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
    (do
      (let* [<the destructuring bits>]
        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 when-match expression, the picture looks very different (formatted for clarity):

[sourcecode lang=”clj” gist=”779635″]

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