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Pattern Matching vs. Destructuring… Electric Boogaloo

Jan 14, 2011

After my previous post comparing pattern matching and destructuring in Clojure 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

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 function1 (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
      (let* [<the destructuring bits>]

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 when-match expression, the picture looks very different (formatted for clarity):

[sourcecode lang=”clj” gist=”779635″] 301 Moved Permanently

301 Moved Permanently


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.


  1. If you really want to understand destructuring, then there is no better way than to experiment with the destructure function. 

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