HCoop / hcoop / debian / mlton.git / blame
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| 5 | <meta name="generator" content="AsciiDoc 8.6.9">\r | |
| 6 | <title>EtaExpansion</title>\r | |
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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>EtaExpansion</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>Eta expansion is a simple syntactic change used to work around the\r | |
| 32 | <a href="ValueRestriction">ValueRestriction</a> in <a href="StandardML">Standard ML</a>.</p></div>\r | |
| 33 | <div class="paragraph"><p>The eta expansion of an expression <span class="monospaced">e</span> is the expression\r | |
| 34 | <span class="monospaced">fn z => e z</span>, where <span class="monospaced">z</span> does not occur in <span class="monospaced">e</span>. This only\r | |
| 35 | makes sense if <span class="monospaced">e</span> denotes a function, i.e. is of arrow type. Eta\r | |
| 36 | expansion delays the evaluation of <span class="monospaced">e</span> until the function is\r | |
| 37 | applied, and will re-evaluate <span class="monospaced">e</span> each time the function is\r | |
| 38 | applied.</p></div>\r | |
| 39 | <div class="paragraph"><p>The name "eta expansion" comes from the eta-conversion rule of the\r | |
| 40 | <a href="LambdaCalculus">lambda calculus</a>. Expansion refers to the\r | |
| 41 | directionality of the equivalence being used, namely taking <span class="monospaced">e</span> to\r | |
| 42 | <span class="monospaced">fn z => e z</span> rather than <span class="monospaced">fn z => e z</span> to <span class="monospaced">e</span> (eta\r | |
| 43 | contraction).</p></div>\r | |
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