r | |
162 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">tuple2</span><span class="w"> </span><span class="n">ab</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">iso</span><span class="w"> </span><span class="p">(</span><span class="k">op</span><span class="w"> </span><span class="n">*`</span><span class="w"> </span><span class="n">ab</span><span class="p">)</span><span class="w"> </span><span class="n">Product</span><span class="p">.</span><span class="n">isoTuple2</span><span class="w"></span>\r | |
163 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">tier</span><span class="w"> </span><span class="n">th</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">pure</span><span class="w"> </span><span class="p">((</span><span class="k">fn</span><span class="w"> </span><span class="p">(</span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="n">ua</span><span class="p">)</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="p">(</span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="n">Fn</span><span class="p">.</span><span class="n">const</span><span class="w"> </span><span class="n">a</span><span class="w"> </span><span class="n">o</span><span class="w"> </span><span class="n">ua</span><span class="p">))</span><span class="w"> </span><span class="n">o</span><span class="w"> </span><span class="n">th</span><span class="p">)</span><span class="w"></span>\r | |
164 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">id</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">pure</span><span class="w"> </span><span class="p">(</span><span class="n">Fn</span><span class="p">.</span><span class="n">const</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="n">Fn</span><span class="p">.</span><span class="n">id</span><span class="p">))</span><span class="w"></span>\r | |
165 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">function</span><span class="w"> </span><span class="n">?</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
166 | <span class="w"> </span><span class="n">pure</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="k">let</span><span class="w"></span>\r | |
167 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">r</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">ref</span><span class="w"> </span><span class="p">(</span><span class="n">Basic</span><span class="p">.</span><span class="n">raising</span><span class="w"> </span><span class="n">Fix</span><span class="p">.</span><span class="n">Fix</span><span class="p">)</span><span class="w"></span>\r | |
168 | <span class="w"> </span><span class="k">in</span><span class="w"></span>\r | |
169 | <span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">!r</span><span class="w"> </span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="p">(</span><span class="n">r</span><span class="w"> </span><span class="n">:=</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">;</span><span class="w"> </span><span class="n">f</span><span class="p">))</span><span class="w"></span>\r | |
170 | <span class="w"> </span><span class="k">end</span><span class="p">)</span><span class="w"> </span><span class="n">?</span><span class="w"></span>\r | |
171 | <span class="k">end</span><span class="w"></span>\r | |
172 | </pre></div></div></div>\r | |
173 | <div class="paragraph"><p>Let’s then take a look at a couple of additional examples.</p></div>\r | |
174 | <div class="paragraph"><p>Here is a naive implementation of lazy promises:</p></div>\r | |
175 | <div class="listingblock">\r | |
176 | <div class="content"><div class="highlight"><pre><span class="k">structure</span><span class="w"> </span><span class="n">Promise</span><span class="w"> </span><span class="p">:></span><span class="w"> </span><span class="k">sig</span><span class="w"></span>\r | |
177 | <span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
178 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">lazy</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">Thunk</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
179 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">force</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"></span>\r | |
180 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">Y</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="n">Tie</span><span class="p">.</span><span class="n">t</span><span class="w"></span>\r | |
181 | <span class="k">end</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
182 | <span class="w"> </span><span class="k">datatype</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t'</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
183 | <span class="w"> </span><span class="n">EXN</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">exn</span><span class="w"></span>\r | |
184 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">THUNK</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">Thunk</span><span class="p">.</span><span class="n">t</span><span class="w"></span>\r | |
185 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">VALUE</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">'a</span><span class="w"></span>\r | |
186 | <span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t'</span><span class="w"> </span><span class="n">Ref</span><span class="p">.</span><span class="n">t</span><span class="w"></span>\r | |
187 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">lazy</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">ref</span><span class="w"> </span><span class="p">(</span><span class="n">THUNK</span><span class="w"> </span><span class="n">f</span><span class="p">)</span><span class="w"></span>\r | |
188 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">force</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
189 | <span class="w"> </span><span class="k">case</span><span class="w"> </span><span class="n">!t</span><span class="w"></span>\r | |
190 | <span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">EXN</span><span class="w"> </span><span class="n">e</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="k">raise</span><span class="w"> </span><span class="n">e</span><span class="w"></span>\r | |
191 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">THUNK</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="p">(</span><span class="n">t</span><span class="w"> </span><span class="n">:=</span><span class="w"> </span><span class="n">VALUE</span><span class="w"> </span><span class="p">(</span><span class="n">f</span><span class="w"> </span><span class="p">())</span><span class="w"> </span><span class="k">handle</span><span class="w"> </span><span class="n">e</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="n">:=</span><span class="w"> </span><span class="n">EXN</span><span class="w"> </span><span class="n">e</span><span class="w"> </span><span class="p">;</span><span class="w"> </span><span class="n">force</span><span class="w"> </span><span class="n">t</span><span class="p">)</span><span class="w"></span>\r | |
