After my [previous post comparing pattern matching and destructuring in Clojure](http://blog.fogus.me/2011/01/12/pattern-matching-vs-destructuring-to-the-death/) a fine gentleman was kind enough to point out an incredibly concise summary: > Pattern matching is a conditional construct and destructuring isn’t. > > -- [a fine gentleman](http://clojure.org) This is absolutely correct, and I missed making that distinction in a clear way. Consider the following, from my previous post: (def $ [1 2 3 4 5]) (when-match [[1 2 ?x 4 5] $] x) ;=> 3 (when-match [[1 2 ?x 400 500] $] x) ;=> nil It appears that conditional branching is occurring somehow based on pattern matching, but what about the following using destructuring: (when-let [[_ _ x _ _] $] x) ;=> 3 Now it would seem that destructuring also acts a conditional, but that is an illusion brought on by the nature of `when-let`. We can see the distinction more clearly if we pull apart what is actually happening behind the scenes using the `destructure` function[^lookat] (formatted for clarity): (destructure '[[_ _ x _ _] [1 2 3 4 5]]) ;=> [V [1 2 3 4 5] _ (nth V 0 nil) _ (nth V 1 nil) x (nth V 2 nil) _ (nth V 3 nil) _ (nth V 4 nil)] That is, the result of `destructure` is a simple sequence of steps to pull apart the vector `[1 2 3 4 5]` piece by piece. Replacing the structure above into the expansion of the previous `when-let` you can see how the actual condition occurs (formatted for clarity): (let* [temp $] (if temp (do (let* [] x)))) That is, not only does the destructuring provide no conditional property, but its not even involved in the expanded conditional logic at all. However, if we expand [Matchure's](https://github.com/dcolthorp/matchure) `when-match` expression, the picture looks very different (formatted for clarity): [sourcecode lang="clj" gist="779635"] 301 Moved Permanently

301 Moved Permanently

nginx/0.7.67 [/sourcecode] While this expansion is pretty big, it's clear that the code produced is walking through the vector `[1 2 3 4 5]` and conditionally checking its contents against the expected pattern. And therein lies the reality of the opening quote. :F [^lookat]: If you really want to understand destructuring, then there is no better way than to experiment with the `destructure` function.