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    <p>We have 3 possible situations, only one at a time is applicable to a collection. </p> <ul> <li>The collections has items and they all pas the predicate - True</li> <li>One of the item in the coll failed the predicate - False</li> <li>The coll is empty - ??</li> </ul> <p>Now if you think about returning false for empty list, it would also feel weird because false is returned when at least one item in the coll failed the predicate but for empty list that didn't happened.</p> <p>I think the rational to choose true for empty coll was based on the fact that the number of cases where this is applicable in real world were more as compared to where it would not make sense. For example: Someone gave me a list of URLs and I need to return their downloaded content, now my function first want to verify that each URL is in proper format and then use map or pmap to download the URLs -</p> <pre><code>(if (every? url? urls) (map download urls) (throw "Invalid url found")) </code></pre> <p>This and more examples of this kind will make working with sequences more seamless.</p> <p>On the other hand I am a statically typed language guy (like Haskell) and would prefer to use types to solve this problem. Rather than returning a true or false I would want this function to return <code>Maybe bool</code> such that:</p> <ul> <li>Every item in the coll pass the predicate - <code>Just True</code></li> <li>One of the item in the coll failed the predicate - <code>Just False</code></li> <li>The coll is empty - <code>Nothing</code></li> </ul>
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    1. POIn clojure, (every? pred coll) returns true if coll is empty, could this be a design flaw?
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    1. COThe first of your situations includes the last one. Maybe you wanted the first one to be "the collections has items and they all pas the predicate"?
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      1. USOmar Antolín-Camarena
    2. COUpdated........
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    3. COThanks for the solutions, I agree possibly in real world we may need to define a third type like Nothing here. From summary part of Wikipedia [Vacuous_truth](http://en.wikipedia.org/wiki/Vacuous_truth) I read "There seems to be no direct reason to pick true; it’s just that things blow up in our face if we don’t".
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      1. USKevin Zhu
 

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