192 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">VALUE</span><span class="w"> </span><span class="n">v</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">v</span><span class="w"></span>\r | |
193 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">Y</span><span class="w"> </span><span class="n">?</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Tie</span><span class="p">.</span><span class="n">tier</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="k">let</span><span class="w"></span>\r | |
194 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">r</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">lazy</span><span class="w"> </span><span class="p">(</span><span class="n">raising</span><span class="w"> </span><span class="n">Fix</span><span class="p">.</span><span class="n">Fix</span><span class="p">)</span><span class="w"></span>\r | |
195 | <span class="w"> </span><span class="k">in</span><span class="w"></span>\r | |
196 | <span class="w"> </span><span class="p">(</span><span class="n">r</span><span class="p">,</span><span class="w"> </span><span class="n">r</span><span class="w"> </span><span class="n"><\</span><span class="w"> </span><span class="k">op</span><span class="w"> </span><span class="n">:=</span><span class="w"> </span><span class="n">o</span><span class="w"> </span><span class="n">!</span><span class="p">)</span><span class="w"></span>\r | |
197 | <span class="w"> </span><span class="k">end</span><span class="p">)</span><span class="w"> </span><span class="n">?</span><span class="w"></span>\r | |
198 | <span class="k">end</span><span class="w"></span>\r | |
199 | </pre></div></div></div>\r | |
200 | <div class="paragraph"><p>An example use of our naive lazy promises is to implement equally naive\r | |
201 | lazy streams:</p></div>\r | |
202 | <div class="listingblock">\r | |
203 | <div class="content"><div class="highlight"><pre><span class="k">structure</span><span class="w"> </span><span class="n">Stream</span><span class="w"> </span><span class="p">:></span><span class="w"> </span><span class="k">sig</span><span class="w"></span>\r | |
204 | <span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
205 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">cons</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
206 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">get</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="p">(</span><span class="n">'a</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="n">Option</span><span class="p">.</span><span class="n">t</span><span class="w"></span>\r | |
207 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">Y</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="n">Tie</span><span class="p">.</span><span class="n">t</span><span class="w"></span>\r | |
208 | <span class="k">end</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
209 | <span class="w"> </span><span class="k">datatype</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">IN</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="p">(</span><span class="n">'a</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="n">Option</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="n">Promise</span><span class="p">.</span><span class="n">t</span><span class="w"></span>\r | |
210 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">cons</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="n">xs</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">IN</span><span class="w"> </span><span class="p">(</span><span class="n">Promise</span><span class="p">.</span><span class="n">lazy</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">SOME</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="n">xs</span><span class="p">)))</span><span class="w"></span>\r | |
211 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">get</span><span class="w"> </span><span class="p">(</span><span class="n">IN</span><span class="w"> </span><span class="n">p</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Promise</span><span class="p">.</span><span class="n">force</span><span class="w"> </span><span class="n">p</span><span class="w"></span>\r | |
212 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">Y</span><span class="w"> </span><span class="n">?</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Tie</span><span class="p">.</span><span class="n">iso</span><span class="w"> </span><span class="n">Promise</span><span class="p">.</span><span class="n">Y</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">IN</span><span class="w"> </span><span class="n">p</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">p</span><span class="p">,</span><span class="w"> </span><span class="n">IN</span><span class="p">)</span><span class="w"> </span><span class="n">?</span><span class="w"></span>\r | |
213 | <span class="k">end</span><span class="w"></span>\r | |
214 | </pre></div></div></div>\r | |
215 | <div class="paragraph"><p>Note that above we make use of the <span class="monospaced">iso</span> combinator. Here is a finite\r | |
216 | representation of an infinite stream of ones:</p></div>\r | |
217 | <div class="listingblock">\r | |
218 | <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="n">ones</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">let</span><span class="w"></span>\r | |
219 | <span class="w"> </span><span class="k">open</span><span class="w"> </span><span class="n">Tie</span><span class="w"> </span><span class="n">Stream</span><span class="w"></span>\r | |
220 | <span class="k">in</span><span class="w"></span>\r | |
221 | <span class="w"> </span><span class="n">fix</span><span class="w"> </span><span class="n">Y</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">ones</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">cons</span><span class="w"> </span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="n">ones</span><span class="p">))</span><span class="w"></span>\r | |
222 | <span class="k">end</span><span class="w"></span>\r | |
223 | </pre></div></div></div>\r | |
224 | </div>\r | |
225 | </div>\r | |
226 | </div>\r | |
227 | <div id="footnotes"><hr></div>\r | |
228 | <div id="footer">\r | |
229 | <div id="footer-text">\r | |
230 | </div>\r | |
231 | <div id="footer-badges">\r | |
232 | </div>\r | |
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