Jay Fields is a software developer at DRW Trading. He has a passion for discovering and maturing innovative solutions. His most recent work has been in the Domain Specific Language space where he's delivered applications that empowered subject matter experts to write the business rules of the applications. He is also very interested in maturing software design through software testing. Jay is a DZone MVB and is not an employee of DZone and has posted 112 posts at DZone. You can read more from them at their website. View Full User Profile

Clojure: Converting a Custom Collection to a Sequence

04.16.2010
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I've been doing a bit of clojure lately and I've often found myself looking to convert a custom collection to a sequence. Unfortunately, several custom collections from libraries that we use don't implement Iterable; therefore, I can't simply use the seq function to create a seq.

In Java it's common to simply use a for loop and a get method to iterate through custom collections that don't implement Iterable.

for (int i=0; i<fooCollection.size(); i++) {
Foo foo = fooCollection.get(i);
}
There are patterns for doing something similar in Clojure; however, I much prefer to work with Sequences and functions that operate on Sequences. Shane Harvie and I added the following snippet to our code to quickly convert a custom collection to a seq.
(defmacro obj->seq [obj count-method]
`(loop [result# []
i# 0]
(if (< i# (. ~obj ~count-method))
(recur (conj result# (.get ~obj i#)) (inc i#))
result#)))
Our first version only worked with a custom collection that had a .size() method; however, the second collection we ran into used a .length() method, so we created the above macro that allows you to specify the (size|length|count|whatever) method.

The obj->seq function can be used similar to the following example.
(let [positions (obj->seq position-collection size)]
; do stuff with positions
)
I'm fairly new to Clojure, so please feel free to provide recommendations and feedback.

From http://blog.jayfields.com/2010/04/clojure-converting-custom-collection-to.html

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