doc/guide/localhost/EtaExpansion

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  26 <h1>EtaExpansion</h1>
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  31 <div class="paragraph"><p>Eta expansion is a simple syntactic change used to work around the
  32 <a href="ValueRestriction">ValueRestriction</a> in <a href="StandardML">Standard ML</a>.</p></div>
  33 <div class="paragraph"><p>The eta expansion of an expression <span class="monospaced">e</span> is the expression
  34 <span class="monospaced">fn z =&gt; e z</span>, where <span class="monospaced">z</span> does not occur in <span class="monospaced">e</span>.  This only
  35 makes sense if <span class="monospaced">e</span> denotes a function, i.e. is of arrow type.  Eta
  36 expansion delays the evaluation of <span class="monospaced">e</span> until the function is
  37 applied, and will re-evaluate <span class="monospaced">e</span> each time the function is
  38 applied.</p></div>
  39 <div class="paragraph"><p>The name "eta expansion" comes from the eta-conversion rule of the
  40 <a href="LambdaCalculus">lambda calculus</a>.  Expansion refers to the
  41 directionality of the equivalence being used, namely taking <span class="monospaced">e</span> to
  42 <span class="monospaced">fn z =&gt; e z</span> rather than <span class="monospaced">fn z =&gt; e z</span> to <span class="monospaced">e</span> (eta
  43 contraction).</p></div>
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