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Eta expansion is a simple syntactic change used to work around the <a href="ValueRestriction">ValueRestriction</a> in <a href="StandardML">Standard ML</a>. <p>
The eta expansion of an expression <tt>e</tt> is the expression <tt>fn z => e z</tt>, where <tt>z</tt> does not occur in <tt>e</tt>. This only makes sense if <tt>e</tt> denotes a function, i.e. is of arrow type. Eta expansion delays the evaluation of <tt>e</tt> until the function is applied, and will re-evaluate <tt>e</tt> each time the function is applied.
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The name "eta expansion" comes from the eta-conversion rule of the <a href="LambdaCalculus">lambda calculus</a>. Expansion refers to the directionality of the equivalence being used, namely taking <tt>e</tt> to <tt>fn z => e z</tt> rather than <tt>fn z => e z</tt> to <tt>e</tt> (eta contraction).
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Last edited on 2006-03-28 00:57:50 by <span title="adsl-71-141-33-187.dsl.snfc21.sbcglobal.net"><a href="StephenWeeks">StephenWeeks</a></span>.
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