StackOverflow2013

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POusing callCC with higher-order functions in R
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copied!<p>I'm trying to figure out how to get R's callCC function for short-circuiting evalutation of a function to work with functions like lapply and Reduce.</p> <p><strong>Motivation</strong></p> <p>This would make Reduce and and lapply have asymptotic efficiency > O(n), by allowing you to exit a computation early.</p> <p>For example, if I'm searching for a value in a list I could map a 'finder' function across the list, and the second it is found lapply stops running and that value is returned (much like breaking a loop, or using a return statement to break out early).</p> <p>The problem is I am having trouble writing the functions that lapply and Reduce should take using a style that callCC requires.</p> <p><strong>Example</strong></p> <p>Say I'm trying to write a function to find the value '100' in a list: something equivalent to</p> <pre><code>imperativeVersion <- function (xs) { for (val in xs) if (val == 100) return (val) } </code></pre> <p>The function to pass to lapply would look like:</p> <pre><code>find100 <- function (val) { if (val == 100) SHORT_CIRCUIT(val) } functionalVersion <- function (xs) lapply(xs, find100) </code></pre> <p>This (obviously) crashes, since the short circuiting function hasn't been defined yet.</p> <pre><code>callCC( function (SHORT_CIRCUIT) lapply(1:1000, find100) ) </code></pre> <p>The problem is that this also crashes, because the short circuiting function wasn't around when find100 was defined. I would like for something similar to this to work.</p> <p>the following works because SHORT_CIRCUIT IS defined at the time that the function passed to lapply is created.</p> <pre><code>callCC( function (SHORT_CIRCUIT) { lapply(1:1000, function (val) { if (val == 100) SHORT_CIRCUIT(val) }) ) </code></pre> <p>How can I make SHORT_CIRCUIT be defined in the function passed to lapply without defining it inline like above?</p> <p>I'm aware this example can be achieved using loops, reduce or any other number of ways. I am looking for a solution to the problem of using callCC with lapply and Reduce in specific.</p> <p>If I was vague or any clarification is needed please leave a comment below. I hope someone can help with this :)</p> <p>Edit One: The approach should be 'production-quality'; no deparsing functions or similar black magic. </p>

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