<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Hi all<div class=""><br class=""></div><div class="">I discovered that memoization can be pretty hard to implement for pure functions in enums and I wanted to ask if anyone knows of a good way to implement it?</div><div class="">To explain the problem further look at this example of a primitive lazy linked list:</div><div class=""><br class=""></div><div class=""><div class=""><div class="" style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;"><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);">public</span><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);">struct</span><span class="" style="font-variant-ligatures: no-common-ligatures;"> LazyList<T> {</span></div><div class="" style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;"><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);">let</span><span class="" style="font-variant-ligatures: no-common-ligatures;"> root: </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);">LLE</span><span class="" style="font-variant-ligatures: no-common-ligatures;"><</span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);">T</span><span class="" style="font-variant-ligatures: no-common-ligatures;">></span></div><div class="" style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;"><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><br class="webkit-block-placeholder"></div><div class="" style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;"><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);">init</span><span class="" style="font-variant-ligatures: no-common-ligatures;">(</span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);">_</span><span class="" style="font-variant-ligatures: no-common-ligatures;"> root: </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);">LLE</span><span class="" style="font-variant-ligatures: no-common-ligatures;"><</span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);">T</span><span class="" style="font-variant-ligatures: no-common-ligatures;">> = </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);">LLE</span><span class="" style="font-variant-ligatures: no-common-ligatures;"><</span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);">T</span><span class="" style="font-variant-ligatures: no-common-ligatures;">>.</span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(49, 89, 93);">End</span><span class="" style="font-variant-ligatures: no-common-ligatures;">) {</span></div><div class="" style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;"><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);">self</span><span class="" style="font-variant-ligatures: no-common-ligatures;">.</span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);">root</span><span class="" style="font-variant-ligatures: no-common-ligatures;"> = root</span></div><div class="" style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;"><span class="" style="font-variant-ligatures: no-common-ligatures;"> }</span></div><div class="" style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;"><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><br class="webkit-block-placeholder"></div><div class="" style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;"><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);">public</span><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);">var</span><span class="" style="font-variant-ligatures: no-common-ligatures;"> hd: </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);">T</span><span class="" style="font-variant-ligatures: no-common-ligatures;">? {</span></div><div class="" style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;"><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);">get</span><span class="" style="font-variant-ligatures: no-common-ligatures;"> {</span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);">return</span><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);">root</span><span class="" style="font-variant-ligatures: no-common-ligatures;">.</span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(49, 89, 93);">val</span><span class="" style="font-variant-ligatures: no-common-ligatures;">()}</span></div><div class="" style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;"><span class="" style="font-variant-ligatures: no-common-ligatures;"> }</span></div><div class="" style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;"><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><br class="webkit-block-placeholder"></div><div class="" style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;"><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><br class="webkit-block-placeholder"></div><div class="" style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;"><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);">lazy</span><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);">private</span><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);">var</span><span class="" style="font-variant-ligatures: no-common-ligatures;"> tail: </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);">LLE</span><span class="" style="font-variant-ligatures: no-common-ligatures;"><</span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);">T</span><span class="" style="font-variant-ligatures: no-common-ligatures;">> = </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);">self</span><span class="" style="font-variant-ligatures: no-common-ligatures;">.</span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);">root</span><span class="" style="font-variant-ligatures: no-common-ligatures;">.</span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(49, 89, 93);">tail</span><span class="" style="font-variant-ligatures: no-common-ligatures;">()()</span></div><div class="" style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;"><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);">public</span><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);">var</span><span class="" style="font-variant-ligatures: no-common-ligatures;"> tl: </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);">LazyList</span><span class="" style="font-variant-ligatures: no-common-ligatures;"><</span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);">T</span><span class="" style="font-variant-ligatures: no-common-ligatures;">> {</span></div><div class="" style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;"><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);">mutating</span><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);">get</span><span class="" style="font-variant-ligatures: no-common-ligatures;"> {</span></div><div class="" style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;"><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);">return</span><span class="" style="font-variant-ligatures: no-common-ligatures;"> </span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);">LazyList</span><span class="" style="font-variant-ligatures: no-common-ligatures;"><</span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);">T</span><span class="" style="font-variant-ligatures: no-common-ligatures;">>(</span><span class="" style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);">tail</span><span class="" style="font-variant-ligatures: no-common-ligatures;">)</span></div><div class="" style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;"><span class="" style="font-variant-ligatures: no-common-ligatures;"> }</span></div><div class="" style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;"><span class="" style="font-variant-ligatures: no-common-ligatures;"> }</span></div></div><div class="" style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;"><span class="" style="font-variant-ligatures: no-common-ligatures;">}</span></div></div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">enum</span><span style="font-variant-ligatures: no-common-ligatures" class=""> LLE<T> {</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">case</span><span style="font-variant-ligatures: no-common-ligatures" class=""> End</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">indirect</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">case</span><span style="font-variant-ligatures: no-common-ligatures" class=""> Node(</span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">T</span><span style="font-variant-ligatures: no-common-ligatures" class="">, () -> </span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">LLE</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">T</span><span style="font-variant-ligatures: no-common-ligatures" class="">>)</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><br class="webkit-block-placeholder"></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">func</span><span style="font-variant-ligatures: no-common-ligatures" class=""> val() -> </span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">T</span><span style="font-variant-ligatures: no-common-ligatures" class="">? {</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">switch</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">self</span><span style="font-variant-ligatures: no-common-ligatures" class=""> {</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">case</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">let</span><span style="font-variant-ligatures: no-common-ligatures" class=""> .</span><span style="font-variant-ligatures: no-common-ligatures; color: #31595d" class="">Node</span><span style="font-variant-ligatures: no-common-ligatures" class="">(v, </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">_</span><span style="font-variant-ligatures: no-common-ligatures" class="">):</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">return</span><span style="font-variant-ligatures: no-common-ligatures" class=""> v</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">default</span><span style="font-variant-ligatures: no-common-ligatures" class="">:</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">return</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">nil</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><br class="webkit-block-placeholder"></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">func</span><span style="font-variant-ligatures: no-common-ligatures" class=""> tail() -> () -> </span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">LLE</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">T</span><span style="font-variant-ligatures: no-common-ligatures" class="">> {</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">switch</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">self</span><span style="font-variant-ligatures: no-common-ligatures" class=""> {</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">case</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">let</span><span style="font-variant-ligatures: no-common-ligatures" class=""> .</span><span style="font-variant-ligatures: no-common-ligatures; color: #31595d" class="">Node</span><span style="font-variant-ligatures: no-common-ligatures" class="">(</span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">_</span><span style="font-variant-ligatures: no-common-ligatures" class="">, tl):</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">return</span><span style="font-variant-ligatures: no-common-ligatures" class=""> tl</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">default</span><span style="font-variant-ligatures: no-common-ligatures" class="">:</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(209, 47, 27);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #3e1e81" class="">assert</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">(</span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">false</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">, </span><span style="font-variant-ligatures: no-common-ligatures" class="">"Can't call tail on empty list"</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">)</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">return</span><span style="font-variant-ligatures: no-common-ligatures" class=""> {</span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">self</span><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><br class=""></span></div><div style="margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures;" class="">For the memoization I need a separate struct which wraps the actual linked list. For this primitive list this would work, but as soon as you want to implement “count”, it gets very complicated: A call of count would require the evaluation of the tail, so it would make sense to memoize the tail (and its computed count property). But this way it is not possible, because a struct does not allow to have a</span> stored property that references itself. And as I cannot have any stored property at all in a enum, I don’t find a way how I could implement memoization for a pure function like “tail” in the enum.</div><div style="margin: 0px; line-height: normal;" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class="">So is it somehow possible to memoize the output of a pure function in an enum?</div><div style="margin: 0px; line-height: normal;" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class="">Greetings</div><div style="margin: 0px; line-height: normal;" class="">Tierry Hoermann</div></div></body></html>